Given that brain circuits, behavioral phenotypes, and environmental mechanisms are likely reciprocally influential (Miller and Rockstroh, 2013), studying how these dynamic relationships evolve across development will be important for establishing developmental pathways to behavioral manifestations of externalizing. The cross-sectional nature of our data prohibits strong conclusions regarding the direction of the proposed effects, and prospective research is needed to ascertain the mechanism(s) by which neural and behavioral phenotypes interact overtime. The observed associations across genetic, neural, and behavioral levels of analysis provides compelling evidence that resting-state connectivity in the identified network is a heritable mechanism that confers risk for externalizing. At the same time, more research is needed to validate this network as an endophenotype, including examining the generalizability of the polygenic associations with network connectivity. Additionally, although this study focused on identifying neurobiological mechanisms related to genetic risk for externalizing, understanding the role environmental factors play in the etiology of externalizing disorders is equally important. There is extensive evidence to suggest that environmental influences exert both direct and interactive effects on externalizing outcomes. For example, a large literature points
