Finally, the importance of studying such policy relevant questions within the context of genetically informative research designs was highlighted by our findings of considerable heritable influences across a range of cannabis-related phenotypes including those, such as opportunity to use cannabis, that have been traditionally perceived as environmental constructs. Nonetheless, our analyses revealed moderate heritable influences on both early age of first opportunity to use cannabis use (72%) and, importantly, on early onset cannabis use itself (80%). In parallel analyses, we also found evidence for moderate to high heritability of a measure of cannabis abuse or dependence. While there were differences between the current study and previous studies of cannabis abuse/dependence (e.g., cohort differences between samples, the inclusion of a comprehensive assessment of withdrawal within the definition of dependence), the parallels between our findings and those of previous studies are striking 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
