Several limitations must be kept in mind when interpreting the results of this study. First, we are unable to determine the direction of causation between conflict and the disorders. It may be that conflict exacerbates the disorders, causing the child to manifest even more severe externalizing symptomatology than he or she otherwise would, or alternately, that the oppositional component of externalizing psychopathology induces parent-child conflict. Finally, it may be that both of these forces work in unison, such that conflict both influences and is influenced by ADHD, CD, and ODD.15 Regardless, the model used herein cannot determine direction of causation. Future research should address this concern, perhaps using a longitudinal twin design. A second limitation concerns the statistically nonsignificant but non-zero contribution made by the second genetic factor (a22) to the latent externalizing factor. It may be that we did not have the power to detect these effects as significant. Should that be the case, future studies may find that other psychosocial variables account for some of the common genetic influence on ADHD, CD, and ODD. A third limitation concerns general limitations of the additive twin model. Specifically, these models do not reveal the contributions of gene-environment correlations or gene × environment interactions. Instead, these effects are indirectly included in the reported genetic and environmental parameter estimates. Also, although this study relies on the equal-environments assumption for interpretation, that assumption was not directly evaluated herein. Such analyses are beyond the scope of the present study. However, the equal-environments assumption appears tenable for many mental disorders.39- 42 A fourth limitation concerns the use of family-reported, as opposed to observer-rated, conflict. Observer ratings are potentially advantageous in that they remove the dispositional characteristics of the reporter(s). However, observer ratings are generally based on only 1 to 2 hours of observation, whereas family reporters have direct and long-term knowledge of their within-family relationships. Regardless, it is unclear what effect observer reports may have on the estimates reported herein. Finally, the value of conflict as a psychosocial indicator may vary with characteristics of the sample. For example, these results apply only to children aged 10 to 12 years and not to younger or older populations. It may be that other psychosocial variables, such as parental monitoring,43 become more salient as children transition through adolescence. Also, estimates of genetic influence often increase with age,44,45 a phenomenon that may arise because individuals have an increasingly greater impact on their environments as they age.46 As such, the results from the present study should not be applied to externalizing behaviors at other ages.47 It also remains unclear how these results might vary by ethnicity.