The results of this study are preliminary, and require replication and extension, but one can begin to see the value of evaluating genetic effects on substance use in a developmental and environmental context. As sample sizes grow, and investigators begin sharing data at higher rates, tests of developmental and environmental moderation become more powerful and feasible. Mendelian randomization tests offer an elegant quasi-experimental way to test the causal role of an environmental exposure, and will become a mainstay in quasi-experimental research.
