COHORT PERSPECTIVES

The birth cohort, or set of people born in approximately the same period of time, has a triple reference as an analytical tool in sociology: (1) to cohorts of people who are aging and succeeding each other in particular eras of history; (2) to the age composition of the population and its changes; and (3) to the interplay between cohorts of people and the age-differentiated roles and structures of society. Diverse sociological studies illustrate the use of these cohort perspectives (i.e., both theoretical and empirical approaches) to investigate varied aspects of aging and cohort succession, population composition, and the reciprocal relationships between cohorts and social structures.

CONCEPTUAL FRAMEWORK

Figure 1 is a rough schematization of the major conceptual elements implicated in these interrelated cohort perspectives as they have relevance for sociology (for an overview, see Riley, Johnson, and Foner 1972; Riley, Foner and Riley 1999).

Aging and Cohort Succession. The diagonal bars represent cohorts of people born at particular time periods who are aging from birth to death—that is, moving across time and upward with age. As they age, the people in each cohort are changing socially and psychologically as well as biologically; they are actively participating with other people; and they are accumulating knowledge, attitudes, and experiences. The series of diagonal bars (as in the selected cohorts A, B, and C) denote how successive cohorts of people are continually being born, grow older through different eras of time, and eventually die.

Age Composition of the Population. The perpendicular lines direct attention to the people simultaneously alive in the society at particular dates. A single cross-sectional slice through the many coexisting cohorts (as in 1990) demonstrates how people who differ in cohort membership also differ in age—they are stratified by age from the youngest at the bottom to the oldest at the top. Over time, while society moves through historical events and changes, this vertical line should be seen as moving across the space from one period to the next. At different time periods the people in particular age strata are no longer the same people; inevitably, they have been replaced by younger entrants from more recent cohorts with more recent life experiences.

Cohorts and Social Structures. Corresponding to the age strata in the population, the perpendicular lines also denote the age-related role opportunities and normative expectations available in the various social structures (e.g., in schools for the young, in work organizations for those in the middle years, in nursing homes for the old, in families for all ages, etc.). People and structures are interdependent: changes in one influence changes in the other. Yet the two are often out of alignment, causing problems for both individuals and society.

This three-fold heuristic schematization, though highly oversimplified, aids interpretation and design of sociological work that takes cohort perspectives into account. (For simplicity, the discussion is limited here to cohorts in the larger society, with entry into the system indexed by date of birth. Parallel conceptualization refers also to studies of cohorts entering other systems, such as hospitals, with entry indexed by date of admission, or the community of scientists, with entry indexed by date of the doctoral degree—e.g., Zuckerman and Merton 1972; here "aging" refers to duration in the particular system.)

AGING PERSPECTIVES

Research on the processes of aging within and across cohorts illuminates the interrelated aspects of people's lives and the particular characteristics and historical backgrounds of the cohorts to which they belong.

Intracohort Perspectives. Many empirical studies and much conceptual work uses the "life-course approach" to trace over time the lives of members of a single cohort (e.g., Clausen 1986). As one familiar example, studies of "status attainment" investigate lifelong trajectories of achievement behaviors, using longitudinal and causal modeling to examine the interconnections among such variables as family background, scholastic achievement, succession of jobs, and employment and unemployment (cf. Featherman 1981). The intracohort perspective is used in many forms—microand macro-level, objective and subjective—in a range of studies on how people as they develop and grow older move through diverse paths in the changing society (Dannefer 1987), how sequences of role transitions are experienced, and how aging people relate to the changing environment. Psychologists as well as sociologists study interindividual changes in performance over the life course (e.g., Schaie 1996), while Nesselroade (1991) looks at intra-individual fluctuations over shorter periods of time.

Longitudinal studies of aging in a single cohort can contribute importantly to causal analysis by establishing the time order of correlated aspects of people's lives and environmental events. However, this perspective is vulnerable to possible misinterpretation through the fallacy of "cohortcentrism," that is, erroneously assuming that members of all cohorts will age in exactly the same fashion as members of the cohort under study (Riley 1978). Yet in fact, members of different cohorts, as they respond to different periods of history, usually age in different ways. For example, the enjoyment of "midlife" experienced at around age 50 by cohort members studied in the 1990s may not be felt by some future cohort until age 85.

