Both genetic factors [3] and peer influences [4–7] have been implicated in adolescent alcohol use. Twin studies indicate that 20–30% of the variation in liability to adolescent alcohol use is genetic in origin [3]. In the first UK-based twin study of substance use, we have found previously that genetic influences accounted for around 30% of the variation in the initiation and 50–60% in the progression of alcohol use [8]. Peer alcohol use (usually assessed by adolescent report rather than directly) has been reported to be one of the strongest predictors ofadolescent alcohol use [5–7], while perception of peers' attitudes to alcohol use is another important risk factor [7,9].
