Consistent with Korhonen et al. [20] who studied the co-occurrence of early externalizing behaviour, based on teacher ratings, and mid-adolescent tobacco and drug use in a partly overlapping sample, we found that common genetic and shared environmental influences explained the co-occurrence between adolescent conduct problems and tobacco, alcohol, and cannabis use in young adulthood. More specifically, we found that shared environmental factors contribute to the association between adolescent conduct problems and subsequent tobacco and cannabis use in females, and with tobacco use in males. Silberg et al. [21] also indicated the importance of common shared environmental influences for conduct disturbance and substance use in both sexes. They further illustrated that family dysfunction and affiliation with deviant peers are implicated in these shared environmental effects. According to Silberg et al., common genetic influences are also involved in the co-occurrence of conduct problems and substance use in both sexes [21]. This is in contrast to our findings where only in males the co-occurrence of conduct symptoms and substance use can be mostly explained by overlapping genetic influences. A highly heritable latent phenotype,
