Trained clinical interviewers administered structured in-person interviews with the twins and their parents independently. Members of each twin-pair were interviewed concurrently by separate interviewers. Lifetime presence of DSM-III-R substance abuse and conduct disorders was assessed via a revised version of the Diagnostic Interview for Children and Adolescents (DICA; (Reich, 2000)) and an expanded version of the Substance Abuse Module from the Composite International Diagnostic Interview (Robins, Babor, & Cottler, 1987). Mothers reported on their twin sons through interviews using the parent version of the DICA (Reich, 2000). A DSM-III-R diagnosis was assigned on the basis of a consensus, “best-estimate” approach (Leckman, Sholomskas, Thompson, Belanger, & Weissman, 1982) combining mother and son interview data. A lifetime study diagnosis was given if either all DSM-III-R symptom criteria were met (definite certainty level) or all criteria but one were met (probable certainty level). Because a single symptom is sufficient for a diagnosis of substance abuse, all substance abuse cases were of definite certainty.
