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 power of the PS is limited by the robustness of the original GWAS, and the PS assumes an additive genetic model without interactions, which might not optimally reflect the underlying genetic architecture of externalizing. Additionally, the PS does not take into account environmental effects. Given the high level of stress exposure in the current sample and externalizing prone samples in general (e.g., Douglas et al., 2010; Luntz and Widom, 1994), examining the influence of potential epigenetic effects in relation to externalizing disorders will be important to explore in future research, such as whether differentially methylated loci associated with externalizing overlap with the externalizing PS.
