The operational criteria set up to identify and distinguish so-called common mental disorders cut across the natural hierarchies existing between symptoms. The consequence is that many people who have one of these disorders also meet the criteria for one or more of the others. This comorbidity has generated much interest, and was even responsible for the name of one of the major US epidemiological surveys (the National Comorbidity Survey) (Kessler et al., 1994). Researchers, then, divide into two camps: those who think the comorbidity represents important relationships between well-validated disorders; and those who think it arises as an artefact of a classificatory system that is conceptually flawed and fails adequately to capture the nature of affective disturbance.
Thus, Kessler (2000) has defended the status of generalized anxiety disorder (GAD) as an independent condition, despite its high comorbidity, arguing that it does, for example, precede major depression, and also outlasts it. However, this would be expected if GAD represented a low threshold disorder that could transmute into a higher threshold disorder with the addition of a few symptoms. GAD and depression certainly share a common genetic diathesis (Mineka et al., 1998). The superimposition of major depression on a long-lasting minor depressive disturbance (dysthymia) has been called double depression (Keller et al.,
1997). The comorbidity of anxiety and depression may arise because anxiety states can transform into depressive disorders with the addition of relatively few symptoms (Parker et al., 1997). Depression/anxiety is equally apparent in adolescents (Seligman & Ollendick,
1998), as is the link between dysthymia and major depression (Birmaher et al., 1996). The idea that there are several distinguishable affective disorders is, to some extent, self-perpetuating, as it prevents clinicians from seeking out the full range of symptoms that are reflected in comorbidity. I imagine that it will turn out to be much more useful to see these comorbidities as an indication of common underlying processes leading to, but not necessarily reaching, a common destination.
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