Comorbidity refers to the co-occurrence of two or more distinct disorders in an individual, where the symptoms of each can often interact in negative ways. This is a common phenomenon in psychology, with each disorder presumed to have its own etiology, pathology, and implications for treatment.

Comorbidity

New diagnoses added to the nomenclature reflect not so much the discovery of a previously unrecognized disease, pathogen, or lesion but are instead to efforts to fill gaps among existing categories. However, these new diagnostic categories can create additional boundary confusions. 

Generalized social phobia

Schizoaffective disorder

Mixed anxiety-depressive disorder

These three diagnoses are examples because they are on the borders of domains of psychopathology that have been regarded as relatively distinct.

Boundary disputes

Excessive diagnostic co-occurrence

Boundary disputes

University of Illinois at Urbana-Champaign

Dilemmas of the categorical model

Research on categorical versus dimensional models

Clinical utility

Categorical or Dimensional

Widiger, T. A., & Samuel, D. B. (2005). Diagnostic categories or dimensions? A question for the Diagnostic and statistical manual of mental disorders--. Journal of abnormal psychology, 114(4), 494. https://doi.org/10.1037/0021-843X.114.4.494

Diagnostic categories or dimension? A question for the Diagnostic and Statistical Manual of Mental Disorders -- Fifth Edition

Clinical utility is the extent to which DSM assists clinical decision makers in fulfilling the various clinical functions of a psychiatric classification.

Clinical utility

Categorical classification divides mental disorders into types based on criterion sets with defining features. Researchers and clinicians following this model would diagnose and conceptualize conditions of disorders as qualitatively distinct from normal functioning and from one another. 

Categorical classification

A dimensional system classifies clinical presentations based on quantification of attributes rather than the assignment to categories and works best in describing phenomena that are distributed continuously and that do not have clear boundaries. 

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