Participants were male-male twin pairs from the Virginia Adult Twin Study of Psychiatric and Substance Use Disorders.14 These pairs came from a sample (birth years 1940-1974) ascertained from registry records containing all twin births. Zygosity was determined using standard twin questions validated against DNA genotyping.15 The first interview was completed largely by telephone in 1993 to 1996 with a 72% response rate. The second wave of interviews, conducted in 1994 to 1998, was largely done face to face, yielding a response rate of 83%. Self-report questionnaires (SRQs) were obtained from 94% of individuals who completed the second interview (ie, 643 complete monozygotic and 430 complete dizygotic pairs plus 623 single twins). The mean (SD) age of male-male pairs at the time of SRQ completion was 37.2 (9.2) years.
