Twins

Twins are siblings carried together in the womb and born at the same time. Similarities and differences between twins can be used to answer questions about the role genes and the environment play in the development of traits such as personality, intelligence, and susceptibility to disease. While results from any single pair of twins cannot provide conclusive answers to such questions, the study of large numbers of twin pairs allows researchers to draw conclusions about inheritance with a significant degree of confidence.

Developmental Mechanisms

Twins are classified as either dizygotic or monozygotic. Dizygotic twins (also called fraternal twins) arise from two separately fertilized eggs, or zygotes. In humans, usually only one egg is released at a time from a woman's ovaries. When two are released, both may become fertilized by separate sperm and implant in the uterus. Dizygotic twins develop separate placentas and amniotic sacs. They may be of the same or different sexes. In the absence of reproductive technology interventions, dizygotic twinning occurs in approximately three of every thousand human births, a rate that increases with maternal age, varies with ethnic group, and is probably influenced by genes that control pituitary function. Various types of assisted reproductive technologies routinely create dizygotic twins, triplets, and higher numbers of offspring.

Monozygotic twins (also called identical twins) arise from a single fertilized egg. At some point after the zygote begins to divide, the cell mass splits into two, creating two embryos from one. Monozygotic twinning occurs in approximately 0.25 percent of human births. Monozygotic twins are always of the same sex. If the cell mass splits before about day five after fertilization, the two embryos will develop with separate placentas and separate amniotic sacs. This occurs in about two-thirds of human monozygotic twins. Between day five and about day nine, splitting leads to two amniotic sacs but one placenta. This occurs in about one-third of Monozygotic twins. Twins that split after day nine will share the amniotic sac. Splitting that late also increases the likelihood that the twins will not separate completely and will develop into conjoined (Siamese) twins.

Monozygotic versus Dizygotic Twins

Because monozygotic (MZ) twins develop from a single fertilized egg, they begin life with exactly the same set of genes. In this respect, they are clones—organisms whose genes are identical. As discussed below, however, they may accumulate genetic and other differences during development.

In contrast, dizygotic (DZ) twins are no more genetically close than any pair of siblings. While it is commonly said that siblings share half their genes, this is incorrect for two reasons. First, the random nature of meiosis and fertilization means that two siblings could end up with many, or few, genes from a particular parent in common. Second, there are many human genes for which there is only one common form, or allele. Therefore, any two people will share many alleles, regardless of their relationship. Only those genes with more than one allele form the basis of human genetic variation. These are the real focus of the question about gene-sharing in siblings. Of these variable genes, siblings (including dizygotic twins) on average share half.

Because dizygotic twins are the same age, they may share more of their environment than would two siblings of different ages. For instance, because they are likely to be engaged in similar activities, dizygotic twins are more likely to have similar environmental exposures (including behaviors, diet, hobbies, exposure to infectious agents, and exposure to chemicals)—whether at home, at school, or in the community—than two siblings of different ages and different activity patterns. It is this similarity of environment but difference of genes that makes them a useful contrast to monozygotic twins, whose environments and genes are largely identical.

Similarities and Differences between Monozygotic Twins

The fertilized egg cell that gives rise to MZ twins begins life with a single set of genes, and so we might predict that every cell that arises from it would be exactly identical. However, small differences between daughter cells may accumulate throughout embryonic development and later in life. The earliest difference may be in the mitochondria each inherits. Mitochondria are the cell's power plants and contain a small amount of DNA. Some of the hundreds of mitochondria in a cell may contain mutations. If the cells that create the two twins carry different mitochondrial genes, even identical twins will be genetically different. Mutations can also accumulate during embryonic development, or after birth, either in the mitochondrial genes or the genes in the nucleus. Such mutations may have a significant effect: Some types of cancer are due to mutations accumulated during one's lifetime, often through exposure to environmental chemicals or radiation.

For the vast majority of genes, though, MZ twins are exactly identical. Nonetheless, twins do experience slightly different environments, even when reared together, and any early differences between them may be accentuated by families members, or by one another, leading to development of very different personalities.

Amazing Twin Similarities

Some of the most tantalizing clues to the genetic basis of human personality and behavior come from studies of MZ twins reared apart since birth. Such twins have the same genes but, presumably, different environments. A major study of more than 100 such twin pairs showed some remarkable coincidences. A pair of twins meeting for the first time at age thirty-nine each arrived wearing seven rings, two bracelets on one wrist, and a watch and one bracelet on the other wrist. Another twin pair discovered they each had dogs named Toy, had married and divorced women named Linda, remarried women named Betty, and named their sons James Allan and James Alan.

While these coincidences are amazing, it is important to remember that many more twin pairs in this study did not have such parallel lives or habits. Such stories are curious and provocative but cannot by themselves tell us about the relative contributions of genetics and the environment in shaping personality, behavior, health, or other aspects of the self.

Twin Studies and Concordance

Insight into such questions can be gleaned by several types of studies that compare twins. Comparison of MZ twins reared apart is one type of study but is hampered by the extreme rarity of such twin pairs. Another type of study, comparing MZ twins to DZ twins, is more commonly done, because there are many hundreds of thousands of such twin pairs worldwide. Data on twins have been collected by numerous research groups who have created large and growing databases (registries) that can be mined for information.

