Fascination with twins and what twin studies tell us about human nature is universal. There is no question that behavioral and medical science advances have happened largely because twins yield a wealth of information just by being themselves.

Twin research takes place at two levels: (1) Studies that are strictly for twins. Such studies examine the pros and cons of twins being separated at school, dressing alike, or sharing their friends. (2) Studies with broad implications. Psychological analyses and medical research are conducted to understand the genetic and environmental factors affecting behavior and health which can be applied to the general nontwin population.

There are two main types of twins: Monozygotic (MZ, identical) and Dizygotic (DZ, fraternal). MZ twins share all their genes having split from a single fertilized egg within the first two weeks following conception (there are exceptions). DZ twins share half their genes, on average, having formed from the separate fertilization of two eggs released simultaneously. There are, however, variations within each type. For example, there are MZ twins who show mirror-image reversals and DZ twins with different fathers.

The classic twin study is simple and elegant. Researchers compare the similarities and differences of identical twins to those of fraternal twins. The greater resemblance between identical twins than between fraternal twins is consistent with the view that genetic factors influence the........

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