The commutative property in comorbid diagnosis: Does A + B = B + A?

The Diagnostic and Statistical Manual of Mental Disorders (4th edition, text revision) assumes an additive model for describing comorbid symptomatology, including the commutativity of disorder descriptions across order of presentation (e.g., A + B = B + A). Given the high prevalence of individuals with comorbid conditions, it is important to investigate if clinicians follow an additive model when conceptualizing disorders. Three studies involving a total of 138 clinicians tested assumptions of commutativity for conceptualizations of three disorders-major depressive disorder (MDD), generalized anxiety disorder (GAD), and antisocial personality disorder (ASPD)-by either fixed-choice or free-response descriptions of all possible pairwise comparisons of the disorders. Clinicians demonstrated less-than-perfect commutativity for all disorder combinations, and this finding was replicated across two samples. In addition, MDD and ASPD tended to overshadow the presence of GAD in combinations. These results challenge the additive assumptions of the current diagnostic system and may suggest the order in which diagnoses are conceptualized influences the resulting symptomatology.