At age 12, participants were asked if they had ever used alcohol when they were not in the presence of an adult, coded 0 (no) or 1 (yes) (n = 1,864 individual twins). Alcohol use behavior was assessed at ages 14 and 17 by asking the participants how frequently they became intoxicated. On the age 14 questionnaire, the item included four response options: (1) Never, I don’t drink alcohol; (2) Less than once a month; (3) About 1 to 2 times a month; and (4) Once or more a week. At age 14, a total of 1,852 participants responded to the item (1,463 of which had initiated alcohol use by age 14). On the age 17 questionnaire, the item included nine response options: (1) Daily; (2) A couple of times a week; (3) Once a week; (4) A couple of times a month; (5) About once a month; (6) About once every two months; (7) 2-4 times per year; (8); Once a year or less; (9) I don’t drink any alcohol. The latter response options were collapsed into four categories
