In examining the association between substance use and reduced educational attainment, it is necessary to account for potential confounders, i.e., factors that might contribute to alcohol/drug use liability and also influence academic performance. Previous research has indicated that both educational attainment (Baker et al., 1996; Heath et al., 1985; Lykken et al., 1990) and substance dependence (see reviews by Agrawal & Lynskey, 2006, 2008; Dick et al., 2009; Heath, 1995; Rose et al., 2009) are heritable. Especially pertinent to the present study, Bergen et al. (2008), found evidence suggesting genetic overlap between drug abuse and educational attainment in a twin study, and Dick et al. (2007) found evidence implicating CHRM2 polymorphisms in association studies examining substance use disorders and IQ/cognition.