Determining a characteristic called concordance plays a crucial role in most such studies. A twin pair is said to be concordant for a trait if both members show it. If neither twin shows the trait, the pair is also concordant, but for the absence of the trait. For instance, twins are concordant for Alzheimer's disease if both develop it. They are discordant if one does have the disease but the other does not.

If a trait is strongly influenced by genes, more MZ twin pairs should be concordant than DZ twin pairs, because MZ twins share more genes. Comparing concordance rates between the two groups, and applying some mathematical analysis, allows researchers to estimate the genetic contribution to a trait, as shown in Table 1.

Twin Registries

Twin studies can have several starting points. Some investigators begin simply by trying to identify twins who will volunteer to be part of a particular research study. Often twins are sought by advertising for twins with the particular disease of interest. This approach has the advantage of simplicity, as twins identify themselves to the research team.

However, twins who volunteer may differ in some important way from those who do not volunteer, and this could affect the conclusions drawn from the study. For example, MZ twins are more likely to volunteer, in general, than DZ twins are. This tendency to volunteer for twin studies among MZ twins is probably because being a twin is a more central part of the identity of MZ pairs than DZ pairs. Also, twins concordant for a particular disease are more likely to volunteer than those without the disease are. If both influences are at work in the same study, more concordant MZ twins than DZ twins may be identified, not because there is an actual difference in concordance between MZ and DZ twins (and thus a genetic effect at work), but because more concordant MZ twins volunteered for the study. If this pattern of volunteerism is mistaken to represent the true pattern of the disease in all twins, an inappropriate conclusion that the disease has genetic causes could result.

Other twin registries attempt to identify all twins within a particular population. One approach is the statewide or national twin registry. All twin births in the region are reported to a central registrar. This results in a more complete picture of all twin pairs in these populations. Examples include the statewide Virginia and Minnesota twin registries in the United States and many national twin registries, including those in the United Kingdom, Australia, the Scandinavian countries, Germany, Belgium, the Netherlands, Italy, and Sri Lanka.

Twin registries have also been assembled from among special populations. Examples in the United States are registries assembled from military records (the World War II Veteran Twins Registry and the Vietnam Era Twin Registry) and from Medicare files (the U.S. Registry of Elderly African-American Twins). In these registries, likely adult twins were identified by searching records to identify individuals with identical dates of birth, birthplaces, and surnames. These individuals were then contacted to verify whether they actually constituted a twin pair. Registries may also be established by identifying twin births within a health maintenance organization (such as the Kaiser Permanente Twin Cohort, in California).

Each registry varies in the amount of contact with registrants. In all, individual contact is strictly monitored to preserve the privacy of each twin. Every research proposal must be approved by a panel to assure the scientific value of the project, the justification for doing the study in twins, and to ensure that the privacy and safety of individual twins will be protected.

Twin registries can be useful starting points for investigating many questions about the genetic and environmental determinants of a trait. Records linkage studies involve no personal contact with the twins. Instead, information in the twin registry is "linked" electronically to information in another database, such as a national health insurance database or a cancer registry. In this way, twins with a particular health problem can be identified, and concordance estimates can be calculated. Similarly, information collected for each twin at registration can later be used to investigate certain kinds of questions without ever contacting the individual twins. On the other end of the spectrum, twins may be asked to volunteer for physical examinations, blood tests, radiological studies, or interviews. Depending on the questions asked, such studies may be useful for comparing concordance, or for identifying risk factors or modifying factors for a trait.

Twin Studies to Investigate the Cause of Parkinson's Disease

An example of the use of investigations in twins to understand more about a disease is provided by recent work in Parkinson's disease. Parkinson's disease (PD) is a progressive neurodegenerative disease causing slowness, tremor, and problems with walking and balance. PD is rare before age fifty but becomes more common thereafter, with increasing age. The cause of PD has long been debated. Both genetic and environmental causes have been suggested, but neither has been definitively shown. Researchers turned to studies in twins to determine the relative contribution of genes and environment to the disease.

The first studies identified twin pairs by recruiting through physicians and PD patient organizations. Studies in the United States, the United Kingdom, and Germany identified 103 pairs, of which only thirteen were concordant for PD. In Finland, forty-two twins with PD were identified by records linkage, but among these was only one concordant pair—a DZ pair. No study had convincingly demonstrated greater monozygotic than dizygotic concordance for the disease, and in all studies the preponderance of twin pairs were discordant for disease. These findings supported an environmental cause of PD. Nonetheless, the advent of molecular genetics prompted great interest in investigations of genetic causes of disease and prompted the resurgence of the hypothesis that all PD had a genetic cause. To address this, a study in a large, unselected cohort—the National Academy of Sciences/National Resource Council (NAS/NRC) World War II Veteran Twins Registry—was undertaken.