Intercohort Perspectives. Broader than the intracohort focus, is a focus on the lives of members of two or more successive cohorts who are growing older under differing historical or sociocultural conditions. Studies of intercohort differences in the late-twentieth century demonstrated for other sciences what sociologists had learned early: the central principle that the process of aging is not immutable or fixed for all time, but varies across and within cohorts as society changes (Riley 1978). Such studies have shown that members of cohorts already old differ markedly from those in cohorts not yet old in such respects as standard of living, education, work history, age of menarche, experience with acute vs. chronic diseases, and perhaps most importantly the number of years they can expect to live. These cohort differences cannot be explained by evolutionary changes in the human genome, which remains much the same from cohort to cohort; instead, they result from a relatively unchanging genetic background combined with a continually changing society (Riley and Abeles 1990, p.iii). Thus the finding of cohort differences has pointed to possible linkages of lives with particular social or cultural changes over historical time, or with particular "period" events such as epidemics, wars, or depressions (e.g., Elder and Rockwell 1979). These linkages are useful in postulating explanations for changes—or absence of changes—in the process of aging.

Studies of cohort differences focus on aging processes at either the individual or the collective level. At the individual level, cohort membership is treated as a contextual characteristic of the individual, and then analyzed together with education, religion, and other personal characteristics to investigate how history and other factors affect the heterogeneous ways individuals grow older (e.g., Messeri 1988; but see Riley 1998). At the collective level, the lives of members are aggregated within each cohort to examine alterations in average patterns of aging. Striking advances have recently been made in the data banks available for intercohort comparisons. Archived data from many large-scale studies now cover long periods of history, multiple societies, and multidisciplinary aspects of the life course; and repeated longitudinal studies are being launched, such as the National Institute on Aging's Health and Retirement Study (HRS) and Asset and Health Dynamics Among the Oldest Old (AHEAD) (cf. Campbell 1994; O'Rand and Campbell 1999).

Cohort perspectives are useful, not only in explaining past changes in aging processes, but also in improving forecasts of future changes. Unlike the more usual straight projections of crosssectional information, forecasts based on cohorts can be informed by established facts about the past lives of people in each of the cohorts already alive (e.g., Manton 1989). Thus, if cohorts of teenagers today are on the average less healthy, less cared for, or less prepared for life than their parents were at the same age (National Association of State Boards of Education 1990), the lives of both offspring and parents will predictably also differ in the future when both have grown older.

COMPOSITIONAL PERSPECTIVES.

Complementing sociological work on cohort differences (or similarities) in the aging process are studies of how cohort succession contributes to formation and change in the age composition of the population. Thus in Figure 1, the perpendicular lines indicate how cohorts of people fit together at given historical periods to form the crosssectional age strata of society; and how, as society changes, new cohorts of people are continually aging and entering these strata, replacing the previous incumbents.

Single Period of Time. In Figure 1, as indicated above, a single vertical line at a given period (as in 1990) is a cross-sectional slice through all the coexisting cohorts, each with its unique size, composition, earlier life experiences, and historical background. This familiar cross-sectional view of all the age strata is often denigrated because its misinterpretation is the source of the life-course fallacy—that is, the erroneous assumption that cross-sectional age differences refer directly to the process of aging, hence disregarding the cohort differences that may also be implicated (Riley 1973). That people who are differentially located in the age composition of society differ not only in age but also in cohort membership was dramatized early by Mannheim ([1928] 1952) and Ryder (1968); yet persistent failure to comprehend this duality has perpetuated numerous false stereotypes (e.g., that intelligence or physical functioning begin inevitable declines at very early ages).

Properly interpreted, of course, a cross-sectional perspective has its special uses: for describing current differences and similarities, social relationships, and interactions among coexisting people who differ in age-cum-cohort membership. Thus, for example, issues of "intergenerational equity" require explication by both age and cohort, as a larger share of the federal budget is reportedly spent on cohorts of people now old than on cohorts of children (Duncan, Hill, and Rodgers 1986; Preston 1984).

