LAUSR

BACKGROUND Peer influence processes have been linked to escalation in substance use during the middle school years, particularly among at-risk youth. In this study, we report on an approach to prevention that attempts to counteract peer influence by interrupting the process of deviant peer clustering, in which socially marginalized youth self-aggregate and reinforce delinquent behavior, including substance use. We aimed to interrupt this process by implementing collaborative, group-based learning activities in school (i.e., cooperative learning). METHODS In a cluster randomized trial in the Pacific Northwest (N = 1460 7th-grade students in 15 schools), we tested whether cooperative learning can reduce alcohol and tobacco use, and whether these effects are mediated by reductions in the amount of alcohol and tobacco use among one's friends. Intervention schools were provided with training in cooperative learning, and data were collected in September/October 2016 (baseline) and March 2017 (follow-up). RESULTS Results indicated that cooperative learning significantly lowered rates of growth in alcohol (β = -0.60 [-0.36|-0.84]; p < .001) and tobacco use (β = -0.58 [-0.21|-0.94]; p = .01) between baseline and follow-up in intervention schools as compared to control schools. These effects were mediated by reductions in the prevalence of alcohol and tobacco use, respectively, among self-selected friends. CONCLUSIONS Cooperative learning was able to significantly reduce the prevalence of both alcohol and tobacco use in friendship networks during the school year. The lower prevalence of alcohol and tobacco use among friends, in turn, reduced individual use at follow-up. This study was registered as trial NCT03119415 in ClinicalTrials.gov.