In the mid-1950s, the Medical Follow-up Agency of the Institute of Medicine of the NAS/NRC established a registry of approximately 32,000 Caucasian male twins, all of whom were born between 1917 and 1927 and were veterans of the U.S. Armed Services. In all, 161 twin pairs were identified, twenty-one of which were concordant for PD, as shown in Table 2. In those few pairs with early-onset PD, concordance was greater in MZ pairs. In those with more typical PD, beginning after age fifty, there was no difference in MZ and DZ concordance.

These findings suggest a strong genetic determinant for early-onset disease but predominantly environmental causes in more typical late-onset disease. One caveat is the narrow age range of the twins, who were sixty-seven to seventy-seven years old when studied. Since PD is a late-life disorder, PD in some twins may have been missed with an examination at only one time point. To overcome this, a second evaluation is in progress.

Risk-Factor Investigations in Twins

Studies of twin pairs discordant for disease can be useful for identifying risk factors for disease. Since both genetic and environmental factors are extensively shared by twins, particularly by MZ twins, case-control studies can be particularly powerful. In such a study, each twin is interviewed with regard to specific environmental factors—such as occupation, lifestyle factors, illnesses or injuries, and diet—prior to the onset of the disease in the affected twin. The presence of these factors in the twin with the disease is compared to the twin without disease. An association of an environmental factor with the disease suggests this factor may be causally related. Factors more common among the unaffected twins suggest that the factor may protect against the development of the disease.

Environmental influences on PD have been investigated by studying discordant twin pairs. PD has repeatedly been found to be more common in people who do not smoke cigarettes. Some have proposed that some people are genetically predisposed to both Parkinson's disease and smoking, while others suggest cigarette smoking somehow prevents the degeneration that leads to PD. In two studies of discordant twin pairs, cigarette smoking was more common in the twin without Parkinson's disease, especially in the MZ pairs. Because monozygotic twins are genetically identical, this pattern tips the scales in favor of a direct biological action of cigarette smoke.

As medicine focuses more on early intervention or prevention, it becomes important to identify those persons at risk for a particular condition. This can be a problem if there is no diagnostic test. In discordant twin pairs, the unaffected twin is more likely to be "at risk" for a particular condition, whether due to shared genes or environment, than would be true for two nontwins. Therefore, studying the unaffected "at risk" twin may help to clarify what features are useful for predicting those who later will develop a particular disease. For example, in the PD twin study, the unaffected twins are being studied prospectively with brain imaging tests that may show early evidence of injury to the brain area damaged in PD. If abnormalities on this test are found to precede the development of PD, this could provide a useful method of early detection. When treatments to slow or stop the onset of PD are available, individuals with imaging abnormalities may receive intervention before symptoms develop.

Results from Twin Studies of Other Disorders and Conditions

The twin study method has been used to try to determine the extent of genetic or environmental influence on a wide variety of traits and conditions. Among these are sense of humor, which appears to be largely environmentally determined, as MZ and DZ pairs have similar concordance. Examples of other diseases in which MZ concordance exceeds DZ concordance, suggesting a significant genetic component, include addictive behaviors such as cigarette smoking and alcohol drinking, mental illnesses such as schizophrenia, as well as stroke and certain types of high blood pressure. Twin studies of many other disorders are ongoing.

Conclusion

Twin studies provide a unique approach to investigating the determinants of a disease or condition. A single twin study cannot absolutely determine the importance of genetic or environmental factors. However, the twin study method, in combination with other approaches, can be a powerful tool for unraveling the causes of disease.

see also Behavior; Fertilization; Gene and Environment; Gene Discovery; Inheritance Patterns.

Caroline M. Tanner

and Richard Robinson

Bibliography

Bouchard, T. J., et al. "The Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart." Science 250 (1990): 223-228.

Segal, Nancy L. Entwined Lives: Twins and What They Tell Us about Human Behavior. New York: Plume, 2000.

Wright, Lawrence. Twins: And What They Tell Us about Who We Are. New York: John Wiley & Sons, 1997.

Internet Resource

Minnesota Twin Family Study. University of Minnesota. <http://www.psych.umn.edu/psylabs/mtfs/default.htm>.

WHY STUDY TWINS?

• To estimate the relative contributions of genes and environment to the cause of disease by comparing MZ to DZ concordance;
• To investigate environmental determinants of etiology in discordant twin pair studies;
• To investigate environmental influences on disease course in concordant twin pair studies;
• To characterize "presymptomatic" or "at risk" states by studying the unaffected twins in discordant pairs.

Twins

Theme Overview

As two children born on the same day to the same mother, twins have a unique sense of identity. They have more in common with one another than any two ordinary people, especially if they are identical twins. Yet twins are also separate beings who may be very different in character. Myths about twins—as partners, rivals, opposites, or halves of a whole— are rooted in this basic mystery of sameness and difference. Twins appear in the myths and legends of many cultures, but they are especially important in African and Native American mythology. In some traditions, two children may be considered twins if they are born to two sisters at the same time.

Major Myths

The mythology of ancient Egypt includes examples of twinship operating in different ways. According to one version of the Egyptian creation myth, the earth god Geb and the sky goddess Nut (pronounced NOOT) were twins and also lovers, locked together in a tight embrace. The great god Ra separated them with air, leaving Nut arched across the heavens above Geb. Nut and Geb are complementary symbols, meaning that the two complete each other, forming a whole.

