This category differs from others in that it includes disorders identifiable on the basis of not only symptoms and course but also the existence of one or other of two causative influences: an exceptionally stressful life event producing an acute stress reaction, or a significant life change leading to continued unpleasant circumstances that result in an adjustment disorder. Although less severe psychosocial stress ("life events") may precipitate the onset or contribute to the presentation of a very wide range of disorders classified elsewhere in this chapter, its etiological importance is not always clear and in each case will be found to depend on individual, often idiosyncratic, vulnerability, i.e. the life events are neither necessary nor sufficient to explain the occurrence and form of the disorder. In contrast, the disorders brought together here are thought to arise always as a direct consequence of acute severe stress or continued trauma. The stressful events or the continuing unpleasant circumstances are the primary and overriding causal factor and the disorder would not have occurred without their impact. The disorders in this section can thus be regarded as maladaptive responses to severe or continued stress, in that they interfere with successful coping mechanisms and therefore lead to problems of social functioning.