Shared environmental influences explained a significant proportion of the variation in the ages of first use of all three substances. By contrast, these influences could be dropped from all models of disorder. This replicates prior research indicating that shared environmental factors explain a notable proportion of the variance in early-initiation phenotypes, and decrease in influence while genetic influences increase as individuals progress toward problems (Dick et al., 2009; Rhee et al., 2003). Recent research, however, demonstrates that this trend may occur to varying degrees across drugs. In particular, the shared family environment explains a significant proportion of the variation in heavy and problematic use of cannabis. A meta-analysis of studies of problematic cannabis use (Verweij et al., 2010) obtained weighted average estimates of the influence of the shared environment of .20 in males and .15 in females. (Notably, we also obtained a family environmental estimate of .15 for CUD in women). Why might family environmental factors contribute more strongly to cannabis-related problems than other substance use disorders? This may be partly due to availability and access, which has been shown
