Atypical depression is defined in the DSM IV as depression that shares many of the typical symptoms of major depressive disorder or dysthymia but is characterized by improved mood in response to positive events. In contrast to those with atypical depression, people with melancholic depression generally do not experience an improved mood in response to normally pleasurable events. Atypical depression also often features significant weight gain or an increased appetite, hypersomnia, a heavy sensation in the limbs, and interpersonal rejection sensitivity that results in significant social or occupational impairment. Despite its name, "atypical" depression does not mean it is uncommon or unusual. The reason for its name is twofold: it was identified with its "unique" symptoms subsequent to the identification of melancholic depression and its responses to the two different classes of antidepressants that were available at the time were different from melancholic depression (i.e., MAOIs had clinically significant benefits for atypical depression, while tricyclics did not). Atypical depression is four times more common in females than in males. Individuals with features of atypical depression tend to report an earlier age of onset (e.g. while in high school) of their depressive episodes. These episodes tend to be more chronic than those of major depressive disorder and only have partial remission between episodes. Younger individuals may be more likely to have atypical features, whereas older individuals may more often have episodes with melancholic features. Atypical depression has high comorbidity with anxiety disorders, carries more risk of suicidal behavior, and has distinct personality psychopathology and biological traits. Atypical depression is more common in individuals with bipolar I, bipolar II, cyclothymia or seasonal affective disorder. Depressive episodes in bipolar disorder tend to have atypical features, as does depression with seasonal patterns. Pathophysiology Significant overlap between atypical and other forms of depression has been observed, though studies suggest that there are differentiating factors within the various pathophysiological models of depression. In the endocrine model, evidence suggests the HPA axis is hyperactive in melancholic depression, and hypoactive in atypical depression. Atypical depression can be differentiated from melancholic depression via verbal fluency tests and psychomotor speed tests. Although both show impairment in several areas such as visuospatial memory and verbal fluency, melancholic patients tend to show more impairment than atypical depressed patients. Furthermore, regarding the inflammatory theory of depression, inflammatory blood markers (cytokines) appear to be more elevated in atypical depression when compared to non-atypical depression.