Comorbidity of major depression and anxiety disorders in twin pairs.

The relationship among major depression only, major depression with anxiety disorders, and anxiety disorders only was investigated in a twin sample (N = 177 pairs). The results suggest that there is an etiological relationship between mixed major depression-anxiety disorders and major depression only but no relationship between these two conditions and anxiety disorders only. When anxiety disorders with panic attacks were analyzed, the relationship between mixed cases and major depression only and the lack of a relationship between mixed cases, major depression only, and anxiety disorders only became even clearer. Furthermore, mixed cases seemed to be more strongly influence by genetic factors than was major depression only.