the relative influences of heritable and environmental factors vary across cultures, the generalizability of our findings may also be somewhat limited. Conducting investigations similar to ours in other populations would reveal the nature of cross-cultural distinctions and commonalities in the degree to which these phenotypes are shaped by genes vs. environment (and gene-by-environment interplay). Such studies will lead to the next critical step in this line of research: the exploration of specific environmental contributions to heavy use and dependence on alcohol and cannabis, which, if they are not consistent across cultural groups, may call for differing approaches to intervention and prevention. Similarly, the modest associations between non-shared environmental influences on the two substances indicate that there are substance-specific environmental risk factors. The development of effective strategies for preventing misuse of alcohol and cannabis will therefore depend in part on identifying the influences on problem use that are unique to each of the two substances.