Similar myths from around the world associate twins with complementary features of the natural world, such as male and female, day and night, and sun and moon. The Xingu (pronounced shing-GOO) people of Brazil, for example, have stories about the twin brothers Kuat and Iae who forced the vulture king Urubutsin to give light to the dark world. Kuat occupied the sun, and Iae the moon. Their wakefulness keeps light in the world, except for a brief time each month when they both sleep and the world experiences nights without a visible moon (also known as a “new moon”).

Twins can also be rivals. Egyptian mythology explores this aspect of twin relationship in the stories about the gods Osiris (pronounced oh-SYE-ris) and Set , twin sons of Nut and Geb. Set was so determined to be born first that he tore his way out of his mother's womb before he was fully formed. He hated his brother Osiris and eventually killed him. In the mythology of ancient Persia, some accounts of Ahriman (pronounced AH-ri-muhn), the spirit of evil, say that he too was a twin who forced his way out of the womb so that he could be born first. Ahriman and his twin and enemy Ahura Mazda (pronounced ah-HOO-ruh MAHZ-duh), the spirit of good, are symbols of opposing moral forces that struggle for control of the universe.

Many myths of the Melanesian islands in the southwest Pacific Ocean tell of twin brothers who are rivals or enemies. Often, one twin is wise and the other foolish, as in the case of To Kabinana and To Karvuvu. The stupidity of To Karvuvu led to unpleasant or dangerous things. For example, he created the shark, thinking it would help him catch more fish. Instead, the shark ate the fish—and people. When To Karvuvu's mother shed her old, wrinkled skin and became young, he wept because he could not recognize her. To calm him she put on her old skin again. Ever since that time, people have had to grow old and die.

Twins often appear as partners or companions who share a bond deeper than ordinary friendship or even brotherly affection. This is illustrated in the myth of Castor and Pollux (pronounced PAHL-uhks). Some versions of their story say that although they were born to the same mother, they had different fathers. Pollux, son of the god Zeus (pronounced ZOOS), was able to live forever; Castor, son of a human, was not. When his beloved brother was killed, Pollux gave up half of his immortality to restore Castor to life. As a result, each twin could live forever, but they had to divide their time between Mount Olympus (pronounced oh-LIM-puhs) and the underworld , or land of the dead. The Greeks identified Castor and Pollux with a constellation, or star group, known as Gemini, the Twins.

Aborigines of Australia also associated this constellation with twins. According to a myth told in central Australia, twin lizards created trees, plants, and animals to fill the land. Their most heroic deed was to save a group of women from a moon spirit who wanted to mate with them. The women went into the sky as the cluster of stars widely known as the Pleiades (pronounced PLEE-uh-deez) while the lizard twins became Gemini.

Because the birth of twins is a rare occurrence, some cultures believed that certain gods and heroes were twins. In Greek mythology , notable sets of twins included the deities Apollo (pronounced uh-POL-oh) and Artemis (pronounced AHR-tuh-miss), and two remarkable sisters, Helen and Clytemnestra (pronounced klye-tem-NES-truh), who were also the sisters of Castor and Pollux. Some myths of community origins featured royal twins, or those born to gods. The Greeks said that Amphion (pronounced AM-fee-uhn) and Zethus (pronounced ZEE-thuhs), twin sons of Zeus, had founded the city of Thebes (pronounced THEEBZ), while the Romans claimed that the founders of their city were the twin brothers Romulus and Remus , sons of Mars.

Twins in African Mythology The theme of twins is common to the creation myths of some West African peoples. To the Dogon of Mali, twins represent completeness and perfection. The symbol of this wholeness is the deity Nummo, who is really a set of twins, male and female. The act of creating the other gods and the world required the sacrifice of one part of Nummo. From that time on, all beings were either male or female, lacking Nummo's divine completeness.

The supreme creator deity of the Fon people of Benin is Mawu-Lisa, a being both male and female who is sometimes described as a pair of twins. Mawu is the moon and the female element of the deity, while Lisa is the sun and the male part. They gave birth to all of the other gods, who also were born as pairs of twins.

Among the Yoruba people of Nigeria, twins are called ibejis after Ibeji, the protector deity of twins. People believe that, depending on how they are treated, twins can bring either fortune or misfortune to their families and communities. For this reason, twins receive special attention. One myth links the origin of twins with monkeys. According to this story, monkeys destroyed a farmer's crops, so he began killing all the monkeys he could find. When the farmer's wife became pregnant, the monkeys sent two spirits into her womb. They were born as the first human twins. To keep these children from dying, the farmer had to stop killing monkeys.

Twins in American Indian Mythology The role of twins in American Indian mythology is complex. Some pairs of twins combine heroism with the mischievous behavior of tricksters. Occasionally, twins represent opposing forces of good and evil. The Huron people of northeastern North America tell of Ioskeha and Tawiskara, twins who fought to decide who would rule the world. The evil Tawiskara, who fought his way out of the womb, used a twig as his weapon against his brother, while Ioskeha used the horn of a stag. Ioskeha, a positive creative force, won the conflict. In the same way, Gluskap (pronounced GLOOS-kahb), the creator god and hero of many northeastern myths, had to defeat Malsum, his evil twin who was the source of all harmful things and the ruler of demons. In Iroquois mythology, Good Mind helps his grandmother, the Woman Who Fell from the Sky, place useful and beautiful items on the earth. His twin, Warty One, creates unpleasant things, such as mosquitoes and thorny bushes.

