We discuss each of these potential areas of post-GWAS study in further detail below, reviewing different types of questions that can be addressed, and studies that provide background rationale for the importance of these areas of investigation, to include how the extant twin literature can be used to guide and inform post-GWAS studies on the effects associated with identified genetic variants. Although not intended to be exhaustive, we provide illustrative examples of studies that have begun to carry out these kinds of tests. Many of our illustrations are drawn from the field of substance use research. Substance use outcomes, the primary area of study of the authors, is a rich area for exploration of these developmental questions for a number of reasons. Substance use outcomes encompass nearly all of the many challenges inherent in studying complex behavioral disorders and outcomes. At the population level, they have a significant genetic component, but are also strongly influenced by the environment (Verhulst et al., 2015; Kendler et al., 2007; Kendler et al, 2003), and twin studies provide further evidence that the heritability can
