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I. SOCIAL ASPECTSWilliam Petersen





In its most general sense “migration” is ordinarily defined as the relatively permanent movement of persons over a significant distance. But this definition, or any paraphrase of it, merely begins to delimit the subject, for the exact meaning of the most important terms (“permanent,” “significant”) is still to be specified. A person who goes to another country and remains there for the rest of his life, we say, is a migrant; and one who pays a two-hour visit to the nearest town is not. Between these two extremes lies a bewildering array of intermediate instances, which can only partly be distinguished by more or less arbitrary criteria (Lacroix 1949).

Permanence of movement

What should be the minimum duration of stay that differentiates a migration from a visit? With respect to international migration, the recommendation of the United Nations (and the practice of a number of countries) is to define removal for one year or more as “permanent,” and thus as migration, while a stay for a shorter period is classified as a visit. Note that the data reflect not behavior but statements about future behavior; and persons have been known to lie to immigration officials or to change their minds.

This kind of ambiguity often makes it difficult to interpret migration statistics. For example, according to a critical analysis of United States im-migration statistics (Kuznets & Rubin 1954, table 7), during the height of the mass immigration of 1890-1910 about forty per cent of the foreign-born returned. Conclusions from uncorrected immigration data, therefore, are likely to be grossly inaccurate. Remigrants—those who leave their country of origin for a period and then return to it—ordinarily differ from the emigrants who remain abroad, but not necessarily according to any consistent pattern. Particularly during an economic depression, some immigrants left the new country when they lost their jobs (Berthoff 1953, p. 73). Sometimes, on the contrary, it was the relatively successful that returned, either to find wives (Borrie 1954) or to retire. The rise of nationalism in the old country often attracted back some of the incompletely assimilated migrants (Saloutos 1956). A small percentage of the subsidized emigrants to Australia and Canada have returned to Britain, and the especially careful studies made of these groups are largely inconclusive (Appleyard 1962a; 1962b; Richmond 1966). Almost by definition, remigrants are less able to acculturate to their new environment than immigrants who remain there, but few valid generalizations can be added to that truism.

The ambiguity pertains also to the definition of internal migrants (Hamilton 1961; Taeuber 1961). Particularly in a federal country like the United States, “permanence” of movement is defined, in effect, by laws stipulating the meaning of domicile with respect to marriage and divorce, suffrage, and other prerogatives reserved to “bona fide residents.” Surveys by the U.S. Bureau of the Census show that each year approximately one person in five moves to a new residence (compare Wilber 1963). However, according to a detailed analysis of one particular community (Goldstein 1954), a sizable proportion of this large percentage is made up of persons who move more than once during a year and who are atypical also in other ways. A study of repeated migration in Denmark suggests that the phenomenon is not restricted to any one country (Goldstein 1964).

More generally, when one speaks of migratory birds, or migrant laborers, or nomads, the connotation is not of a permanent move from one area to another, but rather of a permanently migratory way of life, which often means a cyclical movement within a more or less definite area. Nomads (the word derives from the Greek for pasturing) typically follow their herds back and forth over a region delimited either by natural boundaries or by neighbors sufficiently powerful to repel incursions. Similarly, agricultural laborers often move with the growing season, and shepherds (in what is termed transhumance) alternate between high mountain pastureland in the summer and lowlands in the winter. Commutation, the daily “journey to work” (Liepmann 1944), constitutes a similar cycle within a smaller compass. One must not accept the common notion that such a separation of place of residence from place of work is peculiar to modern industrial societies. Many of the burghers of ancient Athens, fourteenth-century London, and preindustrial cities generally were part-time agriculturists (Petersen 1961, pp. 348-353). In many presently underdeveloped countries, particularly India and Pakistan, a peasant who migrates to the city often leaves his family in the village, to which he therefore returns periodically. In Africa south of the Sahara the temporary separation of male industrial or mine workers from village life has been institutionalized into the standard pattern (Mitchell 1961). In sum, whether short-term re movals should be included in migration depends on the purpose of the statistics being collected. Thus, no particular specification of the duration of stay suits all purposes, and each analyst has to adapt the available data to his needs as best he can.

“Significant” distances

The meaning of migration also varies according to how a “significant” distance is defined. The word derives from the Latin migrare, to change one’s residence, but by current definitions it means rather to change one’s community. A person who moves from one home to another in the same neighborhood, and who therefore retains the same social framework, is not deemed a migrant.

If we regard a nation as a community, then by this criterion all international movements are included under the rubric “migration.” Partly because of this rationale, partly because the two sets of statistics are separately collected, the distinction between international and internal migration sets the framework of most analyses. It is worth emphasizing, therefore, that in a general discussion of the phenomenon the distinction is more or less irrelevant. Not only do some types of migration fall outside of this dichotomy (prehistoric wanderings, for instance), but some of the most important and interesting characteristics of migrants apply whether or not they cross an international border (labor mobility, urbanization, migratory selection, acculturation, etc.). Moreover, there are often greater cultural differences within the boundaries of a nation than between nations.

In practice, geographical distance is generally taken as a rough measure of whether the migrant crosses into another community. Thus, the U.S. Bureau of the Census divides the mobile population between “movers,” who have changed their residence within a single county, and “migrants,” who have crossed a county line, and it subdivides the latter category according to whether they move within a single state, to an adjacent state, or to a nonadjacent state. This kind of classification passes over the fact that a farmer who moves to a town in the same county probably changes his way of life more than one who crosses the nation but remains a farmer. To take a more striking example, the tens of thousands of refugees who fled from East to West Berlin have traversed the most significant boundary line in Europe while remaining within the confines of a single city.

Models of migration

It is reasonable to suppose that the number of migrants within any area homogeneous with respect to all the other factors that affect the propensity to migrate will be inversely related to the distance covered. One can express this relation in an equation, as follows: M = aX/D&, where M stands for the number of migrants, D for the distance over the shortest transportation route, and X for any other factor that is thought to be relevant; a and b are constants, usually set at unity. In one version of this equation, the so-called P1P2/D hypothesis, the populations of the end points of the movement are taken as the X factor (Zipf 1949). Another variation is the familiar proposition that the number of persons going a given distance is directly proportional to the number of employment opportunities at that distance and inversely proportional to the number of intervening opportunities (Stouffer 1940). When “opportunities” were defined operationally as the number of in-migrants, the hypothesis could be validated in a number of instances. According to a detailed comparison of the two, Stouffer’s formulation is better than Zipf’s, since, in effect, measuring opportunities corrects the total population figures for the amount of unemployment in the two areas (Anderson 1955). (For other models, see Lovgren 1956; Thomlinson 1961; Heide 1963; Tabah & Cataldi 1963.)

A proposition about migration between only two points is too simplistic a unit, however, to be a useful building block for more elaborate theories. These have in general started from other premises. An important example is the three-volume study Population Redistribution and Economic Growth: United States, 1870-1950 (Kuznets 1957-1964), in which the available data concerning the regional distribution of the developing national economy and data concerning internal migration are combined into a unified analysis of the interaction between the two.

This kind of analysis is not limited to migration within a single country. A shorter work in the same broad perspective analyzes the post-1945 migration to Switzerland (Mayer 1966). According to several studies of the transatlantic movement, if conditions in the home country build up a propensity to emigrate, the volume, direction, and timing of the movement are set largely by the business cycle in the receiving country (e.g., D. S. Thomas 1941). A later work, however, challenged this interpretation and placed more emphasis on the unity of “the Atlantic economy” and the importance of “push” factors (B. Thomas 1954).

Noneconomic motives

In most of the supposedly general models of migration, it is presumed that movement is generated mainly by economic forces. This may not always be a reasonable postulate. Whether the correlation between business cycles and migratory movements is positive or negative, for example, sometimes depends only on how broadly the study is conceived. While the rise of Europe’s urban-industrial civilization brought a great increase in population and thus a pressure to emigrate, it also resulted in a general rise in the level of aspiration. Young men who were better off than their fathers were nonetheless dissatisfied, and many sought to better themselves overseas. Thus, it may be true to say that for certain periods the dominant motivation of emigrants from particular countries was economic, even though these countries had, by and large, far better conditions than those from which very few persons left. This paradox is not limited to economic factors: religious oppression, or the infringement of political liberty, was often a motive for European emigration, but before the rise of modern totalitarianism those who left came predominantly from precisely those countries least marked by such stigmata. An increasing propensity to emigrate spread east and south from northwest Europe, together with democratic institutions and religious tolerance. The anomaly that those who emigrated “because” of persecution tended to come from countries where there was less of it than elsewhere can be analyzed only by separating personal motivation from social causation. According to a recent survey of British emigrants’ motives, they tended to rationalize their general feeling of insecurity and inadequacy into more specific economic factors (Appleyard 1964).

At least in the United States, internal migration is also less motivated by economic factors than is usually assumed. At one time, the U.S. Bureau of the Census asked a sample of migrants who had moved during one year why they had moved (“Postwar Migration” . . . 1947; compare “Reasons for Moving . . .” 1966, which showed similar responses). Only 22.6 per cent said it was to take a job or to look for work. Family migration constituted 61.7 per cent (i.e., moving with the head of the family or to join him, moving because of a change in marital status); 6.4 per cent said it was because of housing problems. Health, climate, education, and miscellaneous motives accounted for the remaining respondents. This conclusion has been generally validated by the few other studies made of internal migrants’ motivation (e.g., Rossi 1955).

Migratory selection

That migration is both related to economic trends and yet not, in any simple sense, caused by them, should not occasion any surprise. The same is true of many other complex social phenomena. It would be no contribution to substitute for purely economic causes a list of other “factors,” ranging from the spirit of adventure to the development of transportation facilities; nor would it be a great improvement to divide such a list between circumstances at home that repel and those abroad that attract, that is, between “push” and “pull” factors. Given a sedentary population and an inducement to leave home, typically some persons go and some stay behind. Push and pull factors, in short, do not exert their force equally. The self-selection by which migrants differentiate themselves from the sedentary population is called migratory selection (or, by some authors, selective migration). An analysis of this process can afford a better understanding of why a migration takes place.

It is a valuable extension of the Stouffer-Zipf generalization, for example, to go beyond the counting of heads and differentiate among the types of migrants. It is not sufficient, even in an analysis restricted to economically motivated migrations, to posit job opportunities in general: potential migrants with specific skills go to places where there are openings specifically for them. Thus, among white migrants within the United States, those seeking higher-status positions generally have to move greater distances than those with lower levels of skill. And some job-seeking migrants are also strongly motivated by noneconomic factors: among American Negroes important reasons for moving have been to get out of the rural South (hence the high rate of urbanization), and prefer-ably to get out of the South altogether (hence the shift to the North and West). Negroes, therefore, move greater distances than would be expected from the level of skill in the jobs that they typically seek (Rose 1958; compare Stub 1962; Taeuber & Taeuber 1965).

It is possible to analyze migratory selection by a number of demographic and social characteristics in addition to occupation and race; and although the conclusions from different studies vary widely, some tentative generalizations are possible (D. S. Thomas 1938; Petersen 1961, pp. 592-603). In both internal and international movements adolescents and young adults predominate; for not only do the young adapt more easily, but since they are close to the beginning of their working life, they can more readily take advantage of new opportunities. It is feasible, therefore, to analyze migration by cohorts (Eldridge 1964).

One can argue a priori that either the less or the more intelligent will tend to migrate: since the more intelligent will have succeeded at home, the less intelligent will seek their fortunes elsewhere; on the other hand, the more intelligent will respond first to any stimulus to migrate, while the duller will remain behind. Various studies have seemed to validate one or the other of these propositions. It is possible to reconcile the contradiction by postulating that a selection by intelligence is in fact one by actual or potential occupational level (Hofstee 1952; compare Lee 1966). Thus, since urban occupations are generally more demanding, ruralurban movements typically select the more intelligent. This is not true, however, of agriculturists who move to manual jobs in the cities, such as Negroes in the United States (D. S. Thomas 1938, pp. 111-121), or in general of migrants who make no substantial change in vocational level.

Effects on populations

For the two areas concerned, migratory selection determines the significance of the movement almost as much as the number of migrants. Consider the ramifications of what can be taken as the most fundamental question in migration theory: If X persons leave country A and migrate to country B, what changes take place in the size of the two populations (Petersen 1955, chapter 9)? The common-sense answer, that country A is decreased and country B is increased by X, is true only in the short run. If the typically young migrants have their children in their new country, its fertility rate may go up, while that of their native country goes down. Since the remaining population of country A will then be older on the average, its death rate may go up, while that of country B goes down. In short, after a generation the transfer of X persons will in fact amount to X plus a certain proportion based on the migration’s effects on the population structures, and rates of population growth, of the two countries.

At a third level of analysis, however, this increment, and indeed X itself, may be canceled out. For Malthus, thus, emigration was a slight palliative, a partial and temporary expedient, with no permanent effect on population size (Malthus 1798, book 3, chapter 7). This is likely to be true of any country where the mortality of infants and children is high (so that emigration would reduce the mortality slightly), or where marriages and conceptions are put off because of economic pressure (so that a lesser pressure, the consequence of emigration, would result in a higher fertility). If one includes such indirect effects, the change in the population of the immigration country is also difficult to estimate. Immigration to the United States, for example, accelerated urbanization and industrialization; and these changes, in turn, increased the upward social mobility of the native population and thus tended to accelerate the secular decline of the birth rate.

