Researchers have theorized that the mood and anxiety disorders represent different manifestations of psychopathology within a single coherent spectrum. Underlying all of the disorders is a general dimension of negative affect, while (low) positive affect is a component of mood disorders and social phobia, and physiological hyperarousal is common to panic disorder and post-traumatic stress disorder (Clark & Watson, 1991; Mineka, Watson, & Clark, 1998; Watson, 2005; Watson, Clark, & Carey, 1988). Research has shown that the comorbidity among mood and anxiety disorders can largely be accounted for by the personality trait of neuroticism (Battaglia, Przybeck, Bellodi, & Cloninger, 1996; Bienvenu et al., 2001; Clark, Waston, & Mineka, 1994; Khan, Jacobson, Gardner, Prescott, & Kendler, 2005). Empirical evidence from several large scale studies that collected diagnostic data across the range of Axis I syndrome disorders provide evidence that the phenotypic structure is best captured with a two factor model involving Externalizing and Internalizing spectrums (Krueger, 1999; Krueger, Caspi, Moffitt, & Silva, 1998; Slade & Watson, 2006; Vollebergh et al., 2001). Krueger and Markon (2006) conducted a meta-analysis of multivariate comorbidity