Rather than enemies, twins in Native American mythology are often partners in a task or a quest. In myths from the Pacific Northwest, the twins Enumclaw (pronounced EE-nuhm-klaw) and Kapoonis sought to obtain power over fire and rock from the spirits. Their activities became so threatening that the sky god made them into spirits themselves. Enumclaw ruled lightning and Kapoonis controlled thunder.

Hunahpú (pronounced WAH-nuh-pwuh) and Xbalanqúe (pronounced shi-BAY-lan-kay), hero twins of Mayan mythology , descended into the underworld to restore their father to life. They then escaped from the lords of the underworld by outwitting them. Masewi (pronounced mah-SEH-wee) and Oyoyewi (pronounced oh-yo-YEH-wee), culture heroes in the myths of the Acoma Indians of the American Southwest, made a journey to their father, the sun. The theme of twins in search of their father also appears in the myth of Ariconte and Tamendonare of the Tupinamba people of Brazil. Setting out on a quest to learn their father's identity, these twin sons faced many dangerous trials. Each twin died once, only to be brought back to life by his brother. In the end, they learned that they had different fathers, one immortal and one mortal. Because the twins did not know which of them had the immortal father, they protected one another forever.

Navajo myths tell of Monster Slayer (Naayee'neizghani) and his twin brother, Child of Water (To bajish chini). Their father carried the sun across the sky and was too busy to pay attention to his sons. One day the twins went in search of him. After enduring a series of ordeals, they at last found their father, and he equipped them to roam the world fighting monsters.

Twins in Context

It is easy to see why twins would be considered special in many cultures. Currendy, about one in every fifty people around the world is a twin; only one in every five hundred people is an identical twin. For some cultures, the occurrence of twins is rare enough to be thought of as possibly supernatural in origin. At the same time, twins appear frequently enough for most people to be familiar with the notion, which makes twin births ideal for indicating the special qualities of characters in myths and legends. In the United States, the rate of twin births in the twenty-first century has been on the rise, with about one in every seventeen children being born a twin in 2001—most likely due to the use of fertility drugs, which can cause multiple eggs to be available for fertilization at the same time. Studies have also shown that diet may also affect the rate of twin births in a society, with mothers who consume more meat producing twins more often than those who do not eat meat. This—along with the risks associated with bringing two infants to term instead of one—may mean that twins were even more rare in ancient times, and therefore more special.

Twins in Art, Literature, and Everyday Life

As indicated in the previous myths, twins were common fixtures in the stories of cultures around the world. The twins most often found in ancient art are those from the Greek and Roman cultures, such as Castor and Pollux or Apollo and Artemis. In modern times, twins have also appeared in supernatural or mythical roles: one well-known example is Luke Skywalker and Princess Leia Organa from the Star Wars series of films by George Lucas.

Read, Write, Think, Discuss

In modern times, as in many myths, twins are often depicted as having opposite natures or personalities. However, stories of many real-life twins indicate that they share remarkable similarities in thought and behavior, and even consider themselves to have a supernatural bond with each other. Why do you think twins in myth are so often portrayed as opposites?

SEE ALSO Ahriman; Ahura Mazda; Castor and Pollux; Helen of Troy; Hunahpú and Xbalanqúe; Masewi and Oyoyewi; Osiris; Romulus and Remus; Set

Twins

As two children born on the same day to the same mother, twins have a unique sense of identity They have more in common with one another than any two ordinary people, especially if they are identical twins. Yet twins are also separate beings who may be very different in character. Myths about twins—as partners, rivals, opposites, or halves of a whole—are rooted in this basic mystery of sameness and difference. Twins appear in the myths and legends of many cultures, but they are especially important in African and Native American mythology. In some traditions, two children may be considered twins if they are born to two sisters at the same time.

Mythical Twins. The mythology of ancient Egypt includes examples of twinship operating in different ways. According to one version of the Egyptian creation myth, the earth god Geb and the

*See Names and Places at the end of this volume for further information.

sky goddess Nut were twins and also lovers, locked together in a tight embrace. The great god Ra* separated them with air, leaving Nut arched across the heavens above Geb. Nut and Geb are complementary symbols—meaning that the two complete each other, forming a whole.

Similar myths from around the world associate twins with complementary features of the natural world, such as male and female, day and night, and sun and moon. The Xingu people of Brazil, for example, have stories about the twin brothers Kuat and Iae, who forced the vulture king Urubutsin to give light to the dark world. Kuat occupied the sun, Iae the moon. Their wakefulness keeps light in the world except for a brief time each month when they both sleep and the world experiences dark nights.

