The DSM-IV lists 13 anxiety-related complaints, including (amongst others) generalized anxiety disorder, social phobia and panic disorder. Most of the anxiety disorders listed share common cognitive (worry, apprehension), behavioural (avoidance, escape) and physiological (sweating, hyperventilation) features. Similarly, panic attacks commonly occur in the context of anxiety disorders, regardless of the specific diagnosis. There is a high degree of overlap and co-occurrence between anxiety disorders, and in some cases, for example panic disorder and social phobia, the differential diagnosis can in theory be quite complex and convoluted. In addition to the high degree of overlap between anxiety disorders, there is also a high rate of co-morbidity with mood disorders. For instance, panic disorder is occasioned by major depressive disorder in at least 10 per cent of cases, although some estimates of co-morbidity run as high as 65 per cent. In around one third of these cases depressive onset comes first (DSM-IV-TR, 2000).