In conclusion, this study demonstrated that modest to moderate correlations between early adolescent conduct problems and frequency of tobacco, alcohol, and cannabis use in early adulthood can be explained by common genetic and shared environmental effects. Our findings point in the direction of sex differences in the causes of covariation between early conduct problems and later substance use frequency. Future research in genetically informative samples is needed to replicate these sex differences. Moreover, since our findings imply that there may be common biological and environmental processes underlying early adolescent conduct problems and later substance use, research directed at the identification of the specific genetic and environmental factors that underlie conduct problems and substance use will improve our understanding of the underlying mechanisms. This may eventually lead to better prevention and treatment interventions.
