Previous twin research to understand the phenotypic association between CD and substance use involvement has mainly focused on substance use disorders [e.g. 17–19]. However, studies testing the extent to which the association between conduct problems and level of substance use is due to overlapping genetic or environmental influences are relatively scarce. In a cross-sectional study, Miles et al. found that the moderate phenotypic correlation (r=.38 in males and .31 in females) between CD and marijuana use in adolescents was due to overlapping genetic and unique environmental influences on both phenotypes, while shared environmental influences only explained marijuana use [10]. The use of longitudinal data provides the opportunity to also study early manifestations of genetic and environmental influences potentially underlying future substance use. This way, genetically informative designs can improve our understanding of how the relationship between conduct problems and substance use unfolds over time, further elucidating the mechanisms underlying the development of substance use behaviour. For instance, Korhonen et al. [20] found that part of the additive genetic and shared environmental factors underlying externalizing behaviour in early adolescence is also
