Given that individual differences in affiliations with substance-using peers are heritable (Cleveland, Wiebe, & Rowe, 2005), it is possible that OXTR variability might influence selection of friends based on their substance use. Moreover, selection processes might be impacted by intervention. To this end, we investigated the possibility of a 2-way interaction between the intervention and OXTR variation predicting affiliations with substance-using peers. Based upon consistent findings that peer behaviors predict individual behaviors, we expected that affiliations with substance-using friends would be associated with individual adolescents’ own 9th-grade alcohol use. Without evidence to the contrary, we did not expect a direct effect of variation in the OXTR gene on youth alcohol use. Instead, we expected to see an interaction among friend substance use, intervention program participation, and OXTR genetic risk on youth’s own alcohol use. Specifically, similar to Cleveland, et al.’s (2015) finding that the intervention potentiated the impact of high maternal involvement on alcohol use only among adolescents carrying sensitive genotypes, we expected that (1) the intervention would increase the impact of low-risk friends on adolescents’ alcohol use but (2)