Twins can also be rivals. Egyptian mythology explores this aspect of twinship in the stories about the gods Osiris* and Set, twin sons of Nut and Geb. Set was so determined to be born first that he tore his way out of his mother's womb before he was fully formed. He hated his brother Osiris and eventually killed him. In the mythology of ancient Persia*, some accounts of Ahriman, the spirit of evil, say that he too was a twin who forced his way out of the womb so that he could be born first. Ahriman and his twin and enemy Ahura Mazda, the spirit of good, are symbols of opposing moral forces in a dualistic universe.

Twins often appear as partners or companions who share a bond deeper than ordinary friendship or even brotherly affection. This aspect of twinship is illustrated in the myth of Castor and Pollux (called Polydeuces by the Greeks). Some versions of their story say that although they were born to the same mother, they had different fathers. Pollux, son of Zeus*, was immortal; Castor, son of a human, was not. When his beloved brother was killed, Pollux gave up half of his immortality to restore Castor to life. As a result, each twin could live forever, but they had to divide their time between Mount Olympus and the underworld. The Greeks identified Castor and Pollux with a constellation, or star group, known as Gemini, the Twins.

Aborigines of Australia also associated this constellation with twins. According to a myth told in central Australia, twin lizards created trees, plants, and animals to fill the land. Their most heroic deed was to save a group of women from a moon spirit who wanted to mate with them. The women went into the sky as the cluster of stars now called the Pleiades, while the lizard twins became Gemini.

dualistic consisting of two equal and opposing forces

immortal able to live forever

underworld land of the dead

Because twinship is a rare and special state, some cultures said that certain gods and heroes were twins. In Greek mythology, notable sets of twins included the deities Apollo* and Artemis* and two remarkable sisters, Helen and Clytemnestra, who were also the sisters of Castor and Pollux. Some myths of community origins featured royal or even semidivine twins. The Greeks said that Amphion and Zethus, twin sons of Zeus*, had founded the city of Thebes, while the Romans claimed that the founders of their city were the twin brothers Romulus and Remus, sons of Mars*.

Twinship in African Mythology. The idea of twinship is fundamental to the cosmologies and creation myths of some West African peoples. To the Dogon of Mali, twinship represents completeness and perfection. The symbol of this wholeness is the deity Nummo, who is really a set of twins, male and female. The act of creating the other gods and the world required the sacrifice of one part of Nummo. From that time on, all beings were either male or female, lacking Nummo's divine completeness.

The supreme creator deity of the Fon people of Benin is Mawu-Lisa, a being both male and female who is sometimes described as a pair of twins. Mawu is the moon and the female element of the deity, while Lisa is the sun and the male part. They gave birth to all of the other gods, who also were born as pairs of twins.

Among the Yoruba people of Nigeria, twins are called ibejis after Ibeji, the patron deity of twins. People believe that, depending on how they are treated, twins can bring either fortune or misfortune to their families and communities. For this reason, twins receive special attention. One myth links the origin of twins with monkeys. According to this story, monkeys destroyed a farmer's crops, so he began killing all the monkeys he could find. When the farmer's wife became pregnant, the monkeys sent two spirits into her womb. They were born as the first human twins. To keep these children from dying, the farmer had to stop killing monkeys.

Twinship in Native American Mythology. The role of twinship in Native American mythology is complex. Some pairs of twins combine heroism with the mischievous behavior of tricksters. Occasionally, twins represent opposing forces of good and evil. The Huron people of northeastern North America tell of Ioskeha and Tawiskara, twins who dueled to rule the world. The evil Tawiskara, who fought his way out of the womb, used a twig as his weapon against his brother, while Ioskeha used the horn of a stag. Ioskeha, a positive creative force, won the conflict. In the same way, Gluskap, the creator god and culture hero of many northeastern myths, had to defeat Malsum, his evil twin, who was the source of all harmful things and the ruler of demons. In Iroquois mythology, Good Mind helps his grandmother, the Woman Who Fell From the Sky, place useful and beautiful items on the earth. His twin, Warty One, creates unpleasant things, such as mosquitoes and thorny bushes.

Rather than enemies, twins in Native American mythology are often partners in a task or a quest. In myths from the Pacific Northwest, the twins Enumclaw and Kapoonis sought to obtain power over fire and rock from the spirits. Their activities became so threatening that the sky god made them into spirits themselves. Enumclaw ruled lightning, and Kapoonis controlled thunder.

The Stupid Twin

Many myths of the Melanesian islands in the southwest Pacific Ocean tell of twin brothers who are rivals or enemies. Often one twin is wise and the other foolish, as in the case of To Kabinana and To Karvuvu. The stupidity of To Karvuvu has led to unpleasant or dangerous things. For example, he created the shark, thinking it would help him catch more fish. Instead, the shark ate the fish—and people. When To Karvuvu's mother shed her old, wrinkled skin and became young, he wept because he could not recognize her. To calm him she put on her old skin again. Ever since that time, people have had to grow old and die.

cosmology set of ideas about the origin, history, and structure of the universe

patron special guardian, protector, or supporter

trickster mischievous figure appearing in various forms in the folktales and mythology of many different peoples

culture hero mythical figure who gives people the tools of civilization, such as language and fire

Hunahpú and Xbalanqúe, Hero Twins of Mayan mythology, descended into the underworld to restore their father to life. They then escaped from the lords of the underworld by outwitting them. Masewi and Oyoyewi, culture heroes in the myths of the

*See Names and Places at the end of this volume for further information.