In sum, even the simplest question—How many persons migrated?—cannot be fully answered merely by counting heads. Unlike mortality and fertility, migration has no biological dimension: it cannot be analyzed, even in preliminary terms, independently of its cultural context. Accordingly, there are no “laws” of migration in the sense of universal generalizations; the highest level of abstraction possible is the contrast of various types of migrants (Heberle 1955).

Migration typologies

In a study of migrants to Aberdeen—that is, of movement within the single country of Scotland over only a few years—it was found useful to classify respondents into a number of types. These included professionals seeking careers, young persons seeking education, workers taking specific jobs, casual workers looking for employment, former commuters moving for greater convenience, family migrants joining heads of families, and return migrants (Illsley et al. 1963, pp. 238-240). The conclusions to be drawn differed for these various classes.

If this is so for movements within a relatively homogeneous area, then it is manifestly the case for migration in its most general terms, encompassing the whole world and all of human history. In constructing a general typology, one should begin by choosing the criteria by which the types are to be distinguished (Petersen 1964, pp. 271-290). Perhaps the most fundamental is the distinction between innovating migrants, who move in order to achieve the new, and conservative migrants, who move in response to a change in their circumstances, hoping by migrating to retain their way of life in another locus. Within each of these two broad classes, one can distinguish types of migration according to the force impelling the movement. An ecological push results in what might be termed a primitive migration—not a wandering of primitive tribes as such, but one dependent on a people’s inability to cope with natural forces. When the activating agent is the state or some equivalent institution, the movement is forced or impelled migration, depending on whether the prospective migrants retain some power to decide whether to leave or not. A movement of adventurous pioneers, deviant religious or political groups, or similar individually motivated persons can aptly be termed free migration. Its importance is not in its size, which is never large, but in the example it sets for others. If the ensuing flow develops into a broad stream, an established pattern for whole social classes, an example of collective behavior, we speak of mass migration, similar to what has been termed “chain migration” (MacDonald & MacDonald 1964). Then individual motivations become correspondingly less important—indeed, the individuals involved may not be able to give a rational account of their decision to migrate. The motives they ascribe are likely to be trivial or, more probably, the generalities that they think are expected (Hansen 1940a, pp. 77–78).

Uses of typological method

The value of such a typology is in its utility: Does it help in solving analytical problems? The typology suggests, first of all, that migratory selection ranges along a continuum, from total migration at one extreme (food gatherers or nomads) to total nonmigration at the other. Intermediate instances, moreover, cannot be arranged along a single dimension. Sometimes it is the age or the sex or the occupation of the potential migrant that is relevant, but if an ethnic or social minority leaves to escape persecution or is shipped off to concentration camps, then the only pertinent characteristic is how the state defines “Jew” or “kulak,” for example.

For more than a century various governments, concerned about the depopulation (real or supposed) of villages, have sought measures to counteract it. It would increase understanding of the process merely to ask whether this is a conservative or an innovating migration: Do these agriculturists want better conditions within their present way of life, or do they move to cities for the sake of urban amenities? Perhaps the most useful distinction in the typology is that between mass migration and all other types, for it emphasizes the fact that the nineteenth-century exodus from Europe does not constitute the whole of the phenomenon. When this type of migration declined after World War i, largely because of new political limitations imposed by both emigration and immigration countries, this was very often interpreted as marking the end of significant human migration altogether (e.g., Forsyth 1942). It was rather, in large part, a change to neomercantilist migration, in which the welfare of the national state becomes the main criterion for judging whether the movement is desirable and in which state agencies foster or impede, force or prevent, the migration. The “natural” right of the passportless person to move about has been sup-planted by the “natural” right of the state to control that movement (Petersen 1955, chapter 1).

In the present age of total wars and totalitarian regimes, political motivations have set not only “Europe on the move” (Kulischer 1948) but also, partly as reverberations of European influences, much of the rest of the world. To take a notable example, the partition of British India into the nations of India and Pakistan was accompanied by one of the largest migrations in human history, in part induced by terrorists on both sides, in part arranged under state auspices. Many analysts prefer to omit this type of movement from their purview. In a United Nations publication, for example, international migration is defined as “the noncoerced migrations, which constitute the great majority of all migratory movements in normal times, and which are closely related to economic and social factors. . . . Specifically, ’migration’ excludes population transfers, . . . deportations, refugee movements, and the movements of ’displaced persons’” (United Nations 1953, p. 98). That an international body which includes some of the states most responsible for forced migrations should exclude them from its demographic analyses is understandable; but there is no reason why independent scholars should accept this arbitrary and misleading definition.

The number of refugees in the world today depends of course on how that term is defined. Data on “refugees” are compiled mainly by the various agencies set up to aid them, and the resultant totals considerably understate the number of persons who have migrated because of political stress and sought refuge elsewhere. The major world-wide agency, the office of the U.N. High Commissioner for Refugees, has a narrowly restricted prime mandate: to assist persons who do not want to return to their country because of actual or feared racial, religious, or political persecution; and it may also extend its “good offices” to certain other limited categories. This definition does not include several numerically important classes of uprooted peoples: (1) those who have fled from local political disturbances but remain within the boundaries of the same state; (2) those who are forcibly moved about within the boundaries of a single state (see, for example, Conquest 1960); (3) those who have been forced to “return” to what is now defined as “their” country, after having lived “abroad” sometimes for generations. Thousands of refugees remain as hard-core cases from World War i, the Spanish Civil War, and World War n. It has been estimated, probably conservatively, that about forty million persons became refugees in the dozen years following 1945 (Rees 1957); the implication of the figure can be better grasped when it is recalled that the usual estimate for the total migration from all of Europe from 1800 to 1950 is only one and a half times as large, that is, sixty million.

William Petersen

[See alsoPopulation; Refugees; and the biographies ofKulischer; Wlllcox.]


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This article is mainly concerned with international migration, which may be defined, in the strict sense, as a permanent movement of people, of their own free will, from one sovereign country to another. Transfers of this kind, however, account for only a small part of the redistribution of world population in the last three centuries. A comprehensive view of international migration must therefore include forced as well as free move ments, and temporary as well as permanent movements. It is also necessary to distinguish between intercontinental and intracontinental transfers.

Free and forced movements

Up to the beginning of the nineteenth century there were hardly any statistical records of international migration. Nevertheless, it is possible to indicate the main orders of magnitude. The first great Atlantic migration was the traffic in African slaves. It is estimated that over ten million slaves were transported to America between 1619 and 1776 and that 3.4 million of them went to the English colonies in America. The trade was initiated by the Portuguese and the Dutch. Britain also took part, beginning in the 1660s, mainly through the Royal African Company, which existed from 1672 to 1752. The plantation economies producing sugar in the West Indies, tobacco in Virginia, and rice and indigo in South Carolina entailed a growing demand for slave labor from Africa. In the West Indies white immigration was replaced by black on one island after another, until slaves constituted about four-fifths of the population.

In contrast, the white migration across the Atlantic in the seventeenth and eighteenth centuries was comparatively small. In the seventeenth century about 250,000 left the British Isles for the New World, and in the eighteenth century the out-flow was perhaps 1,500,000, of whom about half a million were Ulster Presbyterians. An indication of the volume of Spanish emigration to the New World is given by the fact that 150,000 were recorded as having embarked at the port of Seville between 1509 and 1740, but this is a serious underestimate. The only other prominent group of trans-Atlantic migrants up to 1800 was the 200,000 Germans estimated to have left for America.

The nineteenth century was the great age of mass migration from Europe across the Atlantic of people who went of their own free will, and most of what we know about the economic and social determinants and consequences of international migration is based on the experience of that remarkable period. Between 1846 and 1932 about 52 million people left Europe for oversea destinations. When this redistribution was over, one-eleventh of the population of the world were people of European origin living outside Europe.

One of the most somber features of our time is that while there has been a sharp decline in free international mobility, the relative scale of forced transfers is reminiscent of the eighteenth century. The world picture has been dominated by movements of refugees, as shown by the following rough estimates. The partition of India and Pakistan led to the expulsion of more than 18 million people from their homes. Other outstanding examples of inflows of refugees after World War n are West Germany, 12 million; Japan, 6.3 million; South Korea, 4 million; Hong Kong, 1.3 million; Israel, 1 million; Arab refugees from Palestine, 1 million. About 1.5 million refugees were settled overseas by the International Refugee Organisation and the Intergovernmental Committee for European Migration [see Refugees, article on World Problems]. This is only a partial account, for it is impossible to give estimates of the considerable forced movements which have taken place between countries controlled by the Soviet Union and China. Even this partial estimate of the international transfer of “political” migrants gives a total of 45 million for the ten years beginning in 1945. It is a sobering thought that the number of people expelled from one country to another in the decade after World War II was equal to the entire oversea emigration from Europe in the century ending in 1913.

Temporary and permanent movements

Of the large number of passengers who enter or leave any country in a given period, only a proportion are genuine migrants. Persons to be counted as migrants are those who move from one country to a permanent residence in another, and, in accordance with United Nations standards, the criterion generally adopted is a declared intention to stay in the receiving country for more than one year. Owing to the wide variety of methods used in different countries, it is not easy to obtain accurate statistics. It must be recognized, however, that much of the international mobility of labor which is of economic significance is of a temporary nature. This is particularly true on the European continent, where daily or weekly or seasonal movements over national frontiers occur on a considerable scale. For example, permanent immigration into Switzerland rose steadily from 1946 to 1957 and amounted to 690,000 workers for the period as a whole; during the same period there were 942,000 seasonal workers and 275,000 frontier workers. The net immigration for the period was, however, only 250,000; this demonstrates that a number of the aliens admitted as “permanent” immigrants (a great many being women) are in fact in the temporary category. On the American continent a similar case may be seen in the seasonal traffic across the border between Mexico and the United States.

Intercontinental and intracontinental movements

The distinction between intercontinental and intracontinental movements is illuminating when we are dealing with the phases in the migration of Europeans. With the exception of the movement of Russians into Asia, the story of the outpouring of Europe’s population is largely one of oversea settlement, and it is noteworthy that the migrations which have had the deepest and most enduring effects have been those which were transoceanic and intercontinental. When migrants cross an ocean, there are strict limits to what they can take with them; their traditions, ideals, techniques, and material belongings, when applied in a distant and strange environment, yield a pattern of life quite different from the one they left behind. There is something irrevocable about crossing an ocean. The political, economic, and racial configuration of the United States today is very much the out-come of three transoceanic migrations—the Pilgrim Fathers and their successors, the slaves from Africa, and the European masses in the nineteenth century.

When we consider shifts of population in Asia, we find that intercontinental movements are not significant, for a journey by sea has often meant merely a transfer from one part of the same continent to another. The abolition of slavery in the British colonies in the 1830s saw the beginning of an outflow from the Far East to the countries of America, Oceania, and Africa; and throughout the nineteenth century the recruitment of indentured laborers from India, China, and Japan was a characteristic feature. This species of intercontinental migration came to an end in the 1920s, and its place was taken by interregional movements which have had important demographic and social effects. The chief suppliers of intracontinental migrants have been China, India, Pakistan, Japan, and Korea; the main recipients have been Malaya, Ceylon, Burma, Indonesia, Thailand, Vietnam, Laos, Cambodia, British Borneo, the Philippines, and Manchuria. In most of these countries the immigrants have been mainly Chinese and Indians. In the Far East internal migration has been more important than transfers across national boundaries. For example, in Japan in the period 1920–1940 the net exodus from rural areas to urban areas amounted to 17.5 million persons. This was more than the entire increase in the population of Japan in this period, and it was ten times greater than the net emigration from the country.

The measurement of migration

Countries did not begin to keep records of genuine international migration until the big modern movements had passed their peak. For most of the nineteenth century the available statistics were by-products of acts or regulations introduced to achieve some other purpose. For example, in the United States and the United Kingdom, records of passenger movements were the results of acts passed to regulate shipping. In Britain, statistics began to be furnished under an act in 1803. But it was not until over a century later, in 1912, that the British Board of Trade decided to adopt a sta

Table 1—Intercontinental migration, selected countries and periods (in thousands)
Country ofPeriodNumber ofCountry ofPeriodNumber of
source: Adapted from carr-saunders 1936,p.49.
Austria and Hungary1846-139325,196Argentina1856-19326,405
British India1846-19321,194Brazil1821-19324,431
British Isles1846-193218,020British West Indies1836-19321,587
Italy1846-193210,092New Zealand1851-1932594
Japan1846-1932518South Africa1881-1932852
Norway1846-1932854United States1821-193234,244

tistical classification which defined a migrant as a passenger who declares that he has lived for a year or more in one country and intends to remain for a year or more in another. Statistical tests have shown that, despite their obvious deficiencies, “. . . the Board of Trade statistics of aggregate net passenger movement are a surprisingly good measure of the course of total net emigration from the United Kingdom in the period ending in 1912” (Thomas 1954, p. 52). This would not hold good for recent times, because air travel has become important and Britain persists in not including air migrants in her statistics.

The primary data used by various countries to measure international migration can be grouped under six headings: those yielded by controls at ports, by transport contracts, by population registers, by control at land frontiers, by passports, and by coupons detached from certain documents. In North America, South America, Asia, and Africa the usual practice has been to base the records on controls at frontiers and ports; in Europe, however, countries have adopted one or another of the six systems. Each government has tended to organize its migration statistics in accordance with its own particular policy objectives, without any regard to the need for international comparability. The result has been a bewildering variety of definitions and classifications.

