Gene-environment interaction will be detected at the statistical level when, at the functional level, genetic differences are observed in sensitivity to an environment (Mather & Jinks, 1982). Human studies have reported G × E in the development of CD and antisocial behavior in adoption (Cadoret et al. 1995) and twin studies (Jaffee et al. 2005) in the absence of measured genotypes. Among a community sample of singletons, Caspi et al. (2002) reported findings consistent with the presence of a G × E that exacerbated risk for antisocial behavior when boys with a low-activity MAOA allele were exposed to household maltreatment, defined as maternal rejection, inconsistent presence and identification of any particular primary caregiver, harsh discipline, physical abuse, and sexual abuse among males. The finding in males has been replicated (Foley et al. 2004; Nilsson et al. 2005; Kim-Cohen et al. 2006), although non-replication has also been reported (Haberstick et al. 2005; Young et al. 2006).
