For both peer and adolescent alcohol use no age or gender differences in aetiology were found, nor evidence for sibling interaction for alcohol use. A strong correlation was found between the genetic factors influencing adolescent alcohol use and friends' alcohol use (r = 0.6–0.7). This is consistent with the possibility of rGE (gene–environment correlation). As mentioned previously, 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 make him/her more likely to be selected by others) [16,17]. The occurrence of both active and evocative rGE processes on drinking habits is plausible, where genes influence the choice of friends (who in turn may further affect liability to consume alcohol). Passive rGE is also possible whereby parental genes contribute to both the genotype and the environment of their children. Thus, for example, genetic factors influence parent–child relations [46] and parent–child relations influence peer selection (with poor parent–child relationships quality is associated with affiliation with deviant peers; [47]). Given that
