Twin Studies

Twin StudiesTwin studies can tell us about how genes and environments affect behavioral and physical development. There are two kinds of twins: identical and fraternal. Identical twins result when one fertilized egg splits during the first two weeks of pregnancy. These twins share all their genes and are always of the same sex. They occur in about one third of natural twin conceptions. Fraternal or nonidentical twins result when two eggs released by the mother are fertilized by two sperm from the father. These twins share half their genes, on average, just like ordinary siblings. Fraternal twins can be same-sex or opposite-sex.

The classic twin design involves comparing the similarity of identical and fraternal twins. If identical twins are more alike in intelligence, personality, or physical skills this demonstrates that the trait is probably influenced by genetic factors. Some people have objected that identical twins are alike because people treat them alike, not because of their shared genes. However, careful studies have ruled out this criticism after finding that identical twins who are treated alike are not more similar than identical twins who are treated differently.

There are many ways to study twins. A powerful method is studying identical twins reared apart from birth. Reared-apart identical twins resemble one another only because of their shared genes. Interestingly, research shows that identical twins reared apart and together are about equally similar in personality traits such as aggression and traditionalism. The twin-family method includes identical twins, their spouses, and their children. The children of identical twins are cousins, but they are also “half-siblings” because they have a genetically identical parent. These children’s aunts and uncles are like their “mothers” and “fathers” because they are genetically identical to the children’s own parents. It is possible to compare the behavioral similarity of a twin mother and her daughter (who share genes and environments) and a twin aunt and her niece (who share genes but not environments). Research has shown that parent-child and aunt/uncle-niece/nephew similarity is the same on a spatial visualization test. A more recent research design uses a unique twin-like pair called virtual twins. Virtual twins are same-age individuals who are raised together, but are not genetically related. Virtual twins show modest similarity in intelligence, despite their shared environment, a finding that supports genetic influence.

The multiple birth rate (especially the fraternal twinning rate) has increased from 19.3 to 30.7 multiple births per 1,000 births in recent years. This is primarily due to new reproductive technologies but also to the fact that women are having children at older ages. The increased twinning rate is good news for researchers. However, the downside is that twins are more likely than non-twins to suffer from birth difficulties.

It is likely that twins will continue to play significant roles in psychological and medical research.

Identical twins differing in traits, such as novelty-seeking, schizophrenia, or breast cancer may help identify which genes are expressed and which genes are not expressed. Thus, twin studies can help clarify the origins of behavior in everyone else.


  1. Machin, G. A., & Keith, L. G. (1999). An atlas of multiple pregnancy: Biology and pathology. New York: Parthenon.
  2. Segal, N. L. (2000). Entwined lives: Twins and what they tell us about human behavior. New York: Plume.
  3. Segal, N. L. (2005). Indivisible by two: Lives of extraordinary twins. Cambridge, MA: Harvard University Press.

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