Adolescent and young adult substance use is associated with a wide range of poor mental and physical health outcomes and continues to be an area of substantial public health concern. Substance use has been linked to higher rates of long-term substance abuse and dependence, worse academic and occupational outcomes, impaired cognitive function, poorer physical and mental health, and greater criminal behavior. Therefore, the goal of this two-part dissertation was to better understand the risk and protective factors linked to substance use among adolescents and young adults. Employing a combined sample of 2,386 justice-system-involved youth from the longitudinal Pathways to Desistance and Crossroads studies, Study One examined the associations between a broad range of risk and protective factors and within-person changes in substance use from adolescence through young adulthood, whether any links between predictors and substance use were asymmetric, and whether associations varied by age. The results indicate that offending and peer substance use were the factors more strongly related to different types of substance use. While most predictors did not have asymmetric effects, offending was asymmetrically associated with alcohol use and future expectations were asymmetrically related to marijuana use. Further, decreased offending was more strongly related to lower rates of alcohol use in young adulthood than in adolescence, while having more positive future expectations was related to decreases in marijuana use throughout adolescence and young adulthood. Study Two identified whether the COVID-19 pandemic was related to changes in substance use and whether depressive symptoms, anxiety symptoms, sensation seeking, future expectations, and motivation to succeed moderated those changes. Findings show that alcohol and marijuana use increased during the COVID-19 pandemic and sensation seeking and future expectations moderated the increases in alcohol use, while depressive symptoms, anxiety symptoms, and sensation seeking moderated the increases in marijuana use. Overall, the results from Study One suggest that intervention and treatment efforts should focus on reducing criminal behavior and discouraging friendships with substance-using peers to help youth reduce or desist from using substances. Reducing offending during young adulthood in particular may be effective in discouraging problematic alcohol use. The results from Study Two highlight the importance of providing mental health services, helping high sensation seeking youth find healthy outlets, and providing resources to encourage more optimistic expectations of life after the pandemic to reduce costly iatrogenic effects such as problematic substance use that could have lasting impacts on youth long after the pandemic is over.

Laws legalizing recreational marijuana use have recently been passed in several states across the U.S., sparking debate about their impacts on marijuana use among adolescents and young adults. Critics are concerned that legalization will lead to increased use due to greater access to marijuana. Advocates, on the other hand, argue that legalization will reduce the “forbidden fruit” effect, leading to decreased use. To shed greater light on this debate, the present study investigates the impacts of legalization on marijuana use among young adults who have had prior experiences with the justice system. Importantly, young adults are at higher risk of marijuana use than any other age group and those who are involved in the justice system are at even greater risk. As such, we compare justice-system-involved (JSI) young adults in California (N = 396), where recreational marijuana use was recently legalized, to those in Pennsylvania (N = 372), where recreational use is still prohibited. Results indicate that rates of marijuana use increased significantly in both states, regardless of whether participants lived in a legalized state. While these findings are surprising, they suggest that increases in rates of marijuana use might be better explained by factors beyond legalization laws. In fact, these findings may indicate that increased state legalization of recreational marijuana use is giving rise to a culture of acceptance among JSI young adults.

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