comparable to other recent neuroimaging-genetic studies (Little et al., 2015; Pagliaccio et al., 2015). Further, we cannot say definitively that the current results will generalize to the latent externalizing spectrum. Third, we did not have an independent sample to examine the replicability of our findings. Investigating the replicability of our findings in a larger independent sample with more diverse sample characteristics (e.g., greater representation of women, ethnic minorities, and civilians) is needed before strong conclusions can be drawn. Fourth, the cross-sectional nature of our data prohibits 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. For example, it is possible that the externalizing PS causes an externalizing phenotype (clinical symptoms) that then leads to variability in the neural phenotypes rather than the genes relating directly to neural network functioning. Only longitudinal studies that assess the interactive effects of multiple levels of analysis over time are well positioned to clarify the direction of these effects. Finally, although use of an independently-derived PS represents a less biased approach to genetic analysis than selecting candidate genes, there are limitations to the polygenic approach. For example, the predictive
