Rhetorical flourishes do not justify replacing categorical diagnosis of personality disorders

To the Editor

The review of evidence for the dimensional classification of personality disorders by Gotzsche-Astrup and Moskowitz (2016) relies on rhetoric rather than science to promote one side of clashing scientific research paradigms. The paper concludes that a hybrid dimensional nosology for personality disorders was demoted to the status of a proposal in the Diagnostic and Statistical Manual of Mental Disorders–Fifth Edition (DSM-5) because of extra-scientific mechanisms such as cognitive biases, demarcation disputes and the protection of intellectual turf. However, in order to reach this conclusion, the authors simply ignore the justification given in the DSM-5 itself:

Although the benefits of a more dimensional approach to personality disorders have been identified in previous editions, the transition from a categorical diagnostic system of individual disorders to one based on the relative distribution of personality traits has not been widely accepted … [A]n alternative ‘hybrid’ model has been proposed in Section III to guide future research. (Preface, American Psychiatric Association [APA], 2013)

Far from fighting a rear-guard action against a new paradigm, the DSM-5 notes that there is not yet compelling evidence for a dimensional approach to classification of personality disorders, and proposes a model for pursuing that evidence.

The definitive proof that Gotzsche-Astrup and Moskowitz (2016) are not engaged in a scientific attempt to judge the relative merits of two competing scientific paradigms, as opposed to promoting their own preferred view, is that they make no attempt whatsoever to review the merits of categorical diagnosis. It is unclear how it is possible to judge the relative merits of two competing paradigms when you do not consider the advantages of one of them.

Finally, Gotzsche-Astrup and Moskowitz (2016) do not consider the main criticism of the proposed dimensional model of personality disorder, most powerfully described by Shedler et al. (2010): it is an unwieldy conglomeration of disparate theoretical constructs that is too complex for routine clinical use. While Gotzsche-Astrup and Moskowitz (2016) do refer to single studies suggesting that the dimensional approach can predict particular outcomes, they do not provide a compelling case that the additional complexity of dimensional diagnosis is justified by generally improved reliability, validity or prognostic utility compared with categorical diagnosis. Until it is demonstrated to be clinically useful, it is unlikely that a dimensional approach to diagnosis will be widely used outside research centres.