In this sample of Vietnam-era veteran twins, after controlling for individual-level measured covariates, all of the alcohol, nicotine, and cannabis use and dependence measures were significantly associated with reduced likelihood of completing 16 years of education. However, after additionally controlling for familial contributions to adult educational attainment in cotwin-control analyses, only early alcohol use, alcohol dependence, and daily nicotine use remained significantly associated with reduced educational attainment. Importantly, for all three measures, the odds ratios in the cotwin control analyses (Table 4) fell within the confidence intervals for the odds ratios estimated using the full sample (Table 3, first column), indicating that controlling for family influences did not attenuate the association. Thus, our data were consistent with the possibility that non-genetic causal influences may contribute to the association between daily nicotine use, early alcohol use, alcohol dependence and educational attainment (i.e., population=DZ=MZ>1.0).
