that late adolescent RAPI scores will be higher among those with a diagnosis at age 25 follow-up than among those without (Dick et al., 2011). Lifetime self-report of MAX-D24 is predictive of diagnosis, as well, and both measures offer quantitative measures to grade drinking habits of nonalcoholic individuals (Saccone et al., 2000). The heritability of the logarithm transformation of MAX-D24, analogous to the transformation used in this study, approximates 50% (A. Heath, unpublished data). These variables were recoded as continuous measures, and a logarithm transformation was conducted so that they would better meet the assumptions of normality. Smoking habits were classified into 2 classes, smoking currently and not smoking currently, from the questionnaire administered at ages 23 to 25. We categorized “smoking currently” as smoking at least once a week and “not smoking currently” as smoking less often or not at all. Distributions of demographic characteristics, alcohol use measures and covariates for the studied twins, by zygosity, are shown in Table 1.
