Following the assessment, each interview is reviewed by two individuals with advanced clinical training (supervised by a Ph.D. clinical psychologist) blind to other family diagnoses. Symptom presence is determined through review of severity and frequency criteria for each behavior followed by consensus regarding whether it constitutes a symptom. A symptom is considered present if either the primary caretaker or twin reported it as present, using a best-estimate procedure (Leckman et al., 1982) that we have found results in greater validity than relying on either report alone (Burt et al., 2001). To maximize sensitivity in our population-based sample of adolescents, diagnoses in the twins are made at two levels: definite (all diagnostic criteria satisfied) and probable (all but one diagnostic criteria satisfied). This strategy avoids problems with underreporting of lifetime symptoms in population-based samples (Iacono et al., 1999), and is essential for detecting emerging substance use disorders during adolescence (Martin & Winters, 1998; Pollock & Martin, 1999). Further, it does not appear to inflate our rates of ADHD and CD (Elkins et al., 2007). For example, our DSM-IV lifetime prevalence of
