When assessing twin concordance, use the probandwise not the pairwise rate.

Geneticists and twin researchers have long debated the relative merits of two alternative measures of twin concordance: the pairwise and probandwise concordance rates. The results of this debate are now quite clear, for almost every application the probandwise rate is preferred over the pairwise rate. In a recent review of schizophrenia twin studies, however, Torrey (1992) chose to analyze pairwise rather than probandwise rates. Torrey's use of pairwise rates led him to conclude that the monozygotic twin concordance for schizophrenia is weaker than what is widely accepted, and that, by implication, the magnitude of the genetic contribution to schizophrenia has been overestimated. In this brief commentary, we review the relative strengths and weaknesses of the pairwise and probandwise rates and show that Torrey's conclusion is based upon his incorrect use of pairwise rates. Twin studies of schizophrenia continue to support the existence of a strong genetic influence on the development of schizophrenia.