Twins were interviewed separately and concurrently by different interviewers, who assessed them for psychiatric disorders according to criteria from the DSM-III-R.27 The current analysis uses symptom counts for 11 disorders, including various childhood disruptive disorders, internalizing disorders, and externalizing disorders. Symptoms were assessed by trained staff using structured clinical interviews for DSM-III-R.26,28,29 Interview data were subsequently reviewed in a case conference where diagnosticians had to reach a consensus regarding the presence or absence of a symptom before it was assigned, referring to audiotapes of the interview when necessary. The consensus process yielded uniformly high diagnostic reliabilities (e.g., .92 or greater for all substance use disorder diagnoses21). To simplify our results and represent psychopathology across a span of development, symptoms scores across all ages were averaged into a single score for each disorder.
