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 studies and confirmed that a higher-order Internalizing spectrum split into two, highly correlated lower order factors of Distress (major depression, dysthymia, GAD), and Fear (panic disorder, agoraphobia, social phobia, specific phobia).
