Figure 1.

Multiple potential relationships underlie phenotypic associations between one’s own phenotype and that of one’s peers. As depicted in panel A, these phenotypes could be genetically or environmentally correlated: some of the genes that influence liability to alcohol consumption could also influence affiliation with peers who have alcohol problems. Similarly, environmental factors could be shared across phenotypes. In the current study such factors are unmeasured, but examples include parental monitoring and neighborhood quality. As stated elsewhere, because genetic and environmental correlations exist at a level beyond the manifested phenotypes, a change in one phenotype will not necessarily result in a change to the other.

Panel B depicts the causal processes of social selection and social influence. In the former, an individual’s alcohol consumption directly influences his peers’ alcohol phenotype, because he is selecting those peers to match his own alcohol use or because their use changes to match his. Social influence operates in the other direction: peers’ alcohol use changes one’s own alcohol use, potentially through overt or implied peer pressure or social modeling. Unlike genetic/environmental correlation, the critical implication of these causal paths is that a change in one phenotype will necessarily result in a change to the downstream phenotype. For example, if one’s peers’ alcohol use declines, one’s own alcohol will also decline.