Cumulative stressful life events and Australian women’s depressive symptom trajectories: a longitudinal study of potential non-additive effects

There is strong evidence that accumulation of stressful life events (SLEs) contributes to depressive episodes, but the effect of cumulative SLEs on longitudinal trajectories of depressive symptoms in the general population is less well understood. In a pre-registered analysis involving multi-group Bayesian piecewise growth curve modeling applied to nationally representative data from the Australian Longitudinal Study of Women's Health, we investigated whether, over 36 years, among women aged 45-50 at baseline, the effects of cumulative SLEs on depression symptoms are positive ("sensitizing") or negative ("steeling"), and additive or non-additive (interaction or threshold). Participants were grouped based on the number of SLEs experienced over the first 24 years of the study period: 0-4, 5-9, 10-14, 15-19, or 20-24. Groups were propensity-score-matched at baseline. While groups differed on mental and physical health at baseline, no group exhibited an increase in depressive symptoms following their final SLE. Moreover, groups with the most SLEs exhibited faster declines in depressive symptoms. There was significant heterogeneity of trajectories within groups. The findings contribute to the growing literature supporting the steeling hypothesis, and suggest that, if SLEs increase the severity of depressive symptoms, they do so in small sub-populations that require further research attention.