Background: We used variable threshold models which accounted for age and gender differences to investigate the genetic and environmental influences on DSM-IV conduct disorder (CD) at the level of symptoms, aggressive versus non-aggressive domains, and full-scale. Method: A community sample of 1100 twin pairs (age 11-18) was interviewed using the Diagnostic Interview Schedule for Children. Results: Behavior genetic model fitting suggested that genetic and environmental influences on individual symptoms varied by symptom. The best-fitting models for aggressive and non-aggressive domains, and full-scale CD included additive genetic effects and unique environmental effects only (AE models). These effects could be constrained across age cohorts and sex. The results suggest that using models that incorporate age- and gender-appropriate thresholds specific to each subject we can account for prevalence differences between cohorts. Heritability estimates were 0.49, 0.55 and 0.53 for the aggressive domain, non-aggressive domain, and full-scales, respectively. These results are in contrast to previous research on antisocial behavior measured with the CBCL reporting higher heritability for aggressive versus non-aggressive domains. Conclusions: Results suggest that individual symptoms of CD may be differentially heritable. Additionally, CD assessed using DSM-IV criteria may show differing patterns of heritability compared with estimates obtained for other measures of antisocial behavior such as the CBCL.