Valuable attempts have been made by the International Labor Office to point the way toward a common pattern (see, for example, International Labor Office 1932; 1952). In recent years the problem of improving migration statistics has been thoroughly explored by the United Nations. An inquiry in 1950 showed that only 16 out of 45 countries classified emigrants by country of future residence or destination and only 17 classified immigrants by country of last residence or origin, while only 16 distinguished between continental

Table 2 – World population, by continent, in 1946 and 1957, and balance of intercontinental migration in the intervening period (in millions)
Continent  Total 
or majorPopulation populationNet
* Not available.
source: Adapted from International Labour Office 1959,p.304
Oceania11.815.4+3.6+ 1.0
Table 3 – Population growth and net migration for selected countries, 1946-1957 (in thousands)
    Per centPer cent
    of 1946of
Source: Adapted from International Labor Office 1959, pp. 308, 312.
Canada12,6203,305+ 1,000+7.9+30
United States141,39027,735 +2,200+ 1.6+8
Argentina15,6503,358+800+5.1 +24
Brazil47,31016,000+450+ 1.0+3
Uruguay2,280320+ 110+4.8+34
Venezuela4,3901,812+330+7.5+ 18
Australia7,4601,391+930+ 12.5+67
New Zealand1,660351+ 145+8.7+41

and intercontinental immigrants and 10 between continental and intercontinental emigrants. For the demographer it was disconcerting that only 16 countries gave information on the marital status of migrants, and in nine of these countries this information was not combined with age grouping. As a result of the work of the Economic and Social Council of the United Nations, most of the known statistics have been set out in two comprehensive mono-graphs (United Nations, Bureau of Social Affairs 1953; 1958). These surveys cover 33 countries and include tables classifying migrants by occupation or industry, state of dependency, possession or nonpossession of a contract for employment, and, for the United States, Israel, and South Africa, the amount of money which immigrants bring in with them (for additional detailed information, see United Nations, Department of Economic and Social Affairs 1955).

Since progress in improving official sources on migration is inevitably a slow process, it is all the more necessary to check imperfect time series in the light of the more accurate population census data at decennial intervals. Thus, Kuznets and Rubin (1954) have compared the annual record of immigration into the United States over a long period with the estimates obtained from the census figures on resident foreign-born. Similarly, Keyfitz (1950) has drawn up a population balance sheet for Canada for the century 1851-1950, including the best possible estimates of immigration and emigration in each decade.

Some indications of the scale of intercontinental population movements from the middle of the nineteenth century to the middle of the twentieth can be found in tables 1-3.

Economic determinants of migration

The period 1840-1924 was in several respects unique in the history of migration. The evolution of the Atlantic economy in that era necessitated a considerable movement of population and capital from the Old World, which was relatively well endowed with these factors, to the New World, where they were relatively scarce. Over 45 million people crossed the ocean; the average rate of growth of population in each decade of the nineteenth century was 29 per cent in the United States, 34 per cent in Argentina, and 8 per cent in Europe. The world’s chief provider of capital was Great Britain; of her foreign investments of over $17,000 million in 1913, nearly 70 per cent were located in North America, South America, and Oceania.

The motives which led these millions of people to leave their homelands were infinitely varied; a lengthy catalogue of them would be full of human interest but would not provide an interpretation of the phenomenon. At certain times and in certain places the operative force was political oppression or religious persecution or eviction by tyrannical landlords or the threat of starvation or evasion of military service or the love of adventure or the lure of gold or the attraction of a new country with limitless opportunities. But what has to be explained is why the individual decisions of millions of people resulted in four major upswings with intervening downswings, with an average interval of 15 to 20 years from peak to peak, in oversea emigration from Europe. The emigration upswings took place in 1845-1854, 1863-1873, 1881-1888, and 1903-1913. There can be no possible doubt about the explanation of the first of these: its inception had nothing to do with demand conditions in the United States. Calamity struck in Ireland in 1845, when the potato crop failed and a terrible famine followed; and as if this were not enough, the landlords added to the horrors by violently evicting thousands of peasants from their homes. In another part of Europe, southwest Germany, in the years 1848-1854 a severe crisis in the rural areas (in addition to the prevailing political unrest) brought population pressure to a head, and the only solution was emigration. In that period, of the 2,796,000 European immigrants who landed in the United States, no less than 80 per cent came from Ireland and Germany—1,283,000 from Ireland and 939,000 from Germany. This was essentially a Malthusian evacuation; both its timing and its magnitude were determined by exogenous driving forces in two stricken areas of Europe.

The explanation of the subsequent fluctuations in migration lies in a complicated process of inter-action between the economies of the Old World and those of the countries of new settlement overseas. It is significant that when the receiving countries, notably the United States, Canada, and Australia, were absorbing immigrants on a large scale, they were also experiencing a long upswing in capital construction (such as railroads and housing), which is sensitive to population growth. When this cycle entered its downward phase, with both migration and capital imports dwindling, there was simultaneously an upsurge in capital construction in the United Kingdom, where the rural surplus was now absorbed in urban areas at home. This capital construction was financed by loanable funds which were no longer attracted abroad (Thomas 1954).

Thus, there was an inverse relation between long swings in population-sensitive capital formation in the United Kingdom and in the United States, and in the United Kingdom there was an inverse relation between external migration and internal migration. The mechanism of this inverse long swing between the United Kingdom and the United States can be seen most clearly in the period 1870-1913; it arose because a substantial part of total capital formation was sensitive to the rate of population growth and the rate of population growth was determined by the net migration balance. There are some grounds for thinking that the propensity to emigrate was in some way related to a cycle of births in Europe that caused a periodic recurrence of swollen numbers in the emigration age groups (Thomas 1954).

In the interwar period some of the basic trends of the pre-1913 era were reversed. The United States had become the world’s leading exporter of capital, and the Immigration Restriction Act of 1924 virtually closed the doors to further immigration except on a very modest scale. From the turn of the century, British settlement in Canada, Australia, New Zealand, and South Africa had been expanding, and British emigration to the Empire considered as a proportion of British emigration to the United States, which had been only 43 per cent in 1881-1900, had risen to 245 per cent by 1911-1913. After World War i Britain embarked on a substantial program of Empire settlement, and in the decade 1922-1931, 400,000 emigrants received financial support to enable them to settle in the overseas dominions. However, this outflow did not survive the world depression, which had such severe consequences that it actually reversed the world currents of migration. In 1932, 11 European countries of emigration received a net inward bal ance of 102,000 persons, and Argentina, Australia, New Zealand, the United States, and Uruguay together had a net outflow of 65,000 (International Labor Office 1932). The international migration picture had become a perverse caricature of its former self.

Changes since World War II

The factors determining the volume, quantity, and direction of international migration since World War II are quite different from what they were in the nineteenth century. Profound structural changes have taken place in the Atlantic economy, and they have had far-reaching effects on the pattern of world migration. It is necessary to distinguish between the years immediately after the war and the period beginning in 1952. The correspondence between international flows of people and of private capital, which was the outstanding feature of the nineteenth century, disappeared. In its place there emerged in the years 1945—1952 an international circular flow based on the immense net transfer of $33,800 million of public capital (government loans and grants) from the United States, $22,800 million of which went to Europe. This recovery program made it possible for the exhausted countries of Europe, particularly the United Kingdom, France, the Netherlands, and Belgium, to resume exports of people and capital to the oversea territories with which they had a special relationship. It facilitated the revival of migration and mobility of capital within the British Commonwealth and strengthened the purchasing power of the less developed parts of the world. Perhaps the most significant effect of the recovery program was its contribution to basic capital formation in western Europe. This contribution was the prelude to the remarkable upsurge in economic growth in the 1950s [seeForeignAid].

After 1952 a new balance of economic forces emerged. American economic aid to Europe ended, but the volume of military aid rose considerably and the amount of private investment by American firms in Europe greatly increased, until in 1959 for the first time the flow of new American funds for direct investment was larger in western Europe than in either Canada or Latin America. Rapid economic growth in western Europe has meant a remarkable increase in intracontinental migration and a continued decline in emigration to oversea countries. Net immigration into the European Economic Community amounted to 288,000 in 1960 and 421,000 in 1961. The latter figure was particularly large because of the repatriation of French and Belgian nationals from Africa. Looking at the separate countries, we find that in 1961 West Germany had a net inflow of 421,000 and France 150,000, whereas Italy had a net outflow of 164,000. Over the five years 1956-1960 the net outward migration from Italy to other European countries was at an annual average of 83,000, and emigration from Italy to oversea countries fell from 111,000 in 1956 to 48,000 in 1960. Intracontinental migration is increasing at the expense of the traditional European outflow to countries such as those of Latin America, and this is basically determined by the economic resurgence of western Europe and the consequent change in the economic balance within the Atlantic economy.

New determinants are also operating on other continents. The complex problems of migration on the African continent have been explored in a comprehensive study by the International Labor Office (1958). The spread of industrial development draws Africans long distances from their rural homes, but they are often prevented from becoming members of a settled work force. They find themselves suspended with the maximum of insecurity between the village to which they are attached and the harsh conditions of the industrial labor market. This can have disastrous social consequences: the countryside is denuded of a large part of its labor, family life is broken up, the social structure disintegrates, and there is neither economic nor social security. The General Conference of the International Labor Office in 1955 adopted a recommendation on the protection of migrants in underdeveloped countries which is clearly relevant to Africa.

Most underdeveloped countries are detrimentally affected by current trends in the migration of qualified personnel. In the nineteenth century, skilled manpower tended to accompany capital flows from advanced to less developed countries: in the modern world the bulk of private investment is a circulatory process within the rich sector, and there is a suction of skilled labor from the poorer countries into the more advanced. Of the 9,245 immigrant engineers admitted to the United States in 1953—1956, 50 per cent came from Europe, 25 per cent from Canada, and 22 per cent from “other countries,” which included a number in a low state of economic development. The same was true of natural scientists ([U.S.] National Science Foundation 1958). Unskilled labor is often complementary to skilled, and when there is a decline in the immigration of the latter into a developing country, as has occurred, e.g., in Latin America, the scope for the absorption of unskilled immigrants is automatically curtailed. Some of the international movements of relatively scarce human capital ten to widen the disparity between rates of economic growth in rich and poor countries. Such transfers could be on such a scale that some underdeveloped countries could never begin the process of growth. There seems to be a conflict here between the principle of freedom to migrate and the goal of reducing inequality. However, in assessing the economic effects of the migration of a factor of production, the relevant criterion is not marginal private productivity, but marginal social productivity. Judged by this criterion, some of the international migration of skill in the world today is perverse.

Impact on receiving countries

It used to be argued that immigration into the United States had not added to the American population because its effect had been counterbalanced by an induced decline in the fertility of native-born Americans (see, for instance, Walker 1891). This proposition has been disproved. Modern demographic analysis has demonstrated that the immigrations of the last century had hardly any net effect on the rate of natural increase of the native-born population in the receiving countries. It is estimated that in France between 1801 and 1936 net immigration was 3,960,000 and contributed only a third of the growth of population within the 1936 boundaries of the country during that period. The white population of the United States in 1790 was about 3.2 million; their descendants living in the United States in 1920 have been estimated at 41 million, and in the same year the number of descendants or survivors of immigrants since 1790 came to about 53 million. In that period net immigration into the United States was about 26.5 mil-lion (United Nations, Department of Social Affairs 1953, p. 139).

One of the most striking examples of mass im-migration in recent times is that into West Germany, which absorbed 12 million immigrants in 12 years after the war. Although this influx increased the population of West Germany by one-third, it failed to make up for the gaps in the demographic structure caused by war losses, since the immigrant population had suffered the same kinds of losses in the same age groups (International Labor Office 1959, pp. 27-28).

In recent years there has been a dramatic change of trend in the United Kingdom, a traditional country of emigration. Since 1958 there has been an appreciable net inward movement of migrants. This reversal of trend was caused by the substantial inflow of colored Commonwealth citizens, mainly from the West Indies, India, and Pakistan.

This led the government to introduce legislation to regulate Commonwealth immigration; the Commonwealth Immigrants Act came into force on July 1, 1962. Although the total number of colored people living in the United Kingdom is only about 1 per cent of the population, the newcomers have tended to cluster in certain places, and this has given rise to difficult social problems. This is part of a wider phenomenon—compare, for instance, the situation of the Puerto Ricans in the United States or the north Africans in France. Where poor, overpopulated countries have special relationships with advanced countries, it is natural that an overflow of population will take place so long as channels of mobility remain open.

Impact on sending countries

Mass emigration can in certain circumstances turn into a self-reinforcing process with profound long-run effects on the sending country. This arises from one of the most significant of migration differentials, the fact that the incidence of migration is particularly heavy in the age group 15-30. Given a substantial initial outflow, the sequel can be as follows. There is a decline in the marriage rate and consequently a fall in the size of the age group 0-5; but, since the rate of emigration among children is relatively low, the 0-5 age group becomes a relatively large group ten years later. It is a paradox that countries which are heavy losers through emigration seem to be liberally endowed with teen-agers. The process is self-perpetuating because, after a lag of about 15 years from the original thinning out of the 15-30 age group, the number entering the high emigration age group of 15-20 is relatively high in relation to the total population.

