Why twin studies?
Twins potentially provide us with one of the most powerful techniques available for separating the influences of genetic and environmental factors on psychological and behavioural traits. An identical (monozygotic) twin has exactly the same genes as his cotwin. Fraternal (dizygotic) twins, on the other hand, are no more closely related genetically than are normal siblings, i.e. roughly 50% of their genes are the same. If index cases for a particular trait (e.g. homosexuality) are selected from twins in a given population, then the rate of concordance for that trait between the index case and his or her cotwin can be compared for monozygotic and dizygotic twins. It is assumed that the environment in which each member of a twin pair is reared is virtually identical, as twins are identical in age, and tend to have very similar experiences during childhood (e.g. they get similar attention from their parents, attend the same schools, etc.). Therefore, any difference in the concordance rates
