Cohabitation, Marijuana Use, and Heavy Alcohol Use in Young Adulthood.

Background: Research on the association between cohabitation and substance use has been inconsistent, with some studies indicating an elevated risk among cohabiters and others finding either no difference in risk or a reduced risk of substance use. However, studies of this association have not utilized a causal modeling empirical framework.

Objectives: The purpose of this research was to assess whether cohabitation has a causal effect on two forms of substance use among young adults: marijuana and heavy alcohol use.

Methods: Three waves of data from the National Survey of Youth and Religion (n = 2,202; 2002-2008), a representative sample of young adults in the United States, and an augmented inverse probability weighting (AIPW) model designed for multivalued treatment effects estimation, were used to assess the association between cohabitation and substance use.

Results: The findings indicated that cohabitation was associated with more frequent marijuana use only among females. Much of the effect of cohabitation, though, was due to previous factors, including substance use, that affected whether young adults cohabit or not. Moreover, there was no evidence that cohabitation had a causal impact on heavy alcohol use. Conclusions/Importance: There is little evidence that cohabitation has a causal impact on substance use in general. However, among young women, those who cohabited reported higher levels of marijuana use than those who remained single. Future research should address why this group is at particular risk of substance use.