The influence of mass emigration on age composition in the sending country can be observed most strikingly in the case of Ireland, from which the total outflow from 1850 to 1911 was 4,191,000. By 1951, 30 per cent of the population were aged 45 and over, as compared with 16 per cent in 1841, and 11 per cent were 65 and over, as compared with 3 per cent in 1841. The experiences of Ire-land, Sweden, and Scotland show that substantial emigration tends to reduce the marriage rate, but we cannot be certain about the long-run effects on fertility. There can be little doubt that prolonged emigration helps to explain why there are so many spinsters in Ireland; however, the women who do get married have relatively large numbers of children. As to the death rate, the tendency is for the loss of good lives from the 15-35 age group through emigration to raise the average death rate in that group.


It is broadly agreed that assimilation is best regarded as a mutual process of integration. An American sociologist has well said that “. . . the United States has not assimilated the newcomer nor absorbed him. Our immigrant stock and our so-called ‘native’ stock have each integrated with the other. . . . It will be apparent that this concept of integration rests upon a belief in the importance of cultural differentiation within a framework of social unity. It recognizes the right of groups and individuals to be different so long as the differences do not lead to domination or disunity” (Borrie 1959, pp. 93-94). One of the difficulties of group settlement, e.g., in Latin America, is that there arises a conflict between the immediate interests of the migrant in his group and the long-run objective of cultural integration. Much benefit can be derived from the provision of instruction to the migrant before he has left his own country. The Intergovernmental Committee for European Migration has evolved effective means of selecting, educating, and pretraining European migrants, [SeeRefugees, article on AdjustmentAndAssimilation.]

Experts who have studied migration in Asia stress the importance of assimilation as a necessary condition of the diffusion of skills. In colonial regimes there was a tendency for skilled immigrants to remain a class apart, and they often sought to maintain the relative scarcity of their skill. The great need in Asian countries now is for the importation of skilled personnel who will assimilate easily and thereby facilitate the rapid spread of technical knowledge.

International migration no longer plays the role in economic growth that it did in the nineteenth century. Legislative restrictions, the changes in the economic determinants, and the population up-surge in different parts of the world have all tended to reduce the scale of movement. Countries which have been receiving a relatively large influx of migrants since World War II, e.g., Australia, will soon find that the rate of entry into the working population from the swollen lower age groups will make immigration on the old scale unnecessary. The international circulation of skilled manpower has become relatively more important, Intercontinental migrations have lost most of their significance and have been replaced by intracontinental movements.

Much more interdisciplinary research is needed into the problems of adjustment of immigrants, particularly where they are ethnically different from the population of the host country; the inter-action between external and internal migration; the relation between immigration and the incidence of mental health; the determinants of the rate of increase of immigrant groups in multiracial societies; and the economic and social consequences of the changing pattern of the international circulation of skilled manpower.

Brinley Thomas

[Directly related are the entriesCapital, Human; Refugees. Other relevant material may be found inAssimilation; Population, article on Population Distribution; and in the biographies ofGiniand Kulischer.]


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Migration is a difficult concept to define because it includes people who move for different reasons across different spaces. A migrant can be a person who moves to another city or town within a nation; a refugee who crosses an international border to escape religious or political persecution; a jobseeker who moves to another country for better economic opportunities; a slave who is forcibly moved; or a person displaced by war or natural disaster. Demographers lack a single, operational definition for migration because it occurs under different conditions.

The causes of migration are related to the specific contexts in which they take place. First, the composition of migration streams are characterized by structural forces such as the global economy. Second, sociocultural differentials (gender, class, caste, etc.) have important implications for individual mobility.

Because migration occurs under different circumstances, the demographic implications for receiver populations change accordingly. For example, when large numbers of rural residents migrate to cities, infrastructure problems may arise in urban areas.

Given the variation possible in the migration process, all migrants cannot be analyzed with the same theoretical framework. However, although heterogeneous factors make a universal definition impossible, in general, migration is a process in which an individual or a group shifts their residence from one population (or place) to another. Apart from its spatial dimension, migration also implies the disruption of work, schooling, social life, and other patterns. A migrant is someone who breaks off activities and associations in one place and reorganizes their daily life in another place. A move within the same area is considered mobility, not migration, because the mover can continue day-today life (keep the same job or school, shop at the same stores, and socialize with the same people) without significant disruption (Weeks 1999).

Most demographers argue that migration must involve an essentially permanent territorial shift in residence to be distinguished from mobility. Hence, travelers and commuters are excluded from migration studies because they move across boundaries on a temporary basis and because their movement does not generally cause major change in any population. Categorizing movers strictly in terms of the permanence of their move can be problematic, however, because this method tends to disregard the social context of population movement. Temporary moves are typically absent from the census, and therefore do not register in demographic terms. In this perspective, a software engineer who makes a permanent move from San Francisco to Los Angeles is classified as a migrant, while a Mexican arriving in California as a seasonal agricultural laborer is not. However, the agricultural laborer's move is probably the most socially significant given language barriers, culture shock, and race discrimination encountered.

Just because a mover is not measured as a member of a population does not mean that their movement has had no measurable social impact on populations. Under the United States's bracero program (a system created in 1942 to help solve U.S. agricultural labor shortages), for example, the number of temporary Mexican laborers reached a point that U.S. policy makers reacted with mass deportations and the creation of a restrictive immigration act meant to keep Mexicans out. This policy created severe unemployment problems in Mexico, whose economy was not strong enough to reabsorb these workers. Hence, the program that previously encouraged mobility into the United States contributed to Mexican poverty after it was terminated in 1964. Even though they would not be counted as migrants, the mobility of Mexican agricultural workers left a deep impression on the Mexican economy, and it initiated changes to U.S. immigration policy.

Types of Migration

The relatively permanent movement of people across territorial boundaries is referred to as inmigration and out-migration, or immigration and emigration when the boundaries crossed are international. The place of in-migration or immigration is called the receiver population, and the place of out-migration or emigration is called the sender population. There are two basic types of migration studied by demographers:

  1. Internal migration. This refers to a change of residence within national boundaries, such as between states, provinces, cities, or municipalities. An internal migrant is someone who moves to a different administrative territory.
  2. International migration. This refers to change of residence over national boundaries. An international migrant is someone who moves to a different country. International migrants are further classified as legal immigrants, illegal immigrants, and refugees. Legal immigrants are those who moved with the legal permission of the receiver nation, illegal immigrants are those who moved without legal permission, and refugees are those crossed an international boundary to escape persecution.

Jay Weinstein and Vijayan Pillai (2001) denote a third classification: forced migration. Forced migration exists when a person is moved against their will (slaves), or when the move is initiated because of external factors (natural disaster or civil war). The distinction between internal and international migration is crucial because they happen for different reasons. Because structural barriers are more likely to impede the mobility of a potential international migrant than an internal migrant—international migration involves more administrative procedures, greater expense, and more difficulties associated with obtaining employment, accessing state services, learning a new language, and the like—the motivations behind international migration are usually stronger than those behind internal migration (Weeks 1999).

Theories of Migration

People move for different reasons. These differences affect the overall migration process. The conditions under which a migrant enters a receiver population can have broad implications for all parties involved. The expression migration experience refers to the fact that different causes for migration will produce different outcomes observable from a sociological perspective. For example, a person who moves within a nation will not have the same migration experience as a political refugee. In most cases, refugees need special services from the receiver population such as emergency shelter, food, and legal aid. The psychological trauma of fleeing their homeland and leaving family members behind can also complicate refugees' adjustment to their new environment. Considering that a migrant can be a slave, refugee, or job-seeker, or have some other reason for moving, no single theory can provide a comprehensive explanation for the migration process.

Although a comprehensive theory is unattainable, it remains a crucial task of demographers to explain why people migrate. Theories of migration are important because they can help us understand population movements within their wider political and economic contexts. For example, if outmigration from Third World nations is shown to be a result of economic problems caused by the global economy, then such migration could be managed with better international economic agreements instead of restrictive immigration acts. Indeed, rather than slowing Mexican in-migration to the United States, termination of the bracero program actually increased the amount of illegal immigration because it exacerbated Mexican poverty.

Ernest Ravenstein is widely regarded as the earliest migration theorist. Ravenstein, an English geographer, used census data from England and Wales to develop his "Laws of Migration" (1889). He concluded that migration was governed by a "push-pull" process; that is, unfavorable conditions in one place (oppressive laws, heavy taxation, etc.) "push" people out, and favorable conditions in an external location "pull" them out. Ravenstein's laws stated that the primary cause for migration was better external economic opportunities; the volume of migration decreases as distance increases; migration occurs in stages instead of one long move; population movements are bilateral; and migration differentials (e.g., gender, social class, age) influence a person's mobility.

Many theorists have followed in Ravenstein's footsteps, and the dominant theories in contemporary scholarship are more or less variations of his conclusions. Everett Lee (1966) reformulated Ravenstein's theory to give more emphasis to internal (or push) factors. Lee also outlined the impact that intervening obstacles have on the migration process. He argued that variables such as distance, physical and political barriers, and having dependents can impede or even prevent migration. Lee pointed out that the migration process is selective because differentials such as age, gender, and social class affect how persons respond to push-pull factors, and these conditions also shape their ability to overcome intervening obstacles. Furthermore, personal factors such as a person's education, knowledge of a potential receiver population, family ties, and the like can facilitate or retard migration.

Several theories have been developed to treat international patterns of migration on their own terms, but these too are variants of push-pull theory. First, neoclassical economic theory (Sjaastad 1962; Todaro 1969) suggests that international migration is related to the global supply and demand for labor. Nations with scarce labor supply and high demand will have high wages that pull immigrants in from nations with a surplus of labor. Second, segmented labor-market theory (Piore 1979) argues that First World economies are structured so as to require a certain level of immigration. This theory suggests that developed economies are dualistic: they have a primary market of secure, well-remunerated work and a secondary market of low-wage work. Segmented labor-market theory argues that immigrants are recruited to fill these jobs that are necessary for the overall economy to function but are avoided by the native-born population because of the poor working conditions associated with the secondary labor market. Third, world-systems theory (Sassen 1988) argues that international migration is a by-product of global capitalism. Contemporary patterns of international migration tend to be from the periphery (poor nations) to the core (rich nations) because factors associated with industrial development in the First World generated structural economic problems, and thus push factors, in the Third World.

Migration and the Family

Explanatory frameworks premised on the push-pull hypothesis tend to overemphasize the role of the individual in the migration process. Critics of this perspective argue that the decision to migrate is based on group experience, in particular the costs and benefits to the family. Rather than being an isolated calculation, an individual's decision to migrate is conditioned by multiple social and economic factors. For example, a member of a rural family may be motivated to migrate if urban employment translates into the diversification and amelioration of the family economy, or if rural productive resources are not enough to sustain an extended family. Such out-migration probably would not occur if it was likely to produce an economic deficit for the family unit. Apart from this, the decision to migrate is not calculated from an exclusively economic standpoint. An individual can have an economic opportunity in another place, but not take it up if their departure would cause emotional hardship in the sender community. In Sarah Harbison's words, the family "is the structural and functional context within which motivations and values are shaped, human capital is accrued, information is received and interpreted, and decisions are put into operation" (1981, p. 226).

The family is the crucial agent of an individual's capacity and motivation to migrate. Harbison argues that the complexities of the family structure characterize the migration process because the family unit mediates between the individual and society, and thus it can prioritize its needs over the individual's in many instances. Three factors give the family unit significant importance in the migration process (Harbison 1981; Boyd 1989).

  1. Because migration requires resources for transportation and to establish a new home, family support is paramount, especially because most migrants are young and lack sufficient personal savings to finance a move. In most cases, the family unit is the essential unit of economic production and thus determines the allocation of resources to individuals and specifies economic roles. Apart from the economic needs of the family, differential access to family resources and the social division of labor have important implications for individual mobility. The socioeconomic framework of the family can facilitate or restrict migration. In a situation where males control family resources, and females are assigned strict domestic roles in the division of labor, women's mobility will be structurally limited or at least determined by men.
  2. The family is the primary socializing unit. Through the framework of kinship, customs, values, social obligations, and the like, the family unit conditions the individual to fill a basic role in society. For example, in societies where migration is crucial for the putative well-being of the group, patterns of socialization will develop to prepare certain individuals to migrate. The primogeniture system—under which real estate passes to first-born sons—assisted the mobility of later-born sons because they were often provided an education or military placement in lieu of inheritance.
  3. The family is an enabling economic and social network. The geographic dispersion of kin members partially determines the migration destination. Many people move to where they have family members (rather than where economic opportunities are most fruitful) because they can be relied upon to provide food, shelter, and information, which help them cope with their new environment. The presence of kin will also reduce the psychological impact of culture shock through the perpetuation of old customs in the new place. For these reasons, ethnic groups have tended to concentrate in specific regions and neighborhoods.

Harbison (1981) points out that while the family's function as a subsistence unit, agent of socialization, and support network shapes the motivations and incentives for migration, these are also conditioned by sociocultural factors, such as marriage rules and kinship rights. Of particular importance is the gender division of labor. For the most part, migration studies have been gender-blind, and this obviously has serious implications from both a theoretical and an empirical perspective. Many studies (Bjerén 1997; Chant 1992; Kelson and DeLaet 1999) have shown that women's migration experience is fundamentally different from men's. Because gender is a primary organizing principle of the family and society, it follows that gender structures the migration process to a significant degree.

