Clinicians' conceptualizations of comorbid cases: a test of additive versus nonadditive models.

Comorbidity in psychopathology is a common phenomenon. However, little is known about the way in which clinicians think about comorbid cases. The Diagnostic and Statistical Manual, Fourth Edition, Text Revision (DSM-IV-TR; American Psychiatric Association, 2000) implies an additive model of concept combination, but studies of human cognition find that individuals often combine concepts in nonadditive ways. In this study, 70 clinicians listed symptoms for three disorders and their combinations. Participants produced nonadditive descriptions, termed overextensions, at significant rates. These results challenge the utility of the implicit additive model of the current DSM.