Age at first drink was broken into 3 categories based on the overall sample distribution. “Early” was defined as the lowest quartile (14 or younger), “average” as the middle 50% (15–17), and “late” as the highest quartile (18 or older). We used a categorical variable because it better represents developmental periods (early, middle, and late adolescence) than does a continuous variable, and it allows for the interpretation of age at initiation of alcohol use in relative terms, i.e., earlier vs. later than individuals in the same birth cohort. Analyses were conducted with all participants who reported an age at first drink (available from all respondents who endorsed lifetime use). The sample analyzed in the current study (n=3,532) was composed of 91 monozygotic (MZ) AA pairs, 109 dizygotic (DZ) AA pairs, 80 AA singletons, 785 MZ EA pairs, 602 DZ EA pairs, and 278 EA singletons.
