Continued exploration of the environmental conditions that moderate genetic influences on substance use at different periods in the life course, as well as the ways that those at high risk may select out of protective or into risky environments will help inform treatment and policy initiatives intended to reduce the harms of substance use. Incorporating GxE into designs currently used to examine genome-wide data may provide insight into the specific genetic variants that are influenced by environmental conditions (Mukherjee et al., 2012). Though this approach has proven limited thus far (Boardman et al., 2014), it is likely to be more powerful/successful in a post-GWAS era when the risk variants are known. In addition, extending the rationale of GWAS towards identifying environments (e.g. environmental wide association studies, or EWAS) that are important across developmental periods (Park et al., 2014) may allow us to eventually provide aggregate measures of both environmental and genetic risk, which could prove useful in tailoring prevention or treatment efforts. Additionally, because gene-environment correlation is also important, research designs leveraging information from natural experiments, such as a change
