Externalizing disorders were assessed via structured interviews administered by trained staff. DSM-III-R criteria were used in all analyses as it was assessed at intake and all follow-up assessments. Symptoms of alcohol, nicotine, and illicit drug abuse and dependence were assessed using the Substance Abuse Module of the World Health Organization's Composite International Diagnostic Interview27. Conduct disorder and adult antisocial behavior (the adult criteria for antisocial personality disorder) were assessed using a structured interview comparable to the Structured Clinical Interview for DSM-III-R Axis II. Rates of lifetime diagnoses for the full sample were: 21.3% alcohol dependence and 11.6% drug dependence, 31.0% nicotine dependence, 10.8% conduct disorder, and 7.5% adult antisocial behavior. Externalizing disorders exhibit age and sex effects and even secular trends for drug use that might affect comparisons of familial similarity28,29. Therefore, all symptom counts were regressed on age, age2, sex, the interactions between the age and sex variables, and whether the participant was a member of the parent or offspring generation. The mean correlation among the symptom count variables was 0.42 (range 0.30 to 0.55).
