Multivariate analyses of epidemiological samples studied both cross-sectionally (Krueger, 1999; Krueger et al. 2001) and longitudinally (Krueger et al. 1998) suggest that the distribution of common psychiatric and drug abuse disorders, and the observed patterns of co-morbidity, can be largely explained as resulting from two broad psychopathologic dimensions typically called ‘internalizing’ and ‘externalizing’. Population-based twin studies in Virginia and Norway have verified the existence of these dimensions and found that they result largely from the operation of genetic factors (Kendler et al. 2003, 2011). A range of prior studies have attempted to define the boundaries of these spectra from a phenotypic perspective (e.g. Krueger et al. 2007; Venables & Patrick, 2012). One large-scale twin study reported a multivariate genetic analysis of the relationship between internalizing disordersandthe personality trait of neuroticism (Hettema et al. 2006). However, we are unaware of a previous focused effort to define which disorders and traits are central versus more peripheral to genetically defined internalizing and externalizing spectra. This is an important question because it addresses how, for genetic, epidemiological or clinical studies, we might be best able to assess in patients or subjects their overall genetic propensity to internalizing or externalizing psychopathology.
