In the present paper, we address several limitations in previous research by using a cotwin control design to test the hypothesis that cannabis initiation, early alcohol and cannabis use, daily nicotine use for a month or longer, and lifetime diagnoses of alcohol, nicotine, cannabis, and any illicit drug dependence remain associated with reduced adult educational attainment after controlling for familial influences. First, by examining twin pairs discordant for substance use/dependence, we are able to control for genetic and environmental confounds that might contribute to the association between substance use and educational attainment. Second, because our participants were in their late 30s when their educational attainment was assessed, we are better able to address lifetime educational attainment than are most previous studies, which have focused on high school dropout or educational attainment in 18–25 year-olds whose eventual educational level might not have been reached (although see Bergen et al., 2008; Staff et al., 2008). Third, we examine educational attainment in a veteran cohort that had access to education via benefits of the G.I. bill, thereby alleviating some of the economic barriers to higher education (see Xie, 1992 for a detailed discussion) that might otherwise be confounded with alcohol and drug outcomes.
