Effects of Marijuana Use and Heavy Episodic Drinking on Aggressivity in the Transition to Young Adulthood

Introduction: While trait aggressivity typically declines in the transition to young adulthood, substantial research links aggressivity and heavy episodic drinking (HED), particularly in males. Although marijuana use is increasing in this age group, research on the effects of marijuana use on aggressivity is inconsistent and the combined effects of marijuana use and HED are rarely studied. This study examines the direct and interacting effects of marijuana use and HED on changes in aggressivity in youth and young adults ages 16 to 29. Sex differences are also examined.

Methods: A randomly selected community-based sample of youth (N = 662; 48.3% males) was surveyed biannually for 10 years from 2003 (T1; ages 12 to 18) to 2014 (T6; ages 22 to 29). Self-reported aggressivity was collected from 2007 (T3; ages 16 to 22) onwards. Due to skewness, HED was dichotomized into drinking five or more drinks more often than monthly (1) and never or a few times a year (0)

Results: In the analysis, estimating models for males and females separately, males demonstrated significant direct effects of marijuana use (β = .335, p = .044) and interaction effects between marijuana use and heavy episodic drinking (β = -.621, p = .027). SES showed direct effects on aggressivity (β = -.154, p = .019) in males only. No effects or interactions were observed in females.

Conclusions: These findings point to the importance of the potentially negative consequences of marijuana use and its interactive effects with heavy episodic drinking on the maintenance of aggressivity during the transition to young adulthood, especially for males. The findings have significant implications for public health policy as the widespread legalization of marijuana becomes more likely in both Canada and the United States.