Selection and Influence Effects of Marijuana Use in the Transition into High School

Background: A quarter of American youth have tried marijuana by 9^th grade, and 13% report having used marijuana in the past 30 days (Kann et al., 2018). Given the importance of peers and friendships during the transition into high school, it is important to know how marijuana use in influences friendship ties, as well as how friendship ties influence marijuana use.

Methods: Using four waves of longitudinal social network data collected quarterly during the school year from the spring of 8^th grade to the spring of 9^th grade, we examine how social ties relate to marijuana use. Adolescents across six Western Oregon school districts (n = 1075) were included in the present analysis. Using stochastic actor-oriented modeling, we examined whether classmates with similar past 30-day marijuana use were more likely to select each other as friends, and if a classmate’s past 30-day marijuana use is influenced by friends’ use.

Results: Past 30-day marijuana use increased in prevalence across 9^th grade. Classmates with similar marijuana use were more likely to select each other as friends, and classmates were more likely to adopt similar patterns of marijuana use as their friends. This influence effect was not modified by the friend’s popularity, nor by best friend status. The influence of friends on marijuana use was stronger than the behavior of one’s best friend.

Conclusions: In the transition into high school, adolescents tend to select friends who report similar recent marijuana use. In addition to this selection effect, adolescents are also more likely to adopt the marijuana use patterns of their friends.