Migration and the Global Economy

Before the international economic downturns that began in the 1970s, the composition of international migration generally held to Ravenstein's hypothesis that the primary motivation for migration was economic, and that young males predominated long-distance travel. Since the 1970s, the pattern of migration has transformed with global economic changes. Moreover, the composition of migration streams transformed because earlier waves of migration created support networks that helped more recent migrants overcome intervening obstacles and difficulties associated with adjusting to a new environment (Boyd 1989). As people settled in new places, they became valuable sources of information and economic assistance for prospective migrants to draw upon. As Monica Boyd (1989) notes, while structural forces form the basic incentives for migration, push-pull factors are filtered through social networks that connect sender and receiver populations. Various patterns of migration (e.g., Mexican immigration to the United States) have become institutionalized as these networks took root.

However, while family contacts often determine where migrants move, structural forces remain powerful causal factors. Douglas Massey and colleagues (1994) point out that most empirical evidence suggests that a crucial impetus for international migration is the combination of systemic unemployment in the sender population and good employment prospects in the receiver population. World-systems theorists argue that one effect of globalization has been to keep Third World economies dependent on agriculture and the exportation of raw materials and simple commodities. Slow industrialization and relatively high fertility rates have generated acute unemployment in these nations, and this partially explains why net migration streams have generally flowed in a unilateral (from periphery to core nations) under the global economy. Economic problems associated with globalization have made labor migration an important survival strategy for many Third World families. For some nations labor has become a major export economy, and states have facilitated migration to capitalize on its economic benefits. Shu-Ju Cheng (1999) notes that there were over two million documented Filipino migrant workers worldwide by 1995, and they remitted U.S. $18 billion between 1975 and 1994. Massey and colleagues (1994) point out that Mexican remittances were so great in certain communities that there were more U.S. dollars in circulation than their peso equivalent. In 1995, the total of world remittances from migrant laborers amounted to U.S. $70 billion (Taylor 1999).

Migration connections between rural economies and urban and international labor markets are particularly important for Third World consumption and production. Out-migration from rural populations to external labor markets has stimulated consumption and productivity in many Third World countries. As Massey and colleagues (1994) note, one study of two Mexican rural communities showed that remittances from domestic urban centers and the United States sustained a level of consumption 37 percent higher than gross production therein. J. Edward Taylor (1999) remarks that for every dollar Mexican migrant laborers sent home from the United States, Mexico's Gross National Product (GNP) increased between $2.69 and $3.17. Contrary to neoclassical theory, these studies demonstrate the potential nonunitary impact of labor migration. Besides increasing household consumption, income transfers have had a dynamic impact on some Third World economies. Remittances often initiate economic improvements because they are used for productive investments, and thus increase household incomes, productivity, and the GNP.

See also:Global Citizenship; Home; Homeless Families; Immigration; Industrialization; Kinship; Poverty; Rural Families; Socialization; Socioeconomic Status; Stress; Unemployment; Urbanization; War/Political Violence; Work and Family


bjerén, g. (1997). "gender and reproduction." in international migration, immobility, and development: multidisciplinary perspectives, ed. t. hammer, g. bruch-mann, k. tomas, and t. faist. oxford, uk: berg.

boyd, m. (1989). "family and personal networks in international migration: recent developments and new agendas." international migration review 23:638–70.

chant, s., ed. (1992). gender and migration in developingcountries. london: belhaven press.

cheng, s. a. (1999). "labor migration and international sexual division of labor: a feminist perspective." in gender and immigration, ed. g. a. kelson and d. l. delaet. new york: new york university press.

harbison, s. f. (1981). "family structure and family strategy in migration decision making." in migration decision making: multidisciplinary approaches to microlevel studies in developed and developing countries, ed. g. f. dejong and r. w. gardner. new york: pergamon press.

kelson, g. a., and d. l. delaet, eds. (1999). gender andimmigration. new york: new york university press.

lee, e. (1966). "a theory of migration." demography 3:47–57.

massey, d. s.: arango, j.; hugo, g.; kouaouci, a.; pellegrino, a; and taylor, j. (1994). "an evaluation of international migration theory: the north american case." population and development review 20:699–751.

piore, m. j. (1979). birds of passage: migrant labour inindustrial societies. new york: cambridge university press.

ravenstein, e. g. (1889). "the laws of migration." journal of the royal statistical society 52:245–301.

sassen, s. (1988). the mobility of labour and capital: astudy in international investment and labour flow. cambridge, uk: cambridge university press.

sjaastad, l. a. (1962). "the costs and returns of human migration." journal of political economy 70:80–93.

taylor, j. e. (1999). "the new economics of labour migration and the role of remittances in the migration process." international migration 37:63–88.

todaro, m. p. (1969). "a model of labor migration and urban unemployment in less-developed countries." the american economic review 59:138–48.

weeks, j. r. (1999). population: an introduction to concepts and issues. boston: wadsworth.

weinstein, j., and pillai, v. k. (2001). demography: thescience of population. boston: allyn and bacon.


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Migration is defined as the regular, usually seasonal, movement of all or part of a population of animals. Many different animals migrate, including birds, hoofed animals, bats, whales, seals, and salmon. One-way movement of animals that do not return is called emigration. Emigration is due to different causes and is not considered migration. Also, the regular daily movements that many animals undertake are not considered migration.

Most birds and mammals that migrate follow an annual or seasonal pattern related to cyclic variations in temperature, vegetation, or precipitation. Salmon and some other fishes do not migrate annually. Instead, they return to their place of birth in order to reproduce. Some ethologists do not consider these movements migration but reserve the term for cyclic movements. In some parts of the world, animals will suddenly move into a new area temporarily. These sudden and temporary movements are called irruptions. Irregular movements, such as irruptions, are generally due to population growth during periods of abundant food followed by dispersal when food supplies diminish.

Most migrations involve horizontal movement. Animals move north and south with the seasons or move in a circular pattern to take advantage of cyclic rain patterns or new forage growth. Some animals, however, migrate by changing elevation. Aquatic animals may move from deeper water to the surface according to the season. Many birds, mammals, and insects migrate to higher or lower elevations in mountainous areas. This kind of migration produces the same kind of change in the environment as horizontal migration but involves only small horizontal displacements.

Invertebrate Migration

Some shrimp and crabs migrate for purposes of reproduction. Pregnant females move into shallow water to lay their eggs. The shallow water environment has fewer large predators, so the chances of the baby shrimp growing to maturity is increased. After they mature the shrimp move into deeper water to feed.

Probably the best known insect migrant is the monarch butterfly (Danaus plexippus ). In the summer, these insects move northward as far as Canada's Hudson Bay, where they breed and reproduce. Adults from the last generation of the year migrate southward in autumn through Oklahoma and Texas. Many move as far south as the mountains of central Mexico. After overwintering in the tall firs of these mountains, they follow spring northward, breeding and laying eggs on their preferred milkweed plants. Since a few of the returning butterflies are members of the first generation that developed from the overwintered insects, they can be considered true migrators. Most insects that arrive in the north, however, have been born in route. The longest distance recorded for the complete flight of an individual monarch butterfly is 3,010 kilometers (1,870 miles), truly a remarkable flight for this tiny insect.

Vertebrate Migration

The three categories of migratory fishes are oceanodromous, anadromous , and catadromous . Oceanodromous fish live and migrate entirely in the ocean. The many species of herring with different migration patterns are typical of oceanodromous fish. Anadromous fish live in the sea and migrate to freshwater to breed. Pacific salmon are typical. They hatch from eggs in mountain streams or lakes. The young feed and grow in the freshwater, then migrate to the sea after a year. Adult fish usually remain in the sea for two or three winters, where they grow to full size. Then they undergo dramatic physiological changes and migrate back to the stream where they were born. There the females lay eggs that are fertilized by the males. Then both sexes die. Some Atlantic salmon breed two or three times.

Catadromous fish reverse the behavior pattern of anadromous fish. Catadromous fish spend most of their lives in freshwater, then they migrate to the sea to breed. Eels of the genus Anguilla are the best known. Both European eels and North American eels spawn in an area of the Atlantic Ocean known as the Sargasso Sea. The larval forms of the fish are carried by the Gulf Stream to the shallow waters of the continental shelves. After about two years, when the larval eels are about 8 centimeters (3 inches) long, a metamorphosis occurs. The nearly transparent free-swimming larval eels are transformed into bottom-dwelling, dark-colored, cylindrical fish. Their migration upstream is spectacular, as the young fish gather by millions, forming a dense mass several miles long. In freshwater, the eels grow to full size. They live for several years in freshwater, then undergo a final metamorphosis before they swim back to the Sargasso Sea to spawn.

Terrestrial reptiles and amphibians do not migrate regularly enough to be significant, although some species migrate vertically. Reptiles and amphibians have evolved other strategies to deal with adverse environmental conditions, such as hibernation and estivation. Sea turtles are the exception. Most sea turtles migrate to beaches to lay their eggs. They then disperse back into the ocean. Green sea turtles (Chelonia mydas ) lay their eggs on the coast of Costa Rica in Central America and then disperse through the Gulf of Mexico and the West Indies.

The taxonomic class Aves includes most of the best known migrating species. Most species of birds require a large input of food to maintain their body temperature and other behaviors. Many birds have evolved behaviors that allow them to move to areas where food is more abundant. Birds have evolved a highly efficient means for traveling swiftly over long distances with great economy of energy.

Migratory birds do not differ greatly in gross physiological characteristics from nonmigratory birds. There is a spectrum of birds from completely nonmigratory species to species that fly thousands of kilometers every year.

Since insect populations drop dramatically during the winter, insectivorous species of birds, such as warblers, flycatchers, and wagtails, are highly migratory and typically spend the winter in the tropics. The geographical arrangement of the North American continent determines migration routes for many species of North American birds. Principal routes are known as flyways. They include the Mississippi flyway, the central flyway, the Pacific flyway, and the Pacific oceanic route. Many birds spend the winter in the states that border the Gulf of Mexico, but the principal wintering areas are in Mexico and Central America. Panama has the greatest density of winter bird residents in the world.

Tropical regions do not have the four seasons of temperate regions, but they do have cyclic rainy and dry seasons. Birds of tropical regions migrate according to these wet and dry seasons.

Arctic terns (Sterna paradisaea ) are the world champion migrators. These birds breed in the coastal regions of northern Europe, Asia, and North America. They then fly south and spend the winter in the extreme southern Pacific and Atlantic along Antarctic pack ice 17,600 kilometers (10,940 miles) from their breeding range. American populations of the Arctic tern cross the Atlantic to Europe, then fly south along the coast of western Europe. Arctic terns thus travel farther than any other bird species.

Most terrestrial mammals do not migrate. True migration among terrestrial mammals occurs mostly among large hoofed animals living in habitats with wide fluctuations of climatic and biotic conditions. For example, before the central United States was largely fenced in, American bison (Bison bison ) migrated regularly. Large herds containing millions of animals moved in circular routes to the southern part of their range in winter and back north when spring rains brought fresh grass to the northern part of their range.

In North American Arctic regions, caribou (Rangifer tarandus ) regularly migrate between the open tundra, where they calve, and the forest, where they spend the winter months. In winter, each caribou herd moves independently of the other herds in response to local conditions. Then in the spring, the herds move back onto the tundra. These migrations follow the same routes from year to year.

In contrast to terrestrial mammals, marine and flying mammals typically do migrate because of their inherently greater mobility. The only true flying mammals are the many species of bats. Huge colonies of Mexican freetailed bats (Tadarida brasiliensis ) spend the summer in Texas and adjoining states. The Congress Avenue Bridge in Austin, Texas, is home to the largest urban bat colony in the world, with around 1.5 million bats. It is a huge bat nursery containing females and nursing babies, called pups. Since they are insectivorous, these bats leave around the middle of November for their winter home in Mexico. They return in mid-March when the insect population in Texas increases dramatically. Most of the females in the Congress Avenue colony give birth to a single pup in early June.

Among mammals, the marine mammals are the distance record holders. Antarctic whales, such as humpback whales (Megaptera novaeangliae ), migrate regularly to the tropics. Whales migrate to areas rich in food, particularly the northwestern coast of Africa, the Gulf of Aden, and the Bay of Bengal. Northern whales, such as blue whales (Balaenoptera musculus ), have the same migratory habits as Antarctic whales. They migrate northward along the east coast of the United States, then through Davis Strait to Baffin Bay (north of Canada) or to waters off northern Scotland or the coast of Norway.

Migration Routes

Birds tend to follow well-defined migration routes called flyways. A population of birds may be scattered over thousands of square kilometers. As they begin their migration, the birds may be spread out over a migration front hundreds of kilometers wide. These routes are determined by geographical factors, ecological conditions, and meteorological conditions. Some routes cross oceans. American golden plover fly over open ocean from the Aleutian Islands southwest of Alaska to Hawaii, a distance of 3,300 kilometers (2,050 miles).

Many birds fly at relatively low altitudes. Hawks and other passerines, however, fly at altitudes as great as 4,000 meters (13,120 feet). The highest altitude ever recorded for migrating birds is 9,000 meters (29,520 feet) for geese near Dehra Dun in northwest India.

Some birds fly nonstop. Others are diurnal , flying during daylight hours and resting at night. Pelicans, storks, birds of prey, swifts, swallows, and finches migrate during daylight hours. Other birds reverse the pattern. Cuckoos, flycatchers, thrushes, warblers, orioles, and buntings fly at night and rest during the daylight hours.

Most birds abandon their instinctive territoriality during migration. Even unrelated birds with similar habits sometimes travel together. Some birds migrate in large flocks. Geese, ducks, pelicans, and cranes fly in well-known V-shaped formations that allow each bird to receive lift from the bird just in front.

