The official definition of post-traumatic stress disorder (PTSD) in DSM-III and is subsequent DSM editions is based on a conceptual model that brackets traumatic or catastrophic events from less severe stressors and links them with a specific syndrome. The diagnosis of PTSD requires an identifiable stressor and the content of the defining symptoms refers to the stressor, for example, re-experiencing the stressor and avoidance of stimuli that symbolize the stressor. Temporal ordering is also required: when sleep problems and other symptoms of hyperarousal are part of the clinical picture, they must not have been present before the stressor occurred. The ICD-10 definition of PTSD follows the same model. The defining symptoms alone, without a connection to the stressor, are not regarded as PTSD (Green et al. 1995). Since the introduction of PTSD in DSM-III, the official definition has been adopted in most studies, although discussions about the validity of the definition has continued (Breslau & Davis, 1987; Davidson & Foa, 1993; Green et al. 1995). Although it is widely believed that other disorders (e.g. major depression) can be precipitated by external events, these disorders can occur independent of stressors and do not require a link with a traumatic event in their diagnostic criteria. Previous classifications that separated major depression into stress-related (reactive) or endogenous have been abandoned in newer versions of the DSM, because of lack of evidence of the validity of this distinction.