Male and female (Heath et al., 1999; Heath et al., 2002) same-sex adolescent twins were ascertained from Missouri state birth records for three population-based studies: a study of female like-sex twin pairs born 1975–1986 (ages 12–23 at the time of first interview, mean age = 15.5): “MOAFTS”; and two coordinated studies of male like-sex male twin pairs born 1977–1992 (aged 11–23 at the time of interview; mean age = 15.5): “BOYNIC” and the “MARC”/Project 1. Once families were identified from birth records and traced, a parent and both twins were invited to complete a telephone diagnostic interview. Participants (N=3446 female twins and N=2748 male twins) were interviewed by telephone using astructured diagnostic assessment for DSM-IV (American Psychiatric Association, 1994) nicotine dependence, alcohol dependence, major depression, childhood conduct disorder, social anxiety, and a detailed history of consumption of alcohol and cigarettes. The interview was a modified version of the Semi-Structured Assessment on the Genetics of Alcoholism (Bucholz et al., 1994) and the smoking section was modified from the Composite International Diagnostic Interview (Cottler et al., 1991). Analyses reported here included twins
