Two studies used reports of substance use by sibling pairs and their friends who participated in the National Longitudinal Study of Adolescent Health (Add Health) to investigate the moderating influence of peer substance use on genetic risk on drinking frequency among adolescents. Harden, Hill, Turkheimer, and Emery (2008) found that teens with higher genetic liability had significantly greater frequencies of drinking and smoking if their best friends did as well, indicative of GxE. This held true even after adjusting for the tendency of teens with higher liability to have friends with greater substance use (i.e., a gene-environment correlation). Subsequently, Guo, Elder, Cai, and Hamilton (2009) report that genetic influences on drinking frequency were moderated by peer drinking: heritability of drinking frequency was highest for sibling pairs with high exposure to drinking friends (range 0.87–1.0) and lowest for pairs with low exposure to drinking friends (range 0.0–0.33).
