Demographic, behavioral, and genotypic characteristics of the study samples are presented in Table 1. The sample of ever-drinkers before age 18 used in the primary analyses consisted of 1,550 individuals from 1,151 nuclear families (defined by full-siblings) and 645 extended families. The mean first interview age was 17, 49% were female, and the majority came from high-risk families (89%) and were European American (73%). Before age 18, 74% had a first intoxication and 44% experienced a first DSM-5 symptom of alcohol use disorder. From ages 12 to 17, 39% reported that most or all of their best friends drank alcohol. Consistent with the expected population frequencies of the ADH1B variant, 6% carried at least one copy of the protective A allele (8% in European Americans and 3% in African Americans).
