Participants were 3,762 twins (1,808 male and 1,954 female) from the Minnesota Twin Family Study (MTFS). The MTFS is a longitudinal-epidemiological study examining the development of substance use and related disorders. The current investigation used data from the initial and enriched samples of the MTFS. Detailed information on the recruitment and design of these samples are described elsewhere.21,22 Briefly, the initial sample consists of two parallel cohorts of same-sex twin pairs reared by their biological parents, one first assessed at age 11 (Cohort 1) and one at age 17 (Cohort 2). The enrichment sample consists of twins at risk for the development of substance use problems (Cohort 3). All twins were identified using public birth records for the years 1977–1984 (initial cohorts) and 1988–1994 (enriched cohort) and located using public databases. Over 90% of twins born in these periods were located and over 80% of eligible families agreed to participate. Parents were representative of the Minnesota population of the target birth years in education, socioeconomic status, and history of mental health treatment. Although the samples lack racial and ethnic diversity
