Nonshared environmental mediation of the association between deviant peer affiliation and adolescent externalizing behaviors over time: results from a cross-lagged monozygotic twin differences design.

It has been argued that peers are the most important agent of adolescent socialization and, more specifically, that this socialization process occurs at the child-specific (or nonshared environmental) level (J. R. Harris, 1998; R. Plomin & Asbury, 2005). The authors sought to empirically evaluate this nonshared environmental peer influence hypothesis by examining the association between externalizing behaviors and deviant peer affiliation in a sample of 454 pairs of monozygotic (genetically identical) twins, assessed at ages 14 and 17, within a cross-lagged twin differences design. Results argued against a causal nonshared environmental influence of peer affiliation on the development of externalizing behaviors and in favor of nonshared environmental "selection." In particular, the twin with more externalizing behaviors at age 14 reported increased deviant peer affiliation relative to his or her co-twin 3 years later, regardless of his or her genetic predispositions toward externalizing behavior. Such findings suggest that adolescents with higher levels of externalizing behaviors select or shape (either intentionally or inadvertently) subsequent environmental experiences to involve increased affiliation with deviant peers. Implications are discussed.