Causal relationship between stressful life events and the onset of major depression

Objective: Stressful life events are associated with the onset of episodes of major depression. However, exposure to stressful life events is influenc ed by genetic factors, and these factors are correlated with those that pre dispose to major depression. The aim of this study was to clarify the degre e to which stressful life events cause major depression. Method: The author s assessed the occurrence of 15 classes of stressful life events and the on set of DSM-III-R major depression over a 1-year period in female twins asce rtained from a population-based registry. The sample contained 24,648 perso n-months and 316 onsets of major depression. Stressful life events were ind ividually rated on contextual threat and dependence (the degree to which th e stressful life event could have resulted from the respondent's behavior). The nature of the relationship between stressful life events and major dep ression was tested by 1) discrete-time survival analysis examining the rela tionship between dependence and the depressogenic effect of stressful life events and 2) a co-twin control analysis. Results: While independent stress ful life events were significantly associated with onsets of depression, wh en level of threat was controlled, the association was significantly strong er for dependent events. The odds ratio for onset of major depression in th e month of a stressful life event was 5.64 in all subjects, 4.52 within diz ygotic pairs, and 3.58 within monozygotic pairs. Conclusions: Stressful lif e events have a substantial causal relationship with the onset of episodes of major depression. However, about one-third of the association between st ressful life events and onsets of depression is noncausal, since individual s predisposed to major depression select themselves into highrisk environme nts.