Finding Their Way

Animals use several different techniques to navigate while migrating or to locate the place of their birth. Experiments have demonstrated the ability of animals to orient themselves geographically. Starlings have returned to their nests after being moved 800 kilometers (500 miles) away; swallows have found their way home from more than 1,800 kilometers (1,120 miles). A Manx shearwater (Puffinus puffinus ) flew 4,900 kilometers (3,040 miles) across the Atlantic from Massachusetts to Britain in twelve days. Laysan albatrosses (Diomedea immutabilis ) found their way back to Midway Island in the Pacific after being released at Whidbey Island, Washington. The journey covered 5,100 kilometers (3,170 miles) and took ten days. Experiments with fish and mammals have demonstrated similar homing ability.

Some homing animals use landmarks. The use of landmarks, however, cannot explain how migrants find their way along routes covering many hundreds or thousands of kilometers.

Birds apparently possess a compass sense. This sense is probably related to a sensitivity to Earth's magnetic field. When homing pigeons were released with tiny magnets attached to their necks, they were unable to navigate. Experiments have also shown that the orientation of birds is partly based on celestial bearings. In one well-known experiment, indigo buntings were placed in compartments in a planetarium and shown star patterns. Scratch marks on sensitive paper showed that the birds attempted to move "north" according to the star patterns displayed on the dome of the planetarium.

Experiments have shown that salmon and similar fish apparently use Sun orientation while at sea to find their way back to the general area of the stream in which they hatched. Once in the correct general area, salmon apparently use their sense of smell to locate their home waters.

What Triggers Migration?

Migration is part of the life cycle of animals. Metabolic patterns usually change prior to migration, and fats accumulate in the body tissues. Food consumption increases in the autumn reaching a peak at the beginning of the migration season. These fundamental physiological changes are apparently controlled indirectly by the pituitary gland. The pituitary acts as a sort of internal clock. Variations in temperature and hours of daylight are detected by the pituitary gland. The pituitary then influences the development of gonads and all other metabolic processes, including the thyroid gland. The thyroid gland excretes the chemical substances that cause the physiological changes that prepare the animal for migration. The pituitary serves only to prepare the animal for migration. Actual migration is triggered by appropriate environmental conditions, such as precipitation, availability of food, temperature, and weather conditions. A sudden period of cold weather during autumn may induce the immediate departure of many migrants.

Evolution and Ecology of Migration

Migration as a behavior among birds and mammals probably appeared gradually. Erratic dispersals were probably the precursors of true migration. Such erratic dispersal would have led to greater survival rates and reproductive success among animals that moved to the most favorable places. These originally erratic movements gradually acquired stability through natural selection . In some cases, original habitats were in presentday wintering areas, and animals developed a tendency to leave in spring in order to breed in other territories. Seasonal changes of weather and food supply in these newly settled regions forced the animals to migrate in fall, and they thus retreated to their former range. Many birds now nesting in the Northern Hemisphere, such as hummingbirds, tyrant flycatchers, tanagers, orioles, and swifts, have distinct tropical characteristics. These birds may have gradually spread northward as glacial ice receded.

The evolution of migration must be related to the ecological significance of migration. Migration allows fast-moving animals to exploit variations in resources and to move into areas where they could not remain year-round without the ability to move rapidly. Exploitation of peaks of food production, such as the dramatic increase of insect populations in temperate regions in the springtime, would not be possible without migratory populations.

see also Habitat.

Elliot Richmond


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Billington, Elizabeth T. Understanding Ecology. New York: F. Warne, 1971.

Curtis, Helena, and N. Sue Barnes. Biology, 5th ed. New York: Worth Publishing, 1989.

Miller, G. Tyler, Jr. Living in the Environment, 6th ed. Belmont, CA: Wadsworth, 1990.

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Schmidt-Koenig, Klaus. Migration and Homing in Animals. New York: Springer-Verlag, 1975.

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Terborgh, John. Where Have All the Birds Gone? Essays on the Biology and Conservation of Birds that Migrate to the American Tropics. Princeton, NJ: Princeton University Press, 1989.

Mexican free-tailed bats are among North America's most important animals, from both an ecological and agricultural standpoint. They consume phenomenal volumes of insects nightly, a large number of which are agricultural pests. Serious bat population decline has been observed, largely due to human activities (i.e., the destruction of old buildings or the use of pesticide) and vandalism of important roosting habitats

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Migration is a demographic process that has played an increasingly important role in the changing populations of many countries around the world. Much of the geographic shift in populations occurs from rural areas to urban areas, as well as from less developed countries to more developed countries. Since the mid-twentieth century, an expanding literature has been developed to understand migration. In particular, many scholars have focused not only on the forces encouraging people to migrate, but also on the impact that this migration has had on the migrants themselves and on their places of origin and destination. This interest in the topic of migration emerges not only from the increasing prevalence of people migrating, but also the relevance of migration in shaping demographic, social, economic, and political spheres worldwide.


Migration has been defined generally as a permanent or semi permanent change of residence (Lee 1966, p. 49). This definition places no restriction upon the distance of the move or upon the voluntary or involuntary nature of the act, and no distinction is made between external and internal migration (Lee 1966, p. 49). However, such a broad definition obscures the heterogeneous types of movements that exist based on geographical distance, as well as the factors involved in the different types of movements. For example, some people move within a community, others move across counties within the same state, others move across states, and others move across international boundaries. The U.S. Census Bureau collects information on the migration activities of people using the five-year migration question (location of present residence and location of residence five years earlier). Furthermore, the U.S. Census Bureau defines movers as those who lived in a different house but within the same county in the five-year period, migrants as those that moved at least across counties within the country during the period, and international migrants as those that were living outside of the United States five years earlier. The five-year migration question has numerous shortcomings, most notably that it is based on only two points in time. For instance, persons living in the same county in 1995 and 2000 but who lived in another county in the interim are classified as nonmigrants.

The process of accounting for the continuous movement of people has become complicated by technological advances in the areas of transportation and communications. For example, immigrants increasingly engage in transnational migration, which involves the continuous movement between the community of origin in the home country and the community of destination in the host country (Levitt 1998). This type of movement has important implications beyond the migrants experience because it produces a variety of changes for both the sending and the receiving community.


The theorizing of explanations for the movement of people extends back to the late nineteenth century with geographer E. G. Ravensteins (18341913) article The Laws of Migration (1885). Interest in the development of theoretical perspectives to explain migration resurfaced between the 1950s and 1970s with important works by William Petersen (1958), Everett Lee (1966), and P. N. Ritchey (1976). However, the changing nature of migrationespecially the increasing prevalence of international migrationhas seen the development of theoretical perspectives that place greater attention on immigration since the 1960s (see Massey et al. 1993). This entry describes some of the most prominent perspectives.

World-Systems Perspective The world-system perspective is a structural-macrolevel perspective developed by sociologist Immanuel Wallerstein. This perspective views international migration as the result of the expansion of the market economy throughout the world, as capitalists from core countries make inroads into peripheral and semiperipheral countries (Brettell 2000; Massey and Espinosa 1997; Portes 1998; Sassen 2001; Wallerstein 1974). Accordingly, the flow of capital into peripheral and semi-peripheral countries, such as Mexico, results in a counter-movement of labor from such countries to core countries, such as the United States (Massey and Espinosa 1997). This perspective highlights the links between the movement of capital and the countermovement of labor. To a large extent, emigrants from periphery countries migrate to global cities (Sassen 2001), which are urban centers in which banking, finance, administration, professional services, and high-tech production tend to be concentrated (Massey et al. 1993, p. 447). However, by focusing exclusively on the macrolevel, the world-systems perspective ignores the microlevel factors that encourage people to migrate, while also neglecting the role of politics and the state in social and economic change (Brettell 2000).

Dual Labor-Market Perspective The dual labor-market perspective emphasizes the demands of advanced industrial countries for low-skilled and low-wage labor (Massey et al. 1993). This perspective suggests that immigrants fill jobs that natives shun due to low wages, lack of mobility ladders, lack of benefits, and the arduous and dangerous nature of such jobs. The dual labor-market perspective asserts that the movement of immigrants from less developed to more developed countries is driven by the constant demand for cheap labor in developed countries.

Neoclassical Economics Perspective The neoclassical economics perspective is characterized by attention to both the macrolevel and the microlevel. At the macrolevel, this perspective suggests that migration occurs as a response to a disequilibrium between labor supply and labor demand. Thus, some labor markets have a surplus of labor, which results in high levels of unemployment and low wages. Other labor markets have greater labor demands than the existing labor pool can supply, which results in low levels of unemployment and high wages. This perspective suggests that workers gravitate from labor markets with a greater surplus of labor toward those with a greater demand for labor.

In contrast, the microlevel form of the neoclassical economics perspective focuses on the cost-benefit calculations that individuals undertake when making decisions regarding migration. This perspective treats individuals as utility maximizers who attempt to obtain the highest wages in relation to investments. In considering migration, people weigh the costs of relocation against the benefits that they are likely to receive from moving. People are expected to migrate when the benefits outweigh the costs, whereas they are presumed to remain stable when costs outweigh benefits.

The New Economics-of-Migration Perspective The new economics-of-migration perspective shifts the context to the household and focuses on how such units organize themselves to maximize economic returns while minimizing risks (see Massey et al. 1993). In settings without readily accessible unemployment benefits, bank loans, and insurance to protect against potential failure (e.g., loss of crops due to weather), households are especially vulnerable to a variety of economic and physical calamities. The new economics-of-migration perspective suggests that household members are deployed to undertake a wide variety of employment activities to ensure that the household generates economic resources while minimizing risks. Thus, some household members may remain at home to conduct subsistence agricultural activities, while other household members work locally in the private sector, others migrate to urban areas within the country, and still others move abroad and send remittances to the household. Essentially, the new economics-of-migration perspective proposes that households diversify their investment portfolios to protect themselves against risks and uncertainties.

Social-Network Perspective The social-network perspective emphasizes the interpersonal ties linking potential migrants and former migrants, as well as the migrants communities of origin and destination. In particular, the social-network perspective focuses on social relations that exist among family or community members, with special attention to social capital. Alejandro Portes notes that social capital is made up of those expectations for action within a collectivity that affect economic goals and goalseeking behavior of its members, even if these expectations are not oriented to the economic sphere (1993, p. 1322). This perspective places primary importance on the social networks that potential migrants have with people who have previously migrated as the most important factor influencing peoples decision to migrate (Massey and Espinosa 1997). Hence, social networks affect the decision of individuals to migrate based on interpersonal ties. Migration is facilitated by sharing information about destination communities, reducing the expected costs and risks of migration, and increasing the expected benefits of migration (Rivero Fuentes 2003). In contrast to the neoclassical economics perspective, the social-network perspective recognizes the nonrational elements involved in peoples migration decision. However, sociologists have also been careful to recognize the shortcomings of this perspective. For example, social capital may prevent the creation of new networks by excluding outsiders, restricting individual freedom, and promoting downward leveling norms (Portes 1998).

Cumulative Migration (or the Migration Syndrome) Perspective Closely connected to the social-network perspective is the cumulative migration (or migration syndrome) perspective. According to this perspective, originating from the social-movements framework, every act of immigration has the potential to facilitate the migration of other people by decreasing the cost of immigration (Massey et al. 1993). Carried to its fullest, migration becomes cumulative and natural when it becomes embedded in the culture of sending communities. In such instances, it is completely expected that certain segments of the population (e.g., the young in many parts of Mexico, especially males) will emigrate. In the case of Mexico, for example, many communities have relatively few working-age men because they have followed the trek of friends and relatives to the United States.


While migration represents an important life-changing event for migrants, it also poses important demographic, social, economic, and political implications for sending and receiving communities. In demographic terms, international migration is responsible for changing the racial and ethnic composition of the communities of destination. For example, the American Latino and Asian populations have expanded tremendously through immigration into the United States since the 1960s (Alba 1999). In addition, with respect to the social implications of immigration, much discussion has focused on issues such as language and racial and ethnic boundaries as examined through the assimilation and multiculturalism models (Alba 1999).

Furthermore, migration has tremendous economic implications for communities, states, and countries around the globe. For example, Peggy Levitt and Rafael de la Dehesa (2003) have pointed out that much public policy on immigration is due to the recognition of migrants and their remittances as crucial elements in the states economy. Because of this, many states are reinventing themselves by expanding the boundaries of citizenship and nationality, granting migrants the right to vote, establishing bureaucratic reforms, and making investments that allow states to become more efficient in aiding migrants within and across their own borders.


As noted earlier, and as predicted by contemporary theoretical perspectives on migration, people generally move from less developed to more developed countries. The future points increasingly to this scenario. Of the 2.8 billion people that are projected to be added to the worlds population between 2005 and 2050, nearly all (98.6%) are expected to be added to the populations of developing countries (Population Reference Bureau 2005). Developed countries, with increasingly aging populations and low fertility rates, are projected to account for only 1.4 percent of the worlds population growth between 2005 and 2050. Given continued economic disparities across the developed and developing worlds, increasing global population shifts are likely.

SEE ALSO Immigrants to North America; Migrant Labor


Alba, Richard. 1999. Immigration and the American Realities of Assimilation and Multiculturalism. Sociological Forum 14 (1): 325.

Brettell, Caroline B., and James F. Hollifield. 2000. Migration Theory: Talking across Disciplines. In Migration Theory: Talking across Disciplines, eds. Caroline B. Brettell and James F. Hollifield, 126. New York: Routledge.

