Historically, the classification of anxiety disorders and depression has varied as a function of how the medical profession has viewed the association between the 2. As seen in the first and second editions of the Diagnostic and Statistical Manual of Mental Disorders (DSM), anxiety disorders and depression were thought to be more similar than they were different, as it was believed that they both represented a common underlying neurotic process. A nosologic shift occurred in 1980, when DSM-III was published with mutually exclusive classifications for anxiety and depressive disorders.

This new classification system represented an attempt within the psychiatric community to dissociate the anxiety disorders from depression, but it may have gone too far in separating them. In the late 80s and early 90s, another change occurred and was consistent with the clinical observation that individuals could present with anxiety and depression, singly or jointly. This shift is evident in DSM-III-R in 1987 and DSM-IV in 1994, which each allow clinicians to diagnose individuals with single disorders or multiple disorders concurrently.

Based on this new classification system, empirical observations have yielded rates of lifetime and 12-month comorbidity between each anxiety disorder and major depression that range from 10% to 30%; when one considers the comorbidity between depression and any of the anxiety disorders, rates of comorbidity exceed 50%. In many cases, this comorbidity has sobering clinical implications. As reviewed by Lydiard and Brawman-Mintzer, comorbid depression in the context of an anxiety disorder may be associated with more severe and chronic anxiety disorders, greater social and vocational impairment, higher rates of alcohol and substance abuse, an increased risk of suicide, and a poorer response to acute and long-term treatment.

Given the sobering clinical implications of coexisting anxiety and depression, accurate diagnosis of comorbid conditions and effective treatment for both disorders are essential to improving treatment outcome for individuals experiencing concurrent anxiety and depression.