Twins were administered the Diagnostic Interview Schedule for Children—IV (DISC–IV; Shaffer, Fisher, & Lucas, 1997), a structured, face-to-face psychiatric interview that assesses Diagnostic and Statistical Manual of Mental Disorders (4th ed. [DSM–IV]; American Psychiatric Association, 1994) Axis I disorders. A different field tester interviewed each member of the twin pair whenever possible. Rare exceptions to this procedure were necessary due to the costs of out-of-state travel, in cases where families had relocated outside Colorado during the time between Wave 1 and Wave 2. We used data from the DISC–IV to generate lifetime symptom counts for conduct disorder and ADHD (combining symptoms from the attention and hyperactive/impulsive subsections). Using the instructions provided by the instrument’s authors, computer algorithms were developed to determine the presence or absence of each symptom for each disorder. The outcome measure used in the current analysis was a count of the DSM–IV criteria met, based on the DISC–IV responses. Because questionnaire data had been collected on the LTS sample since early childhood, longitudinal parent- and teacher-reported behavior problems were also utilized. Composite scores were developed by combining