Brown, David L. 2002. Migration and Community: Social Networks in a Multilevel World. Rural Sociology 67 (1): 123.

Conway, Karen Smith, and Andrew J. Houtenville. 1998. Do the Elderly Vote with Their Feet? Public Choice 97: 663685.

De Jong, Gordon F., Rex H. Warland, and Brenda Davis Root. 1998. Family Interaction and Migration Decision Making. Research in Rural Sociology and Development 7: 155167.

Lee, Everett S. 1966. A Theory of Migration. Demography 3: 4757.

Levitt, Peggy. 1998. Social Remittances: Migration Driven Local-Level Forms of Cultural Diffusion. International Migration Review 32 (4): 926948.

Levitt, Peggy, and Rafael de la Dehesa. 2003. Transnational Migration and the Redefinition of the State: Variations and Explanations. Ethnic and Racial Studies 26 (4): 587611.

Massey, Douglas S., and Kristin Espinosa. 1997. Whats Driving Mexico-U.S. Migration? A Theoretical, Empirical, and Policy Analysis. American Journal of Sociology 4 (102): 939999.

Massey, Douglas S., et al. 1993. Theories of International Migration: A Review and Appraisal. Population and Development Review 19 (3): 431466.

Petersen, William. 1958. A General Typology of Migration. American Sociological Review 23 (3): 256266.

Plane, David A., and Peter A. Rogerson. 1994. The Geographical Analysis of Population with Applications to Planning and Business. New York: Wiley.

Population Reference Bureau. 2005. 2005 World Population Data Sheet. Washington, DC: Author.

Portes, Alejandro. 1998. Social Capital: Its Origins and Applications in Modern Sociology. Annual Review of Sociology 24: 124.

Portes, Alejandro, and Julia Sensenbrenner. 1993. Embeddedness and Immigration: Notes on the Social Determinants of Economic Action. American Journal of Sociology 98(6): 13201350.

Portes, Alejandro, and Min Zhou. 1993. The New Second Generation: Segmented Assimilation and Its Variants. Annals of the American Academy of Political and Social Sciences 530: 7496.

Ravenstein, E. G. 1885. The Laws of Migration. Journal of the Statistical Society 48 (2): 167235. Ritchey, P. N. 1976. Explanations of Migration. Annual Review of Sociology 2: 363404.

Rivero Fuentes, Estela. 2003. Engendering Migrant Networks: The Case of Mexican Migration. Demography 40 (2): 289307.

Sassen, Saskia. 2001. The Global City: New York, London, Tokyo. 2nd ed. Princeton, NJ: Princeton University Press.

Wallerstein, Immanuel. 1974. The Modern World-System. Capitalist Agriculture and the Origins of the European World Economy in the Sixteenth Century. New York: Academic Press.

Rogelio Saenz

Maria Isabel Ayala

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migration of animals

migration of animals, movements of animals in large numbers from one place to another. In modern usage the term is usually restricted to regular, periodic movements of populations away from and back to their place of origin. A single round trip may take the entire lifetime of an individual, as with the Pacific salmon; or an individual may make the same trip repeatedly, as with many of the migratory birds and mammals. The animals may travel in groups along well-defined routes; or individuals may travel separately, congregating for breeding and then spreading out over a wide feeding area, as do some of the seals.

Types of Migration

Seasonal migrations occur in many species of insects, birds, marine mammals, and large herbivorous mammals. These migrations often provide the animals with more favorable conditions of temperature, food, or water. Many birds and a few bats of cold and temperate regions migrate to warmer areas during the winter. Herbivores of cold regions, such as wapiti (elk), caribou, and moose, have summer and winter ranges; many herbivores of warm regions, such as the African antelopes, migrate seasonally to avoid drought. These migrations may involve a change of latitude, of altitude, or both.

In many cases the chief function of seasonal migration is to provide a suitable place for reproduction, which may not be the place most suitable for the feeding and other daily activities of adults. Hundreds of thousands of gnus (wildebeests) of E Africa take part in annual migrations to calving grounds. Many fishes migrate to spawning grounds, and in some cases this involves a change from saltwater to freshwater (e.g., salmon) or vice versa (e.g., freshwater eels). Sea turtles, seals, and many sea birds come ashore to breed, and most amphibians gather near water at the breeding season. Fur seals and many whales make ocean voyages of thousands of miles to their breeding grounds, the former coming ashore on islands. Such migration is seriously affected by the increasing rate of destruction of natural habitats.

The term emigration refers to irregular movements out of an area, with no return. When such emigration is the result of sudden, explosive population increase, it is called an irruption. Irruptions are common among small rodents, notably lemmings, and various species of birds and insects. The mass movements of the so-called migratory locusts of N Africa (Locusta) and North America (Melanoplus) are actually irruptions; however, the N African desert locust (Schistocerca) makes true migrations between its winter and summer breeding grounds.

Another type of one-way travel is the regular dispersal of the young of most species. The simplest type of regular migration is the diurnal movement of some marine microorganisms from one depth to another in response to light changes. Certain marine invertebrates, such as the palolo worm (see Annelida), have a monthly migration pattern influenced by the phases of the moon.

Initiation of Migration

Various factors determine the initiation of migration. In some cases external pressures—temperature, drought, food shortage—alone may cause the animals to seek better conditions. For example, most of the mule deer of Yellowstone Park, Wyo., migrate between summer and winter pastures, but those living near hot springs, where grazing is available all year, do not. In many species migration is initiated by a combination of physiological and external stimuli. In birds the migratory instinct is related to the cycle of enlargement of the reproductive organs in spring and their reduction in fall. Experiments have shown that variation in day length is the chief external stimulus for this cycle: light received by the eye affects production of a hormone by the anterior pituitary gland, which stimulates growth of the reproductive organs.

Orientation and Navigation

Much work has been done on orientation and navigation in migrating animals, although the subject is still not well understood. Studies of salmon indicate that they depend on the olfactory sense to locate and return to their stream of origin. Herbivorous mammals often follow well-established trails and probably also use their sense of smell. Bats, whales, and seals use echolocation to navigate in the dark or underwater; in addition, some whales appear to take visual bearings on objects on the shore in their migrations.

Migratory birds are believed to use the stars, sun, and geographic features as guides. The probability that stellar navigation is used has been strengthened by experiments in planetariums indicating that birds navigate at least in part by the stars. Night-migrating birds are sometimes disoriented in prolonged heavy fog. Day-flying birds navigate by the sun and also make some use of geographic features, particularly of shorelines. Many birds also have the ability to orient themselves to the earth's magnetic fields and this appears to contribute to their ability to navigate, but the mechanisms by which this happens are not well understood. Most migratory birds travel within broad north-south air routes known as flyways. There are four major flyways in North America, called the Pacific, central, Mississippi, and Atlantic flyways. The space within the flyway used by a particular group of birds is called a corridor. Bird migration is not always in a north-south direction. Many European birds migrate in an east-west direction, wintering in the more temperate British Isles, and many mountain-dwelling birds descend to lower altitudes in winter. The breeding grounds of a bird species are regarded as its home territory. Some migratory birds winter only a few hundred miles from their breeding grounds, while others migrate between the cold or temperate zones of the two hemispheres. The longest journey is made by the arctic tern, which alternates between the Arctic and the Antarctic.

The monarch butterfly has a north-south migration pattern that resembles that of many birds. One monarch population that inhabits northeastern and midwestern North America averages c.12 mph (19 kph) as it heads for the winter to Mexico's Sierra Madre mountains. Monarchs start the return trip in the spring, but they breed along the way and then die; the new generation completes the journey. Studies suggest that monarchs are able to use both the sun and the direction of the earth's magnetic field to navigate.

Tools for Studying Migration

The movements of migrating animals are often studied by tagging individuals. Bird banding has been carried on extensively since the 1920s; more recently there has been tagging of fishes, butterflies, and marine mammals. Use is now made of radar, sonar, and radio for following migrations, particularly those of marine animals. Radio transmitters attached to whales or seals emit signals that can be picked up by weather satellites at regular intervals.


See R. R. Baher, The Evolutionary Ecology of Animal Migration (1978); D. J. Aidley, Animal Migration (1981).

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Across time and cultures individuals migrate to improve their lives, seek better opportunities, or flee unbearable conditions. In Russian history, migration highlights social stratification, underscores the importance of social management, and provides insight into post-Soviet population change. Migration motivations in Russia were historically influenced by direct governmental control, providing a unique case for assessing barriers to migration and a window into state and society relations.

The earliest inhabitants of the region now known as Russia were overrun by the in-migration of several conquering populations, with Cimmerians, Scythians (700 b.c.e.), Samartians (300 b.c.e.), Goths (200 c.e.), Huns (370 c.e.), Avars, and Khazars moving into the territory to rule the region. Mongol control (1222) focused on manipulating elites and extracting taxes, but not in-migration. When Moscow later emerged as an urban settlement, eastern Slavs spread across the European plain. Ivan III (14621505) pushed expansion south and west, while Ivan IV (15301584) pushed east towards Siberia. Restrictions on peasant mobility made migration difficult, yet some risked everything to illegally flee to the southern borderlands and Siberia.

The legal code of 1649 eradicated legal migration. Solidifying serfdom, peasants were now owned by the gentry. Restrictions on mobility could be circumvented. Ambitious peasants could become illegal or seasonal migrants, marginalized socially and economically. By 1787 between 100,000 and 150,000 peasants resided seasonally in Moscow, unable to acquire legal residency, forming an underclass unable to assimilate into city life. Restricted mobility hindered the development of urban labor forces for industrialization in this period, also marked by the use of forced migration and exile by the state.

The emancipation of serfs (1861) increased mobility, but state ability to control migration remained. Urbanization increased rapidlyaccording to the 1898 census, nearly half of all urbanites were migrants. The Stolypin reforms (1906) further spurred migration to cities and frontiers by enabling withdrawal from rural communes. Over 500,000 peasants moved into Siberia yearly in the early 1900s. Over seven million refugees moved into Russia by 1916, challenging ideas of national identity, highlighting the limitations of state, and crystallizing Russian nationalism. During the Revolution and civil war enforcement of migration restrictions were thwarted, adding to displacement, settlement shifts, and urban growth in the 1920s.

The Soviet passport system reintroduced state control over migration in 1932. Passports contained residency permits, or propiskas, required for legal residence. The passport system set the stage for increased social control and ideological emphasis on the scientific management of population. Limiting rural mobility (collective farmers did not receive passports until 1974), restricting urban growth, the exile of specific ethnic groups (Germans, Crimean Tatars, and others), and directing migration through incentives for movements into new territories (the Far East, Far North, and northern Kazakhstan) in the Soviet period echoed previous patterns of state control. As demographers debated scientific population management, by the late Soviet period factors such as housing, wages, and access to goods exerted strong influences on migration decision making. Attempts to control migration in the Soviet period met some success in stemming urbanization, successfully attracting migrants to inhospitable locations, increasing regional mixing of ethnic and linguistic groups across the Soviet Union, and blocking many wishing to immigrate.

With the collapse of the Soviet Union in 1991, migration restrictions were initially minimized, but migration trends and security concerns increased interest in restrictions by the end of the twentieth century. Decreased emigration control led to over

100,000 people leaving Russia yearly between 1991 and 1996, dampened only by restrictions on immigration from Western countries. Russia's population loss has been offset by immigration from the near abroad, where 25 million ethnic Russians resided in 1991. Legal, illegal, and seasonal migrants were attracted from the near abroad by the relative political and economic stability in Russia, in addition to ethnic and linguistic ties. Yet, the flow of immigrants declined in the late 1990s. Refugees registered in Russia numbered nearly one million in 1998. Internally, migration patterns follow wages and employment levels, and people left the far eastern and northern regions. Internal displacement emerged in the south during the 1990s, from Chechnya. By the late 1990s, the challenges of migrant assimilation and integration were key public issues, and interest in restricting migration rose. While market forces had begun to replace direct administrative control over migration in Russia by the end of the 1990s, concerns over migration and increasing calls for administrative interventions drew upon a long history of state management of population migration.

See also: demography; immigration and emigration; law code of 1649; passport system


Bradley, J. (1985). Muzhik and Muscovite: Urbanization in Late Imperial Russia. Los Angeles: University of California Press.

Brubaker, Rodgers. (1995). "Aftermaths of Empire and the Unmixing of Peoples: Historical and Comparative Perspectives" Ethnic and Racial Studies 18 (2):189218.

Buckley, Cynthia J. (1995). "The Myth of Managed Migration." Slavic Review 54 (4):896916.

Gatrell, Peter. (1999). A Whole Empire Walking: Refugees in Russia During World War I. Bloomington: Indiana University Press.

Lewis, Robert and Rowlands, Richard. (1979). Population Redistribution in the USSR: Its Impact on Society 18971977. New York: Praeger Press.

Zaionchkovskaya, Zhanna A. (1996). "Migration Patterns in the Former Soviet Union" In Cooperation and Conflict in the Former Soviet Union: Implications for Migration, eds. Jeremy R. Azrael, Emil A. Payin, Kevin F. McCarthy, and Georges Vernez. Santa Monica, CA: Rand.

Cynthia J. Buckley

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In biology, the term migration refers to the regular, periodic movement of animals between two different places. Migration usually occurs in response to seasonal changes and is motivated by breeding and/or feeding drives. Migration has been studied most intensively among birds, but it is known to take place in many other animals as well, including insects, fish, whales, and other mammals. Migration is a complex behavior that involves timing, navigation, and other survival skills.

