The diagnostic system used by many mental health practitioners in the United States—known as the Diagnostic and Statistical Manual of Mental Disorders—assumes that symptoms of two disorders that occur at the same time are additive and that the order in which the disorders are presented doesn't matter. But new research suggests that order actually plays a significant role in determining how clinicians think about psychiatric disorders.
In an article published in Clinical Psychological Science, a journal of the Association for Psychological Science, researchers Jared Keeley, Chafen DeLao, and Claire Kirk of Mississippi State University draw from existing research on conceptual combination to investigate how clinicians diagnose psychiatric disorders that occur together.
They predicted that for disorders with overlapping symptoms—such as major depressive disorder (MDD) and generalized anxiety disorder (GAD)—clinicians would describe the disorders pretty much the same way, regardless of which disorder was presented first.
But for two disorders that are quite different—such as generalized anxiety disorder (GAD) and antisocial personality disorder (ASPD)—the researchers predicted that the order in which symptoms are presented would significantly influence clinicians' descriptions of the disorders.
Keeley and colleagues also predicted that the features of one disorder would overshadow the features of another, providing evidence for a "dominance" effect.
In two different studies, the researchers asked clinicians to identify the symptoms that would describe each of three disorders individually (MDD, GAD, ASPD) and the symptoms that would describe paired combinations of the three disorders.
In both studies, the clinicians were inconsistent in their descriptions of disorder pairs—for example, the symptoms they identified for a combination of MDD + ASPD were not necessarily the same as those identified for a combination of ASPD + MDD.
And in one of the two studies, the researchers found that the order of symptoms mattered more for clinicians' descriptions of disorders that were different than for disorders that overlapped, partially confirming their original hypothesis.
Together, these results seem to contradict the assumption that order doesn't matter in psychiatric diagnoses. Findings from a third study indicated that clinicians' descriptions of the symptoms involved in GAD were dominated by their descriptions of both MDD and ASPD, while symptoms of ASPD and MDD had equal weight.
Keeley and colleagues offer several possible explanations for their findings. Clinicians could be straying from the additive guidelines of the Diagnostic and Statistical Manual of Mental Disorders. Alternatively, their clinical experiences may have led to "rater drift," such that the criteria that they use to evaluate symptoms have drifted over time.
But it's also possible that practitioners are actually ahead of the curve. Keeley, DeLao, and Kirk argue that clinicians could be "accurately modeling an aspect of psychopathology that our current diagnostic system has yet to accommodate."
While it remains to be seen whether these findings have implications for the actual treatment of psychiatric disorders, Keeley and colleagues believe that these three studies may help researchers and practitioners in trying to bring the classification of psychiatric disorders and actual clinical practice closer together.
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"The Commutative Property in Comorbid Diagnosis: Does A + B = B + A?" Clinical Psychological Science, 2012.