Acoma Indians of the American Southwest, made a journey to their father, the sun. The theme of twins in search of their father also appears in the myth of Ariconte and Tamendonare of the Tupinamba people of Brazil. Setting out on a quest to learn their father's identity, these twin sons faced many dangerous trials. Each twin died once, only to be brought back to life by his brother. In the end, they learned that they had different fathers, one immortal and one mortal. Because the twins did not know which of them had the immortal father, they protected one another forever.

Navajo myths tell of Monster Slayer (Naayéé'neizghání) and his twin brother Child of Water (To bájísh chini). Their father carried the sun across the sky and was too busy to pay attention to his sons. One day the twins went in search of him. After enduring a series of ordeals, they at last found their father, and he equipped them to roam the world fighting monsters.

See also Ahriman;Ahura Mazda; Castor and Pollux; Clytemnestra; Helen of Troy; HunahpÚ and XbalanqÚe; Masewi and Oyoyewi; Nummo; Osiris; Romulus and Remus; Set.

Twins

Definition

Twins occur when two babies are born at the same birth.

Description

Identical, or monozygotic, twins are of the same sex and are genetically identical and physically similar, because they both come from one ovum (egg), which, after fertilization, divides in two and develops into two separate fetuses. Fraternal, or dizygotic, twins occur when the mother produces two eggs in one monthly cycle and both eggs are fertilized. The conceptions may take place on two separate occasions and could involve different fathers.

Fertilized egg division which produces twins can either happen early or late in development. In the case of early separation, the two fetuses either share an amniotic sac or each has a separate amniotic sac. If the fetuses share an amniotic sac, they also share a placenta. If the two fetuses have separate amniotic sacs, they can either share a placenta or have two separate placentas. Twins can also result from a fertilized egg that divides slightly later in development. In this case, the twins share an amniotic sac and a placenta. It is from these cases of late separation that conjoined (Siamese) twins sometimes develop.

Fraternal twins, who are no more genetically alike than ordinary siblings, may be of the same or different sex and may bear some similarity of appearance. Fraternal twinning appears to be passed on by the female members of a family . If the mother is a fraternal twin herself, has fraternal twin siblings, or fraternal twin relatives on her side of the family, she is more likely to give birth to fraternal twins. If she has already given birth to fraternal twins, her chances of giving birth to fraternal twins again are four times greater than those of a woman who has not had fraternal twins. In vitro fertilization increases a woman's chances for having multiple birth.

The number of twins born in the United States rose between the early 1980s and the early 2000s. In 1980, there were 69,339 sets of twins born, and in 2002 there were 125,134 sets of twins born in the United States. According to data gathered by the Centers for Disease Control (CDC), there is considerable variation among the states in number and rate of twin births. In 1994, for example, the twin birth rate ranged from 19.8 per 1,000 live births in Idaho and New Mexico to 27.7 per thousand in Connecticut and Massachusetts. One factor that may influence the distribution of multiple births is whether the state provides insurance coverage for procedures such as in vitro fertilization (IVF) and other treatments to improve fertility. These procedures increase the chance of multiple births.

Ethnicity is another factor that may correlate to the twin birth rate. For 1994, the twin birth rate among non-Hispanic white mothers was 24.3 per 1000 live births; among non-Hispanic black mothers, 28.3 per 1000; and among Hispanic mothers, 18.6 per 1000. There are also significant differences internationally in the number of twins born with the rate in Belgium almost six times the rate in China.

The CDC also studies whether maternal age has any correlation with the rate of twin births. The data seem to suggest that mothers in states with rates of twin births higher than the overall rate for the United States are older on average, and mothers in states with rates of twin births lower than the overall rate for the United States are younger. Again, as in vitro fertilization is more widely done, the incidence of multiple births will increase.

Infancy

Parents should avoid giving twins very similar names. Twins should be treated as two individuals and not as a package. They may need to be fed at different times and may develop skills at different rates. It is important to spend time with each twin separately so that they become used to being separated from each other for short times and know that they are each valued as individuals.

Toddlerhood

To help twins understand who they are as individuals, parents should avoid dressing both twins the same. It is preferable that each child receive toys that are geared towards their individual interests rather than each receiving the same toy.

School age

Sibling rivalry can be more intense in twins than in siblings of different ages. This is not unusual, because teachers, coaches, and even parents tend to compare twins. All children compare themselves to their siblings, and having others do this regularly can add to the pressure and stress of being a twin. Parents should consider arranging to have the twins put in different classes in school to help foster individuality. Each twin will probably have different skills, interests, and friends, and they should be encouraged to peruse activities separately if their interests diverge. Helping teachers, coaches, babysitters , and friends understand that it is important to treat the twins as two separate people can be very important. Friends should be encouraged to give separate gifts for birthdays and holidays, taking each child's special interests and talents into account.

