In addition to the structured interviews, ratings were obtained from up to 3 teachers on a 30-item scale of overall externalizing behavior (e.g., items similar to the criteria for oppositional defiant disorder and conduct disorder; 76.3% of the sample had ratings from at least one teacher and 61.3% had ratings from at least two teachers). The internal consistency reliability for the teacher rating was .92; the inter-rater reliability (intraclass correlation) for the mean of 2 raters was .71. Minnesota schools have a policy of placing twins in separate classrooms whenever possible, thereby minimizing the likelihood that members of a twin pair would be rated by the same teacher. All statistical analyses utilized an EXT composite variable that was calculated by taking the mean z-score of the symptom counts of adult antisocial behavior, nicotine dependence, alcohol abuse/dependence, drug abuse/dependence, and the teacher rating of externalizing behavior.
