Parents completed the Achenbach Child Behavior Checklist (CBCL; Achenbach & Rescorla, 2001) separately for each twin, while the twins completed the Semistructured Clinical Interview for Children and Adolescents (SCICA; McConaughy& Achenbach, 2001), the corresponding interview for youth aged 6–18 years. Twins were interviewed in separate rooms by different interviewers. In the current study, we made use of the Diagnostic and Statistical Manual of Mental Disorders (DSM)-oriented Conduct Problems (CP) scale (Achenbach & Rescorla, 2001; McConaughy & Achenbach, 2001), which comprises 17 CBCL items and 19 SCICA items (with nearly identical item content) viewed as ‘very consistent’ with the DSM-IV diagnostic category of Conduct Disorder (e.g. stealing, fighting, setting fires, cruelty to animals, etc.). Further validation work (Achenbach et al. 2001) indicated that the DSM-oriented CP scale accurately captures conduct-disordered behavior and DSM-IV diagnoses. Internal consistency reliabilities for the CBCL scales were adequate (α = 0.80 and 0.76 for mother- and father-informant reports, respectively). Roughly 10% of SCICA interviews were videotaped to obtain inter-rater reliability (the average intraclass correlation across raters was 0.88).