The term migration also applies to the movement of humans from one country to another for the purpose of taking up long-term or permanent residency in the new country.

Types of migration

Four major types of migration are known. In complete migration, all members of a population travel from their breeding habitat at the end of that season, often to a wintering site hundreds or even thousands of kilometers away. The arctic tern is an example of a complete migrant. Individuals of this species travel from the Arctic to the Antarctic and back again during the course of a year, a round-trip migration of more than 30,000 kilometers!

In other species, some individuals remain at the breeding ground year-round while other members of the same species migrate away. This phenomenon is known as partial migration. American robins are considered indicators of the arrival of spring in some areas but are year-round residents in other areas.

Differential migration occurs when all the members of a population migrate, but not necessarily at the same time or for the same distance. The differences are often based on age or sex. Herring gulls, for example, migrate a shorter and shorter distance as they grow older. Male American kestrels spend more time at their breeding grounds than do females, and when they do migrate, they don't travel as far.

Irruptive migration occurs in species that do not migrate at all during some years but may do so during other years. The primary factors determining whether or not migration occurs are weather and availability of food. For example, some populations of blue jays are believed to migrate only when their winter food of acorns is scarce.

Migration Pathways

Migratory animals travel along the same general routes each year. Several common "flyways" are used by North American birds on their southward journey. The most commonly used path includes an 800 to 1,100 kilometer flight southward across the Gulf of Mexico. In order to survive this difficult journey, birds must store extra energy in the form of fat. All along the migration route, but particularly before crossing a large expanse of water, birds rest and eat, sometimes for days at a time. The

birds start out again on their journey only when they have added a certain amount of body fat.

Although most migrants travel at night, a few birds prefer daytime migrations. The pathways used by these birds tend to be less direct and slower than those of night migrants, primarily because of differences in feeding strategies. Night migrants can spend the day in one area foraging for food and building up energy reserves for the night's nonstop flight. Daytime migrants must combine travel with foraging, and thus tend to keep to the shorelines, which are rich in insect life, capturing food during a slow but ever-southward journey.


Perhaps the most remarkable aspect of migration is the navigational skills employed by the animals. Birds such as the albatross and lesser golden plover travel hundreds of kilometers over the featureless open ocean. Yet they arrive home without error to the same breeding grounds year after year. Salmon migrate upstream from the sea to the very same freshwater shallows in which they were hatched. Monarch butterflies began life in the United States or Canada. They then travel to the same wintering grounds in Southern California or Mexico that had been used by ancestors many generations before.

How are these incredible feats of navigation accomplished? Different animals have been shown to use a diverse range of navigational aids, involving senses often much more acute than our own. Sight, for example, may be important for some animals' navigational skills, although it may often be secondary to other senses. Salmon can smell the water of their home rivers, and follow this scent all the way from the sea. Pigeons also sense wind-borne odors and may be able to organize the memories of the sources of these smells in a kind of internal map. It has been shown that many animals have the ability to sense the magnetic forces associated with the north and south poles, and thus have their own built-in compass. This magnetic sense and the sense of smell are believed to be the most important factors involved in animal migration.

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migration, of people, geographical movements of individuals or groups for the purpose of permanently resettling.

Early History

Migrations have occurred throughout history and have played an important part in the peopling of all the areas of the earth. Primitive migrations were usually in search of food, but could also result from physical changes, such as the advance of the continental ice sheets, and invasion by other peoples. The most important migrations in European history were the Gothic invasions (3d–6th cent.; see Germans), the Arab invasions (7th–8th cent.; see Arabs), the westward migration of the Golden Horde of Jenghiz Khan (13th cent.), and the invasions of the Ottoman Turks (14th–16th cent.; see Ottoman Empire; Turks).

Later Migrations

From the 17th to the 20th cent. migration involved individuals and families rather than nations or mass groups. The basic motive was economic pressure, as areas of low population density attracted people from high-density areas where economic opportunity was low. The desire for religious and political freedom has also been important, and national policies have played a part. In the largest international migration in history, c.65 million people migrated from Europe to North America and South America between the 17th cent. and World War II, while another 17 million went to Africa and Australia.

Nearly 12 million people, most from Mexico or Asia, migrated to the United States in the 1970s and 80s. Within the United States, migration patterns have traditionally been from east to west. Migration from north to south since the 1960s has resulted in the ascendancy of the Sun Belt, a region extending from Florida to S California. This trend has been supported by the southward migration of many blacks. Government regulation of migration became significant in the 20th cent. (see immigration).

Modern Migration Trends

Normal internal migration has been characterized by a population shift from rural to urban areas. In the United States, the portion of the population that lives in urban areas has risen steadily from 30% in 1910 to more than 70% in 1990; in Brazil, the percentage of urban dwellers has risen from 30% to 75% since 1940. Within urban areas, a large population shift from central cities to suburbs has occurred in the last half of the 20th cent. The development of totalitarianism and World War II resulted in a new pattern of forced mass migration within Europe. Over 30 million people were forcibly moved or scattered by the Nazis. In the postwar period c.10 million Germans and persons of German descent were forcibly expelled from Eastern Europe.

Other forced migrations since World War II have included the partitioning of India and Pakistan, which uprooted 18 million, and the establishment of the state of Israel, which created about one million refugees (see refugee). After the fall of Saigon (now Ho Chi Minh City) at the end of the Vietnam War in 1975, more than 600,000 fled Vietnam in the face of political persecution; many fled by boat and became known as the "boat people." In South Africa, under the policies of apartheid, blacks were forced to live in designated "homelands" from 1959 to 1994. The Soviet invasion of Afghanistan in 1979 led to the migration of millions of Afghans to neighboring Pakistan and Iran.

In the 1980s and 90s war and civil strife continued to force massive refugee migration in many parts of the world. In Somalia and Ethiopia, civil war combined with long-term drought have resulted in large migrations of peoples (often from rural to urban areas and to neighboring countries) in an attempt to avoid famine. Hundreds of thousands of Kurdish refugees (see Kurds) have migrated from Iraq to Turkey and Iran in the wake of the civil war that followed the Persian Gulf War in 1991. The disintegration of Yugoslavia in the 1990s caused the dislocation of many peoples, especially Bosniaks, Croats, Serbs in areas other than Serbia, and Kosovars. In Rwanda and Burundi, millions of people, primarily Hutus, fled as ethnic civil war wrenched those nations in the mid-1990s; many of them fled to Zaïre (now Congo), where their presence aggravated civil and international strife.


See A. A. Brown and E. Neuberger, Internal Migration (1977); M. Greenwood, Migration and Economic Growth in the United States (1981); G. J. Lewis, Human Migration (1982); W. Weidlich and G. Haag, ed., Interregional Migration (1988); R. King, ed. Atlas of Human Migration (2007) and as author, People on the Move: An Atlas of Migration (2010); I Goldin et al., Exceptional People: How Migration Shaped Our World and Will Define Our Future (2011).

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"migration." The Columbia Encyclopedia, 6th ed.. . 12 Dec. 2017 <>.

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Many animals move from one place to another at certain times of the year or during a particular period of their life cycle. These movements are often referred to as migration. Some animals, such as many species of insects, migrate only once during their lifetime, often just before they reproduce. Other animals, including many species of birds and many marine animals, such as sea turtles and whales, migrate long distances to their breeding grounds many times during their lives.

Animals migrate for several reasons. In some cases, animals can only reproduce in a particular habitat, such as sea turtles and sea birds that must return to land in order to lay their eggs. In other instances, animals are forced to leave an area when conditions in the environment deteriorate. Many bird species that nest in Canada and the northern regions of the United States migrate south as winter approaches. The ultimate reason these birds migrate south is because their food supply (including insects and fish) will not be available during the cold months. However, these birds normally begin migrating south before their food supply has disappeared, and often even before it has begun to decline.

In actuality, it is the changing length of the days (photoperiod) that stimulates hormonal and behavioral changes that result in migration. Such an environmental cue is often referred to as the proximate (or immediate) cause of migration, whereas the inevitable decline in food supply is referred to as the ultimate cause. A bird that waited until its food actually disappeared would not have sufficient body fat reserves to migrate a long distance. Thus, scientists believe that natural selection favored birds that used predictable environmental cues, such as the seasonal change in day length, to initiate migration before their food source disappeared.

Seasonal migrators exhibit obligatory migration, meaning they must migrate every year. For other animals, the decline in the conditions of their environment is not so predictable. For example, owls that live in the tundra and Canadian forests feed on small rodents that are abundant some winters and scarce during others. During winters when rodent populations are high, these owls remain in Canada and do not migrate. However, if rodent populations are low, these owls will migrate down into the northern regions of the United States. Animals such as these owls exhibit facultative migration, meaning migration is optional for them.

Scientists have been fascinated by how animals are able to navigate during their migration. Studies have shown that migrating species are able to use a wide variety of mechanisms to navigate, including the stars, the sun, olfactory (chemical) cues, and Earth's magnetic field. Some species learn their migration routes by first traveling with experienced individuals, but other species are able to migrate and navigate successfully without prior experience, an ability that still perplexes scientists. Migration requires a lot of energy and many individuals die during migration. Despite these heavy costs, the potential benefits of migration are great, which is why migration behavior has evolved in so many species.

see also Behavior Patterns; Bird; Field Studies in Animal Behavior; Tundra; Turtle

Mark A. Davis


Alcock, John. Animal Behavior: An Evolutionary Approach, 6th ed. Sunderland, MA: Sinauer Associates, Inc., 1997.

Berthold, Peter. Bird Migration: A General Survey. Oxford: Oxford University Press, 1993.

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migration The seasonal movement of complete populations of animals to a more favourable environment. It is usually a response to lower temperatures resulting in a reduced food supply, and is often triggered by a change in day length (see photoperiodism). Migration is common in mammals (e.g. porpoises), fish (e.g. eels and salmon), and some insects but is most marked in birds. The Arctic tern, for example, migrates annually from its breeding ground in the Arctic circle to the Antarctic – a distance of some 17 600 km. Migrating animals possess considerable powers of orientation; birds seem to possess a compass sense, using the sun, pole stars, and (in cloud) the earth's magnetic lines of force as reference points (see navigation).

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"migration." A Dictionary of Biology. . 12 Dec. 2017 <>.

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migration Periodic movement of animals or humans, usually in groups, from one area to another, in order to find food, breeding areas, or better conditions. Animal migration involves the eventual return of the migrant to its place of departure. Fish migrate between fresh and salt water or from one part of an ocean to another. Birds usually migrate along established routes. Mammals migrate usually in search of food. For thousands of years, the deserts of central Asia widened inexorably resulting in the human migration of pre-historic tribes to China, the Middle East and Europe. Another migration occurred in the 14th century when the Maori left their overpopulated homes in the islands of central Polynesia to settle in New Zealand.

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"migration." World Encyclopedia. . 12 Dec. 2017 <>.

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1. The movement of individuals or their propagules (seeds, spores, larvae, etc.) from one area to another. Three cases may be distinguished:
a. emigration, which is outward only
b. immigration, which is inward only
; and
c. migration, which in this stricter sense implies periodic two-way movements to and from a given area and usually along well-defined routes. Such migratory movement is triggered by seasonal or other periodic factors (e.g. changing day-length), and occurs in many animal groups.

2. In plant succession, specifically the arrival of migrating propagules (migrules) at a newly denuded area.

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"migration." A Dictionary of Plant Sciences. . 12 Dec. 2017 <>.

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1. The movement of individuals or their propagules (seeds, spores, larvae, etc.) from one area to another. Three cases may be distinguished: (a)emigration, which is outward only;(b)immigration, which is inward only; and(c)migration, which in this stricter sense implies periodic movements to and from a given area and usually along well-defined routes. Such migratory movement is triggered by seasonal or other periodic factors (e.g. changing day-length), and occurs in many animal groups.

2. In plant succession, specifically the arrival of migrating propagules (migrules) at a newly denuded area.

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"migration." A Dictionary of Ecology. . 12 Dec. 2017 <>.

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migration The movement of animals from one area to another. Three cases may be distinguished: (a) emigration (outward only);(b) immigration (inward only); and(c) migration, which in this stricter sense implies periodic two-way movements to and from a given area, usually along well-defined routes. Such migratory movement is triggered by seasonal or other periodic factors (e.g. changing day length), and occurs in many animal groups.

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"migration." A Dictionary of Zoology. . 12 Dec. 2017 <>.

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migration A method of reconstructing a seismic section so that dipping reflection events are repositioned to lie beneath their true surface locations and at corrected vertical two-way travel times. There are several migration methods. These include, for example, wave-equation, dip-moveout, ray-trace, finite-difference, wavefront-common-envelope, diffraction, and frequency-domain.

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"migration." A Dictionary of Earth Sciences. . 12 Dec. 2017 <>.

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migration XVII. — F. migration or L. migrātiō, -ōn-, f. migrāre, prob. based on *mei- change (cf. MUTABLE); see -ATION.
So or hence migrate (-ATE3) XVII.

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"migration." The Concise Oxford Dictionary of English Etymology. . 12 Dec. 2017 <>.

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the persons, mammals, or birds that take part in migratory movements abroad, collectively.

Examples: migration of birds, 1704; of salmon, 1704; of souls of men, 1727.

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"Migration." Dictionary of Collective Nouns and Group Terms. . 12 Dec. 2017 <>.

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This entry includes three subentries:

Migration in World History
United States

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