Common problems

Twins often have a harder time developing their own independent identities than other children. Twins are more likely to have low birth weights or be delivered prematurely than single babies.

Parental concerns

Raising twins can be more challenging than raising two single children. The children may need to eat, sleep , and be changed at different times when they are infants. It can also be more expensive, because things like car seats and cribs must be purchased at the same time instead of reused for the second child. Some stores have special discounts for parents of twins.

When to call the doctor

Parents should call the doctor if one or both of their children seems ill, just as they would for any other child or children.

Resources

BOOKS

Noble, Elizabeth, with Leo Sorger. Having Twins and More: A Parent's Guide to Multiple Pregnancy, Birth, and Early Childhood, 3 ed. Boston, MA: Houghton Mifflin, 2003.

Pearlman, Eileen M., and Jill A. Ganon. Raising Twins: What Parents Want to Know, and What Twins Want to Tell Them. New York: Harper Resource, 2000.

PERIODICALS

Brown, Judith E., and Marcia Carlson. "Nutrition and Multifetal Pregnancy." Journal of the American Dietetic Association 100 (March 2000): 343.

ORGANIZATIONS

National Organization of Mothers of Twins Clubs. PO Box 438, Thompsons Station, TN 37179–0438. Web site: <www.nomotc.org>.

Tish Davidson, A.M.

KEY TERMS

Dizygotic —From two zygotes, as in non-identical, or fraternal twins. The zygote is the first cell formed by the union of sperm and egg.

Monozygotic —From one zygote, as in identical twins. The zygote is the first cell formed by the union of sperm and egg.

Placenta —The organ that provides oxygen and nutrition from the mother to the unborn baby during pregnancy. The placenta is attached to the wall of the uterus and leads to the unborn baby via the umbilical cord.

Twins

Two children or animals born at the same birth.

Identical, or monozygotic, twins are of the same sex and are genetically and physically similar because they both come from one ovum, which, after fertilization, divides in two and develops into two separate individuals. Fraternal, or dizygotic, twins occur when the mother produces two eggs in one monthly cycle and both eggs are fertilized. The conceptions may take place on two separate occasions and could involve different fathers. Fraternal twins, who are no more genetically alike than ordinary siblings, may be of the same or different sex and may bear some similarity of appearance. Twin pregnancies occur on the average in one out of every 80 to 100 births. However, the incidence of twins reflects the number of twin babies born per thousand completed pregnancies, and it is a fact that many more twins are conceived than are born.

The causes of identical twinning are not fully understood. Factors affecting the frequency of twin and other multiple births include the mother's race and age, and the number of previous births. The rate of twin births in Japan is 0.7 percent, while the Yoruba of Nigeria have a rate as high as 4 percent. Dizygotic twinning appears to be a sex-linked genetic trait passed on by female relatives in the same family . The chances of having fraternal twins are increased about five times if a woman is a fraternal twin, has fraternal twin siblings or fraternal twin relatives on her side of the family, or has already given birth to fraternal twins (one in twenty chance). While the rate of identical twin births is stable for all ages of childbearing women, the chance of any mother bearing fraternal twins increases from the age of 15 to 39 and then drops after age 40. For women of all ages, the more children they have had previously, the more likely they are to bear twins. Since the 1960s, fertility drugs have also been linked to the chances of producing twins. The majority of research indicates that fathers' genes have little effect on the chances of producing twins.

There are four types of monozygotic twins, determined by the manner in which the fertilized egg, or zygote, divides and the stage at which this occurs. Two independent embryonic structures may be produced immediately at division, or the zygote may form two inner cell masses, with each developing into an embryo. A late or incomplete division may produce conjoined, or Siamese twins. As the zygote develops, it is encased in membranes, the inner of which is called the amnion, and the outer one the chorion. Among monozygotic twins, either or both of these membranes may be either separate or shared, as may the placenta. Together, the arrangement of these membranes and the placenta occurs in four possible permutations. Among dizygotic twins, each one has separate amnion and chorion membranes, although the placenta may be shared. Ascertaining zygosity, or the genetic make up of twins, can be done by analyzing the placenta(s) to determine if it is a single placenta with a single membrane or a double placenta, which account for one-third of identical twins and all fraternal twins. In the case of same-sex twins with two placentas, a DNA or blood test can determine whether they share the same genes or blood groups.

The scientific study of twins, pioneered by Sir Francis Galton in 1876, is one effective means of determining genetic influences on human behavior. The most widely used method of comparison is comparing monozygotic and dizygotic twins for concordance and discordance of traits . Concordant traits are those possessed by either both or neither of a pair of twins; discordant traits are possessed by only one of the pair. Monozygotic twins who are discordant for a particular trait can be compared with each other with reference to

other traits. This type of study has provided valuable information on the causes of schizophrenia .

Another common type of twin research compares monozygotic twins reared together with those reared apart, providing valuable information about the role of environment in determining behavior. In general, monozygotic twins reared apart are found to bear more similarities to each other than to their respective adoptive parents or siblings. This finding demonstrates the interaction between the effects of environment and genetic predispositions on an individual's psychological development.

See also Nature-Nurture Controversy.