Consider, for example, major depression, which emerged as one of the three most prevalent diagnoses in all five classes across both samples. While at odds with the traditional idea of depression as a coherent entity, further consideration reveals deeper ties to the extant literature. Longstanding debates exist regarding depression subtypes, and a number of schemes have been proposed including unipolar versus bipolar, episodic versus chronic, and reactive versus endogenous. The occurrence of depression in the context of differing patterns of comorbidity may indicate variability in the nature and etiology of depressive syndromes. Thus, whereas recurrent depression in the context of dysthymia and GAD may reflect a core dysphoric liability, episodes of depression in the context of chronic antisocial behavior and substance abuse may reflect the cumulative impact of adverse consequences over time. Indeed, in this context, it is worth noting that there have been recent calls to demarcate melancholia as a distinct mood disorder from major depression in the upcoming version of the DSM [39]. A similar argument can be made with regard to the syndrome of PTSD, which has been conceptualized as including both internalizing and externalizing variants [40, 41].
