Our measures of externalizing disorders included DSM-III-R43 symptoms of adult antisocial behavior and alcohol, nicotine, and illicit drug dependence. Adult antisocial behavior (i.e., the adult criteria for antisocial personality disorder) was assessed using a modified version of the Structured Clinical Interview for Axis II Personality Disorders44. Substance use disorders were assessed using the Substance Abuse Module of the Composite International Diagnostic Interview45. The drug assessment included amphetamines, cannabis, cocaine, hallucinogens, inhalants, opioids, PCP, and sedatives. The drug class for which the participant endorsed the most symptoms was used as their number of drug dependence symptoms. Mothers also reported on the presence of symptoms of all substance use disorders in their twin offspring using the parent version of the Diagnostic Interview for Children and Adolescents-Revised46. A symptom was considered present if reported by either the twin or mother. Symptoms of abuse were included in the alcohol and drug symptom count variables. All interview data were reviewed in a case conference consisting of at least two advanced graduate students in clinical psychology. Consensus between the diagnosticians regarding the presence or absence of
