In contrast, for all cannabis measures, nicotine dependence, and any illicit drug dependence, the association between substance use and educational attainment was accounted for by non-causal shared environmental factors (i.e., DZ=MZ=1.0 in cotwin-control analyses). This finding is fairly consistent with drug use disorder findings from the Mid-Atlantic Twin Registry reported by Bergen et al. (2008), who found that the DZ regression coefficient was slightly larger than the MZ regression coefficient, although neither was statistically significant. Although they suggested that this was evidence for shared genetic influences between drug abuse and educational attainment, because neither regression was statistically significant these findings could also be interpreted as evidence of a non-causal shared environmental influence. Bergen et al. (2008) did not report findings for alcohol use/dependence or nicotine dependence.
