Another environmental influence that has received much attention in the risk-behavior literature is affiliation with deviant peers. Numerous studies have provided support for the importance of peers in the development of substance use and abuse (Andrews et al., 2002; Bauman and Ennett, 1994; Curran et al., 1997).The degree to which an adolescent’s peers use alcohol or illicit drugs has been identified as a strong predictor of that adolescent’s own substance use behavior (Chassin et al., 2004). Emerging evidence suggests that deviant peer affiliation may moderate the genetic disposition of youth for substance use. For example, twin data have demonstrated that drinking by friends impacts the genetic effects on alcohol use; specifically higher levels of drinking by friends has been found to bring about higher levels of genetic contribution to this behavior whereas lower levels of drinking by friends suppressed the level of genetic contribution to alcohol use (Guo et al., 2009). Similar findings have been reported by Dick and colleagues (2007) who reported that as peer alcohol use increases, heritable factors associated with an adolescent’s own alcohol involvement also increase.
