Previous research suggests that a highly heritable general externalizing factor contributes to individual differences in a range of behaviors marked by impulsivity and disinhibition, such as rule-breaking and aggression. Relational aggression has been found to be moderately heritable and to share common genetic and environmental influences with physical aggression, though no prior studies have examined the etiological commonality between relational aggression and a more broadly construed externalizing domain. Using a sample of 499 like-sex twin pairs, the present study confirmed that six measures of externalizing-related behaviors formed two distinct subfactors of rule-breaking and aggression. A confirmatory factor analysis revealed that relational aggression correlated nearly equally with both factors. Factors of aggression, rule-breaking, and relational aggression were fit in an independent pathway model which revealed that relational aggression was influenced by additive genetics and non-shared environmental effects (a2 = .31 and e2 = .69). A majority of the heritability of relational aggression (72%) was shared with the aggression and rule breaking factors, whereas the majority of the non-shared environmental influence (88%) was specific to relational aggression. Findings suggest that relational aggression should be included in the broader externalizing domain, and that it may be its own distinct subfactor of externalizing, rather than simply being an alternate form of aggression.