Plato versus Aristotle: categorical and dimensional models for common mental disorders

There are two quite different ways of conceptualizing common mental disorders: as categorically distributed or as dimensions. In the former case, individuals with "subthreshold" disorders may contain a small proportion of "true cases"; while in the latter, they are seen as having the same basic disorder, but in insufficient degree to warrant a diagnostic label. The categorical school postulates the phenomenon of "comorbidity" to account for those who are both depressed and anxious. The dimensional school regards them as cases of "anxious depression," and observes that the 2 dimensions are highly correlated with one another. This article considers 3 levels of severity of cases satisfying research diagnostic criteria for depression: mild, moderate, and severe. At ascending levels of initial severity, there is increasing disability, and a worse outcome at 12 months. The only positive treatment effect is that favoring antidepressants over sedatives at 3 months, and this effect is related to initial severity-being much greater for the "severe" group of depressions. Initial severity is linearly related to disability, and there is no disjunction at the usual division between "non-cases" and "cases" of depression. Clinicians are obliged to use categorical concepts, as they must decide who is sufficiently ill to justify treatment. But in our efforts to understand the relationships between social and biological variables, dimensional models are far more appropriate.