be in part genetically mediated. Indeed, the choice of delinquent peers is significantly heritable (Button et al., 2007; Walden et al., 2004) and genetically correlated with conduct problems, reflecting active genotype–environment correlation compounded with the reciprocal influence of peers on the individual's behavior, in effect rendering peers part of extended phenotype. It is conceivable that, considering human communication capabilities, the homophilic groups can also be virtual—not necessarily in the cyberspace or electronic media only, but via the perception of what behavior is appropriate given a certain degree and modality of one's behavioral deviation. It is thus not necessary for an individual with that behavioral deviation (e.g., CD) to formally belong to a deviant group (e.g., a gang) in order to manifest the respective set of symptoms including drug use, in part guided by such an indirect peer phenotypic assortment process.
