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.

twins are of two types: monovular (identical), from the union of one sperm and one ovum, and binovular (non-identical) resulting from the fertilization of two separate ova. The cell produced by fertilization is called a zygote (from the Greek for ‘yoked’), so they are also known as monozygotic and dizygotic. Dizygotic twins are physically and genetically as dissimilar as any siblings. Monozygotic twins, having resulted from the cleavage of a single ‘conceptus’ — the splitting and separation of an early embryo — are therefore, with rare exceptions, genetically identical.

The incidence of multiple pregnancies varies in different racial groups. To quote ‘Hellin's law’ (1895): ‘twins occur in 1/89 births, triplets 1/(89)², quadruplets 1/(89)³ and so on’. The formula is roughly correct, although twins occur in Caucasians 1/80 to 1/90, in Asiatics 1/150 or less, and black Africans 1/50 with the highest incidence of twinning amongst the Yoruba people of Nigeria for whom 1 in 25 births are of twins. It is the rate of non-identical (dizygotic) twinning that varies around the world: identical (monozygotic) twins occur at a similar rate of 1 in 300 births in all populations. These statistics are based on clinical findings in viable pregnancies. However the initial ‘hidden’ twinning rate is probably higher: with increasing use of ultrasound in early pregnancy it is found that before 12 weeks one of the twins may die and be absorbed leaving an apparent singleton. In Australia the rate of twinning has increased approximately 25% over the past 20 years, partly due to a significant increase in the percentage of births to women aged 35 and over, and partly to the treatment of infertility by ovulation stimulation or assisted conception by gamete intra-fallopian transfer (GIFT) or in-vitro fertilization (IVF).

Twin pregnancy is more prone to complication than single pregnancies and possible hazards of premature birth and poor growth in the womb necessitate increased antenatal surveillance. If twins are identical and they share a single placenta, one baby can steal blood from the other, causing a condition known as ‘Twin– twin transfusion syndrome’.

Multiple pregnancies carry a greater risk of losing a baby before, during, or after birth than singleton pregnancies: multiple pregnancies overall account for more than 10% of all perinatal deaths; the greater the number, the greater the risk. Cerebral palsy in survivors is six times more common in twins than singletons.

The birth of twins has been a source of fascination in many cultures throughout history and the twin image has been incorporated in myths, folklore, and religions. The Old Testament of the Bible tells of Isaac's wife, Rebekah, who eventually conceived after nineteen years of marriage. Twin boys were born. The first was red and hairy, and he was named Esau, meaning ‘red’. His brother was born holding Esau's heel and so he was called Jacob, meaning ‘he who grasps the heel’. Ancient Rome was founded, according to legend by Romulus and Remus, the twin progeny of Mars, god of war, and a mortal princess. In some African communities, twins were regarded with great favour; in others, with great suspicion. The Yoruba in Nigeria were well aware of the high mortality associated with twinning in the past, and they made small wooden sculptures, ‘ibeji’ that had spiritual significance if one of twins died.

Jim Neilson

661. Twins (See also Doubles.)

1. Alcmena’s sons born in single delivery but conceived by two men. [Rom. Lit.: Amphitryon ]
2. Antipholus identically named sons of Aegeon and Emilia. [Br. Lit.: Comedy of Errors ]
3. Apollo and Artemis twin brother and sister; children of Leta and Zeus. [Gk. Myth.: NCE, 125–126]
4. Bobbsey Twins two sets of twins share adventures. [Children’s Lit.: Bobbsey Twins’ Mystery at Meadowbrook ]
5. Castor and Pollux sons of Leda and Zeus, placed in heaven as constellation Gemini. [Gk. Myth.: Zimmerman, 52]
6. Comedy of Errors based on Plautus’s Menaechmi, with two sets of twins. [Br. Lit.: Comedy of Errors ]
7. de Franchi, Lucien and Louis one twin instinctively feels what happens to other. [Fr. Lit.: The Corsican Brothers ]
8. Dioscuri (Castor and Pollux) Spartan brothers. [Gk. Myth.: Avery, 408; Leach, 314]
9. Donny, the Misses twin principals of Greenleaf boarding school. [Br. Lit.: Bleak House ]
10. Dromio Dromio of Ephesus; Dromio of Syracuse. [Br. Lit.: Comedy of Errors ]
11. Gemini (Castor and Pollux) zodiacal twins; [Gk. Myth.: NCE, 1056]
12. Katzenjammer Kids early comic strip featured incorrigible twins. [Comics: “The Captain and the Kids” in Horn, 421]
13. Man in the Iron Mask Bastille prisoner learns that he is the twin brother of Louis XIV; conspirators planned to substitute him for the king. [Fr. Lit.: Dumas Vicomte de Bragellonne in Magill I, 1063]
14. Menaechmi comedy, by Plautus, about mistakes involving identical twins. [Rom. Lit.: Menaechmi ]
15. Mike and Ike short lookalike twins with derbies. [Comics: Horn, 492]
16. Perez and Zerah born to Tamar; conceived by father-in-law, Judah. [O.T.: Genesis 38:29–30]
17. Romulus and Remus suckled by she-wolf; founded Rome. [Rom. Myth.: Wheeler, 320]
18. Siamese twins Eng and Chang (1814–74), the original pair, were connected at the chest. [Medical Hist.: Brewer Dictionary, 828]
19. Tweedledum and Tweedledee identical characters in children’s fantasy. [Br. Lit.: Through the Looking-Glass ]
20. two circles symbol of twins; in particular, Castor and Pollux [Gk. Myth.: Jobes, 343]

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 (twinz) pl. n. two individuals who are born at the same time and of the same parents. dizygotic (or fraternal) t. twins resulting from the simultaneous fertilization of two egg cells; they may be of different sexes and are no more alike than ordinary siblings. monozygotic (or identical) t. twins resulting from the fertilization of a single egg cell that subsequently divides to give two separate fetuses. They are of the same sex and otherwise genetically identical. See also conjoined twins.

twins Two individuals born to the same mother at the same time. Twins can develop from the same egg (see identical twins) or from two separately fertilized eggs (see fraternal twins).

twins see multiple birth .