Results of the multivariate analyses showed that, in males and females, both additive genetic effects and shared environmental effects contribute to the variance in adolescent conduct problems and tobacco, alcohol and cannabis use in early adulthood. Because the multivariate models include more information, the estimates obtained from the multivariate analyses should be regarded as superior to the univariate estimates. The heritability estimates for conduct problems based on the multivariate models 57% in males, 41% in females) are roughly similar to previously reported estimates in partly overlapping samples [17, 20, 30] and in accordance with estimates from previous research [9–11]. Heritability estimates for tobacco (60% in males, 37% in females), alcohol (43% in males, 37% in females), and cannabis use (43% in males, 33% in females) are also broadly comparable to estimates from previous studies [13–16].
