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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

These identical twins in San Francisco make an effort to act and dress alike. (Alison Wright. Photo Researchers, Inc. Reproduced with permission.)

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.

Additional topics

Psychology EncyclopediaChild Development