Journal of Abnormal Child Psychology, Vol. 34, No. 1, February 2006, pp. 29–41 ( C  2006) DOI: 10.1007/s10802-005-9004-9 Deviant Peer Affiliation and Problem Behavior: A Test of Genetic and Environmental Influences Bernadette Marie Bullock, 1,4 Kirby Deater-Deckard, 2 and Leslie D. Leve 3 Received May 4, 2004; revision received June 30, 2005; accepted July 11, 2005 Published online: 21 March 2006 This study uses a multitrait, multimethod (MTMM) approach to investigate the genetic and environ- mental etiologies of childhood deviant peer affiliation (DPA) and problem behavior (PROB). The variability of genetic and environmental estimates by agent and method is also examined. A total of 77 monozygotic and 72 dizygotic twin pairs and each twin’s close friend were assessed. The informants included parents, teachers, and twins, and the methods involved questionnaire reports and coder ratings of videotaped dyadic interactions between each twin and their close friend. Twin intraclass correlations and univariate models attributed DPA and PROB to genetic, and shared and nonshared environmental effects. Parameter estimates differed by rater and method, however. Re- sults accentuate the imperative to attend to method effects inherent in MTMM behavioral genetic research. KEY WORDS: behavior genetic; deviant peer affiliation; problem behavior; children; methodology. Peers are important agents of socialization, influencing many domains of psychosocial and cognitive development (Hartup, 1996; Hartup & Stevens, 1997; Parker & Asher, 1997). In adolescence, affiliation with antisocial friends is one of the strongest correlates of delinquency and substance use (Elliott, Huizinga, & Ageton, 1985; Loeber, Stouthamer-Loeber, Van Kammen, & Farrington, 1991; Patterson, Reid, & Dishion, 1992). Furthermore, interpersonal dynamics between antisocial friends are linked to an escalation in deviant behaviors (Dishion, Andrews, Kavanagh, & Soberman, 1996). In particular, observations of positive reinforcement of deviant talk within peer dyads (e.g., discussing aggressive or illegal activities) are linked to escalations in substance use, delinquency, violence, and risky sexual behavior in adolescence and early adulthood (Capaldi, Dishion, Stoolmiller, & Yoerger, 2001; Dishion et al., 1996; Dishion, Capaldi, Spracklen, & Li, 1995;Dishion, Eddy, 1 Child and Family Center, University of Oregon, Eugene, Oregon. 2 Department of Psychology, Virginia Polytechnic Institute and State University, Blacksburg, Virginia. 3 Oregon Social Learning Center, Eugene, Oregon. 4 Address all correspondence to Bernadette Marie Bullock, PhD, Child and Family Center, University of Oregon, 195 West 12th Avenue, Eugene, Oregon 97401-3408; email: bullock@uoregon.edu. Haas, Li, & Spracklen, 1997). Although environmental mechanisms are indeed influential, they co-occur within the context of genetic influences (Plomin, 1994). The first aim of the current study was to test whether and how genetic and environmental factors operate in accounting for child deviant peer affiliation and problem behavior. Similar to psychosocial research, behavioral genetic studies are subject to methodological problems including rater bias and measurement error (O’Connor & Plomin, 2000). For behavioral genetic research, estimates of ge- netic, and shared and nonshared environmental effects can vary significantly by rater and method (Eaves et al., 1997; Leve, Winebarger, Fagot, Reid, & Goldsmith, 1998; Miles & Carey, 1997), leading some to rely on multimethod latent indicators in their models. Unfortunately, combin- ing data across informants and methods may diffuse dis- tinct patterns of behavior that emerge by context (Bank, Dishion, Skinner, & Patterson, 1990). The second aim of this research was to examine estimates of ge- netic, and shared and nonshared environmental vari- ance using a multitrait, multimethod (MTMM) as- sessment strategy to further the understanding of how rater and method effects might vary when con- sidering child deviant peer affiliation and problem behavior. 29 0091-0627/06/0200-0029/0 C  2006 Springer Science+Business Media, Inc.