Cruz and colleagues (2012) examined social influence using a genetically informative twin and family sample. Such studies allow the partitioning of variance into that attributable to genetic versus environmental factors, and they enable the researcher to control for genetic/environmental correlation (Figure 1A, also known as shared liability). They found that, after controlling for the effects of genetic and shared environmental correlations, which they refer to as selection, peer network substance use predicted drinking behavior in adolescents. Likewise, another genetically informative study (Harden et al., 2008) found that genetic factors influencing the target’s substance use were also related to the substance use of the target’s peers. Once these influences were accounted for, peer behavior predicted target substance use. Thus, there is prior evidence from genetically informative studies that both genetic/environmental correlation and social influence play a role in determining an individual’s substance use. However, these studies did not test whether social selection (Figure 1B) contributed to the association between one’s own substance use and that of their peers.
