Analyses within twin pairs discordant for substance use and dependence are shown in Table 4. The interaction between zygosity and daily nicotine use was significant, and thus was included in the final model along with the main effect of daily nicotine use. Since no other interactions were significant (p-value range: 0.21–0.77), the interaction term was not included in the other conditional logistic models. Compared to their cotwins who did not use alcohol before age 18, individuals who used alcohol before age 18 had significantly lower educational attainment before and after covariate adjustment. Similar results were observed for alcohol dependence and daily nicotine use, with a trend for nicotine dependence (p=0.07 in the adjusted model). However for all cannabis measures and any illicit drug dependence, at risk individuals did not have significantly lower educational attainment than their low-risk cotwin.
