Age of first drink was defined using the Semi-Structured Assessment for the Genetics of Alcoholism question “How old were you the first time you had your very first whole drink?” Peer drinking was defined using the Semi-Structured Assessment for the Genetics of Alcoholism question “When you were 12–17, how many of your best friends used alcohol?” Answers were none, a few, most, or all. For analysis, we divided responses into two categories: most friends drink and most friends do not drink.
