The common themes that are shared by dissociative or conversion disorders are a partial or complete loss of the normal integration between memories of the past, awareness of identity and immediate sensations, and control of bodily movements. All types of dissociative disorders tend to remit after a few weeks or months, particularly if their onset is associated with a traumatic life event. More chronic disorders, particularly paralyses and anaesthesias, may develop if the onset is associated with insoluble problems or interpersonal difficulties. These disorders have previously been classified as various types of "conversion hysteria". They are presumed to be psychogenic in origin, being associated closely in time with traumatic events, insoluble and intolerable problems, or disturbed relationships. The symptoms often represent the patient's concept of how a physical illness would be manifest. Medical examination and investigation do not reveal the presence of any known physical or neurological disorder. In addition, there is evidence that the loss of function is an expression of emotional conflicts or needs. The symptoms may develop in close relationship to psychological stress, and often appear suddenly. Only disorders of physical functions normally under voluntary control and loss of sensations are included here. Disorders involving pain and other complex physical sensations mediated by the autonomic nervous system are classified under somatization disorder. The possibility of the later appearance of serious physical or psychiatric disorders should always be kept in mind.