This study used data from the third wave of interviews of adult male–male twin pairs from the Virginia Adult Twin Study of Psychiatric and Substance Use Disorders (Kendler & Prescott, 2006), and originally contained male–male and male–female twin pairs born between 1940 and 1974. Initially, 6814 twins participated in the first wave of interviews (MM1) (participation rate=72.4%). An 82.6% follow-up response rate was obtained for the wave 2 interviews (MM2). The third interview wave (MM3) (1998–2004) was completed solely with members of male–male twin pairs, with interviews completed by 1796 twins, representing 75.1% of the entire cohort and 77.8% of those eligible (excluding those who died and were lost to follow-up). Subjects were aged 24–62 years (mean age=40.3 years, S.D.=9.0). The sample for the present analysis consisted of 469 monozygotic (MZ) and 287 dizygotic (DZ) twin pairs (including two triplet sets subdivided into all possible pairings) and 290 twins whose co-twin did not participate.
