The shared phenotypic variability and shared genetic etiology for externalizing behavior and traits in the general population implies that risk for exhibiting these behaviors may also share underlying neurobiology. This “cross-cutting” conceptualization is consistent with recent efforts of the U.S. National Institute of Mental Health to move away from the historical diagnostic categories of mental illness and towards dimensional constructs of psychopathology (Sanislow et al., 2010). The purpose of this Research Domain Criterion (RDoC) approach is to harmonize clinical research with neuroscience approaches in order to identify both psychological and biological mechanisms that underlie the traits shared across many clinical disorders, including externalizing (Patrick et al., 2013). Given the large number of genes included in the set analyzed here, we can only speculate about which neural systems may be impacted by variation within these genes. Likely candidate systems include those involved in self-regulation, motivation, affect, and reward processing, including the prefrontal cortex, striatum, and limbic systems (Aron, 2011; Bechara, 2005; Coccaro, Sripada, Yanowitch, & Phan, 2011; Jentsch et al., 2014; Jentsch & Taylor, 1999; Patrick et al., 2013; Verdejo-García &
