Twin pairs for this volunteer sample were born 1964–1971, were identified in 1980–1982 through mass media appeals and school systems, were raised together, and included a broad spectrum of sociodemographic groups (34). As described elsewhere (3,34), twins were not assessed as children, but completed a mailed questionnaire in 1989 and a telephone diagnostic interview between 1996 and 2000. Of the 8020 twins (4010 pairs) identified in childhood, 6257 individuals (78.0%) were interviewed as young adults (2761 complete pairs: 698 MZ female, 494 MZ male, 513 DZ female, 395 DZ male, and 661 DZ unlike-sexed pairs; and 735 twin singletons, N=371 female). Mean age at interview was 30.0 years for women (N=3454) and 29.9 years for men (N=2803).
