into whom is being affected by interventions and why. Other approaches offer advantages as well. Although they are agnostic to the identity of specific genes, twin studies provide the basis for determining the existence and extent of genetic influences on infinite phenotypes, ranging from the obvious, such as height, to the somewhat surprising, such as affiliations with substance-using peers. GWA approaches are also powerful tools for discovery, identifying specific genes rather than the variance accounted for by genetic variance. But given the large sample sizes required to make them most effective and the averaged environments across which they scour the genome for “hits”, they do not provide insight into how genes work or, of course, how environments impact behaviors. In comparison, adding assays of specific genetic variance to behavioral science data sets allows researchers the opportunity to make prior predictions for how genes with known characteristics may transact with experiences to impact human behavior. Of course, not all of these hypotheses will be correct. Our predictions for OXTR impacts on peer selection were supported. Our predictions for OXTR moderating peers’ direct effects and their interactions with interventions were not.
