and there is now a convergence of evidence to suggest that between 51% and 58% of liability to develop cannabis dependence within Western societies where cannabis is commonly available can be attributed to heritable influences (Verweij et al., 2010). It therefore remains feasible that some component both of the association between age of onset of cannabis use and the subsequent use and misuse of other illicit drugs and of the comor-bidity between cannabis dependence and other psychiatric conditions (including drug dependence) may be due to a shared genetic liability between cannabis use/dependence and these other phenotypes. In subsequent studies based on these data, we hope to able to help elucidate such issues.
