Factor analytic work has consistently demonstrated that internalizing and externalizing factors capture a large degree of covariation among common mental health disorders (see Krueger & Markon, 2006 for a review). A similar two-factor model also fits genetic covariation among mental health disorders (Kendler et al. 2011). These two distinct classes of psychopathology have been consistently identified across research groups, samples, and analytic approaches. It is often overlooked, however, that these two factors demonstrate moderate phenotypic (r = 0.41–0.66; Gjone & Stevenson, 1997; Lahey et al. 2004; Krueger & Markon, 2006; Lahey et al. 2012) and genetic (rG = 0.52–0.53; Kendler & Myers, 2014) correlation, suggesting that common risk processes may be involved in both. Focusing on transdiagnostic risk factors, including personality and genetic factors, is consistent with the aim of NIMH’s Research Domain Criteria (RDoC) Project to identify fundamental risk factors that span multiple disorders (Insel et al. 2010; Sanislow et al. 2010). Identifying these common risk factors is pertinent to improving etiologic models of how distinct disorders co-occur, as well as their diagnostic classification and criteria. Further, such risk factors may inform intervention efforts aimed at more effectively addressing co-occurring problems.
