For participants in each of the two cohorts, lifetime presence of DSM-III-R (American Psychiatric Association, 1987) disorders, the diagnostic system in place at the time the MTFS was initiated, was assessed via in-person, structured interviews by trained clinical interviewers. Members of each twin pair were assessed concurrently by separate interviewers. Symptoms of nicotine dependence, alcohol dependence, and illicit drug dependence (which included amphetamines, cannabis, cocaine, inhalants, opioids, PCP, and sedatives) were assessed using an expanded version of the Substance Abuse Module of the Composite International Diagnostic Interview (Robins, Babor, & Cottler, 1987). The substance for which the participant reported the maximal number of symptoms was used for their drug dependence variable. An interview designed by MTFS staff (Holdcraft, Iacono, & McGue, 1998) was used to assess symptoms of conduct disorder and adult antisocial behavior (i.e., the adult criteria for Antisocial Personality Disorder). Mothers reported on the presence of conduct disorder and substance use disorder symptoms in her twin children through interviews using the parent version of the Diagnostic Interview for Children and Adolescents (Reich, 2000). A symptom was considered present
