Teasing Apart the Developmental Associations Between Alcohol and Marijuana Use and Violence

This study explored the longitudinal associations of alcohol and marijuana use and violence over ages 11-20 in the youngest sample of males from the Pittsburgh Youth Study (N = 503). We examined trends in alcohol and marijuana use and violence, how they covaried both concurrently and over time, and whether frequent substance use predicted violence and vice versa in multivariate models controlling for common risk factors. The analyses focused on frequent alcohol or marijuana users, those who scored in the highest 25% of frequency. Throughout adolescence, substance use was more prevalent than violence. Most substance users did not engage in violence, and the proportion of substance users who engaged in violence was smaller than the proportion of violent offenders who were also substance users. Concurrently, frequent use of alcohol and marijuana were both significantly associated with violence. Longitudinal associations between frequent drinking and violence were weak, whereas longitudinal associations between frequent marijuana use and violence were more consistent. However, the relationship between frequent marijuana use and violence (and vice versa) was spurious; it was no longer significant when common risk factors such as race/ethnicity and hard drug use were controlled for. We conclude that the marijuana-violence relationship is due to selection effects whereby these behaviors tend to co-occur in certain individuals, not because one behavior causes the other; rather, both are influenced by shared risk factors and/or an underlying tendency toward deviance.