Since the 1960's there has been a large volume of systematic research on the role of life events in psychiatric disorder. The life event literature now clearly indicates that the risk of depression is considerably increased following stressful life events. Type of life event is only weakly related to type of disorder. Those events, which are more generally stressful, are also more likely to produce disorder. Some disorders are more strongly associated with life events than others. Depression is one of these, with the possible exception o bipolar disorder .
The diathesis stress model shows that a wide range of psychological disorders result from the impact of an environmental event (stressor/stress factor) on a person who had a predisposing vulnerability .
Fig. 1. Diathesis stress model 
As the level of vulnerability increases, the degree of stress needed to precipitate a disorder decreases. Vulnerability is affected by biological and environmental factors.
Life events and onset of depression
There is a large body of evidence showing that stressful life events are closely associated with the onset of major depressive episodes [3, 4, 5]. Table 1 summarises findings of 27 retrospective controlled comparisons of psychiatrically treated depressed patients, one employing two comparison groups. 16 studies from 9 countries employed general population controls including two studies of elderly patients, which were conducted by Murphy and Emmerson et al [6,7]. All found more events reported prior to depressive onset, although in one study, with small numbers, the difference was not significant. One study conducted by Bebbington et al  found greater life events in depressed people in the community than in outpatients.
Table 1 Controlled comparisons of Life Events and Onset of Clinical Depression 
Nature of ControlsAuthorExcess any EventsExcess SeparationsExcess Other Types of Events
General PopulationPaykel et al  YesYesVarious, especially undesirable...