One of the strongest predictors of substance use in adolescents is affiliation with delinquent peers (Ary et al., 1999; Dishion and Loeber, 1985; Duncan et al., 1998; Fergusson et al., 2002; Fergusson and Horwood, 1997), which correlates approximately 0.24 with alcohol use and 0.40 with marijuana use (Duncan et al., 1998). Furthermore, adolescents’ perceptions of their peers’ behavior may be an even more salient risk for their own behavior than the peers’ actual behavior (Iannotti and Bush, 1992). The nature of the association between substance use and delinquent peer affiliation in adolescents is unclear. The relationship may be causal, mediated by social learning, facilitation (e.g. providing the alcohol or the marijuana), and reinforcement (Fergusson and Horwood, 1996), or it may be due to common risk factors predisposing to both affiliation with delinquent peers and substance use problems. For example, there has been some evidence that genetic factors influence a person’s tendency to affiliate with deviant peers (Manke et al., 1995; Rowe and Osgood, 1984); possibly through selection or attraction of delinquent peers, and that the same genetic factors may also