Across Time. Comprehension of the underlying dynamics of the age strata requires going beyond the single cross-sectional snapshot to a sequence of cross-sections (the moving perpendicular line in Figure 1), as successive cohorts interact with historical trends in the society (Ryder 1965; Riley 1982). Historical change means not only that new cohorts are continually entering the system through birth or immigration, while others are leaving it through death or emigration (e.g., men tend to die earlier than women, and blacks earlier than whites). Historical change also means that the members of all existing cohorts are simultaneously aging and thus moving from younger to older strata. As successive cohorts move concurrently through the system, they affect the age strata in several ways. They can alter the numbers and kinds of people in particular strata, as each cohort starts the life course with a characteristic size, genetic makeup, sex ratio, racial and ethnic background, and other properties that are subsequently modified through migration, mortality, and environmental contact. The succession of cohorts can also affect the capacities, attitudes, and actions of people in particular strata as the members of each cohort bring to society their experiences with the social and environmental events spanned by their respective lifetimes.

The most significant alterations in the age composition of modern societies stem from the dramatic and unprecedented increases in the longevity of successive cohorts. Age pyramids diagramming the age composition of the United States in 2010 compared with 1955, for example, demonstrate that entirely new strata have been added at the oldest ages (Taeuber 1992)—strata of old people who are healthier and more competent than their predecessors (Manton, Corder, and Stallard 1997). The advent of these "new" old people is already having untold consequences: Individuals now have time to spread education, work, family activities, and leisure more evenly over their long lives, and wider structural opportunities are needed in society for the age-heterogenous population.

SOCIAL STRUCTURAL PERSPECTIVES

Cohorts, as described above, are composed of people, who age and fit together in strata to form the composition of the population; but cohorts also shape, and are shaped by social structures—the surrounding families, communities, work organizations, educational institutions, and the like. Against the backdrop of history, social structures, like lives, tend to change, and two "dynamisms"—changing structures and changing lives—are in continuing interplay, as each influences the other. Thus full understanding of cohorts requires understanding their reciprocal relations with structures (as in Foner and Kertzer 1978; Mayer 1988). Toward this end, some studies examine how the processes of aging and cohort flow relate to structures, while other studies examine the congruence—or lack of congruence—between age composition and social structures.

Aging and Structures . Because cohorts differ in size and character, and because their members age in new ways (the diagonal lines in Figure 1), they exert collective pressures for adjustments—not only in people's ideas, values, and beliefs—but also in role opportunities throughout the social institutions.

As one example, the influences of cohort differences in size were defined early by Joan Waring's (1975) powerful analysis of "disordered cohort flow." This disordered flow has been dramatically brought to attention as the Baby Boom cohorts first pressed for expansions in the school systems and the labor force, and will become the twenty-first century "senior boom" that will exacerbate the inadequacy of roles for the elderly. Later, as large cohorts were followed by smaller successors, ways were sought to reduce these expanded structures again. Meanwhile, as structures changed, the lives of people moving through these structures also changed.

In another example, the influences of cohort differences in norms has been analyzed as the process of "cohort norm formation" (Riley 1978). As members of a cohort respond to shared historical experiences, they gradually and subtly develop common patterns of response, common definitions, and common beliefs, that crystallize into new norms and become institutionalized in altered social structures. For instance, over the past century many individual women in successive cohorts have responded to common social changes by making many millions of separate but similar personal decisions to move in new directions: to go to college, have a career, or form their families in innovative ways. Such decisions, beginning in one cohort and transmitted from cohort to cohort, can feed back into the social structures and gradually pervade entire segments of society. Thus, many new age norms have become expectations that women should work, and have stimulated the demand for new role opportunities at work and in the family for people of all ages.

Age Composition and Structures. At any given period of history, the coexisting cohorts of people that form the age strata coincide with the existing role structures (both indicated by the perpendicular lines in Figure 1). People who differ in age and experience confront the available agerelated opportunities, or lack of opportunities, in work, education, recreation, the family, and elsewhere. However, people and structures rarely fit together smoothly: there is a mismatch or "lag" of one dynamism behind the other.

