THE CLASSICAL twin method has played a major role in influencing the direction of psychiatric thought on the relative importance of heredity and environment in the pathogenesis of schizophrenia. Fundamentally, this method is a comparison of a group of monozygotic or "identical" twins with a group of dizygotic or "fraternal" twins. The percentage of pairs where both twins are affected by a particular disease is compared in the two groups. This percentage is called the pairwise concordance rate. Since monozygotic twins are assumed to be genetically identical, while dizygotic twins no more so than nontwin siblings, comparisons of concordance rates between these two groups have been used to estimate the genetic contribution to a given illness.

In the past, studies showing a high concordance rate for schizophrenia in monozygotic twins and a much lower one in dizygotic twins have been accepted as