Research has indicated that differential parental treatment is linked to differences in externalizing symptomology (EXT) across siblings, even those siblings who are genetically identical. However, the direction of causation and longitudinal significance of this relationship remains unclear. Thus, in the present study, the authors examined 486 monozygotic twin pairs, assessed at ages 11, 14, and 17 years, within a cross-lagged twin differences design. Results revealed that differential parentchild conflict at age 11 years uniquely contributed to differential sibling EXT 3 years later but only in the most discordant twin pairs. In the full, unselected sample, this relationship was not significant. These results suggest that markedly different parent-child conflict has an environmentally mediated impact on child behavior through mid-adolescence, findings that yield insights into environmental influences on behavior.Keywords parent-child relationships; environmental influences; twin differences; externalizing disorders In 1987, Plomin and Daniels published a seminal and widely cited article in which they noted that the largest environmental contributions to behavioral outcomes occur at the child-specific (i.e., nonshared environment) level rather than at the family-wide (i.e., shared environment) level. They consequently reasoned that, to the extent that parents influence children's outcomes, they likely do so at this child-specific level. To examine this provocative hypothesis, they charged researchers with the following tasks: (a) identify differential experiences among siblings in the same family; (b) relate these differential experiences to differences in sibling behavior; and (c) determine the causal nature of the relationship between differential experiences and differences in sibling behavior.To date, researchers have established both that there are significant sibling differences in parental treatment within families (Feinberg & Hetherington, 2001;Reiss et al., 1995) and that these differences are predictive of child and adolescent outcomes (Anderson, Hetherington, Reiss, & Howe, 1994;Conger & Conger, 1994;Feinberg & Hetherington, 2001;Kowal, Krull, & Kramer, 2004). For example, parent-child conflict appears to contribute to adolescent adjustment largely at the child-specific level (Anderson et al., 1994), results that appear to Correspondence (Conger & Conger, 1994). Specifically, researchers found that the adolescents treated in a more hostile fashion by their parents in early adolescence exhibited relatively more delinquent behavior during mid-adolescence, even when controlling for the association between hostility and delinquent behavior during early adolescence. Although these studies certainly establish a role for differential parental treatment in child outcomes, their interpretative significance is limited by two issues. First, the mechanisms through which differential parental treatment/parent-child relationships affect child outcomes have yet to be firmly resolved. Although they still r...