Structural Lag. While people sometimes lag behind structures as technology advances, more frequent in modern society is the failure of structural changes to keep pace with the increasing numbers of long-lived and competent people (Riley, Kahn, and Foner 1994). Cohorts of those who are young today have few "real-world" opportunities; those in the middle years are stressed by the combined demands of work and family; and those who have reached old age are restive in the prolonged "roleless role" of retirement. Cohorts of people now old are more numerous, better educated, and more vigorous than their predecessors were in 1920 or 1950; but few changes in the places for them in society have been made. Capable people and empty role structures cannot long coexist. Thus, implicit in the lag are perpetual pressures toward structural change.

Age Integration. Among societal responses to structural lag are current tendencies toward "age integration" (Riley, Foner, and Riley 1999, p.338). With pressures from the expanded numbers of age strata, many age barriers dividing education, work and family, and retirement are gradually becoming more flexible. People of different ages are more often brought together, as lifelong education means that old and young study together, as new entrepreneurships hire employees of mixed ages, as in many families four generations are alive at the same time, or as the age segregation of nursing homes is replaced by home health care with wide access to others. Where such tendencies may lead in the future is not yet known. But the interdependence between cohorts and structure is clear.

RESEARCH METHODS.

When aspects of these broad cohort perspectives are translated into empirical studies, a variety of research methods are required for specific objectives: from analyses of historical documents and subjective reports, to panel analyses and mathematical modeling, to rigorous tests of specific hypotheses. This brief overview can only hint at the diverse research designs involved in analyses of the multiple factors affecting lives of people in particular cohorts; or in the shifting role opportunities for cohort members confronting economic, religious, political, and other social institutions (for one example, see Hendricks and Cutler 1990).

Cohort Analysis. The tool most widely used in large-scale studies is "cohort analysis," which takes the intercohort aging perspective—in contrast to "period analysis" (Susser 1969), which takes the cross-sectional perspective. (The difference is illustrated in Figure 1 by comparison of the diagonal cohort lines, in contrast to comparison of a sequence of vertical compositional slices). In his 1992 formulation of the technical aspects of cohort analysis, Ryder defines the term as "the parameterization of the life cycle behavior of individuals over personal time, considered in the aggregate, and the study of change in those parameters over historical time" (p. 230). He conceptualizes the cohort as "providing a macro-analytic link between movements of individuals from one to another status, and movements of the population composition from one period to the next." His classic work on "demographic translation" sets out the mathematical procedure for moving between the cohort and the period "modes of temporal aggregation" (Ryder 1963, 1983).

Central to this method is the identification problem confounding many attempts at cohort analysis. This problem occurs from efforts to interpret the separate effects of three concepts—cohort, period, and age (C, P, and A)—when only two variables, such as date and years of age, are indexed. Apart from various procedures that assume one of the parameters is zero, the most appropriate solution to this problem is to specify and measure directly the three concepts used in the particular analysis (Cohn 1972; Rodgers 1982; Riley, Foner, and Waring 1988, pp. 260–261). After all, as Ryder puts it (1992, p. 228), the cohort (C) is a set of actors, the age (A) is their age, and the period (P) stands for the social context at the time of observation..

Wide Range of Methods. Ryder's exegesis of this particular method illustrates, through its strengths and limitations, the utility of the tripartite conceptualization of cohort perspectives as a heuristic guide. The strengths in Ryder's formulation focus exclusively on the significance of cohorts as a macro-analytic vehicle for social change. In the broader cohort perspectives outlined here, many other methods are useful for specific objectives where cohort analysis is inappropriate. Some employ a cross-sectional approach. Others utilize intercohort comparisons to focus on aging processes at the individual as well as the collective levels. Still others complement demographic analyses of populations with examination of the related structures of social roles and institutions.

Thus, despite its signal contributions, neither cohort analysis nor any other single method can comprehend the full power of cohorts as ingredients of aging processes, age composition, and the complex interplay with social structure.

(see also: Structural Lag)

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Matilda White Riley