A possible explanation for the association is gene–environment correlation (rGE). Conceptually, rGE concerns the co-occurrence of genetic influences increasing risk for alcohol use in adolescents with risk environments (e.g. involvement with substance-using peers). Three types of rGE have been posited: passive (where the parental genes cause the correlation), active (where the subject's own genes make him/her seek certain environments) and evocative (where a subject's own genes increase the chances he/she is selected by others) [16,17]. It has been hypothesized that rGE plays an important role in the continuity of devious behaviour over time [18]. The study of gene–environment correlation is important because it can identify the environments that put individuals with a genetic liability for alcohol problem use at an increased risk. Twin studies using bivariate genetic analyses represent an important first step towards disentangling the mechanisms of gene–environment correlation. Genetic influences have been reported for adolescent alcohol use [3], peer behaviour [19] and friendship maintenance and formation [20]. It seems plausible that the correlation between peers and adolescent alcohol use is explained partially by an overlap in genetic factors.
