Much evidence validates the link between onset of a major depressive episode (MDE) and prior stressful life events, particularly major undesirable events.[1–3]
However, the predominant research focus is on episodic (acute) life events, and commonly ignores the co-occurring effects of chronic, ongoing stressful conditions on depression. Examining episodic events that have relatively discreet beginnings and endings may tell only part of the story of the stress–depression association, as stressful life events occur within the ongoing conditions of a person’s life. For many, daily life experiences include continuing negative environmental circumstances (chronic stress) such as poor working conditions; financial difficulties; absent, intermittently or chronically unfulfilling or conflictual intimate relationships with romantic partners, parents, children, or friends; continuing health problems; and other ongoing burdens. Failure to assess and evaluate chronic stress in the stressor–depression relationship may mean that an important source of variability in the nature and strength of the association has been neglected.
It is also possible that depressive phenomena attributed to discrete stressful life events may reflect unmeasured associations with the ongoing chronic stress. Moreover, increasing research attention to the effects and mechanisms of stress includes topics such as the neurobiology of stress, gene–environment interactions, allostatic load, stress sensitization or “kindling,” and stress generation.[5–9]
Research in these topics suggests somewhat different neurobiological and psychosocial processes, properties, and consequences of acute versus chronic stress.
Failure to distinguish between the effects of acute versus ongoing stressful conditions may impede efforts to fully understand the mechanisms by which depression results from stressors. Many stress–depression studies using questionnaires and checklists do not clearly distinguish between acute and chronic stress. Some of the early work using the well-validated Life Events and Difficulties Schedule[1,10]
distinguished between severe “ongoing difficulties” and severe life events in their association with depression, but commonly operationalizes “provoking agents” as the occurrence of either
a severe life event or
an ongoing difficulty. There is a large volume of research on the depressive effects of chronic adversities such as marital conflict, parent–child discord, chronic illness or illness of a close family member, and social disadvantage (e.g., poverty, racial discrimination, neighborhood dysfunction, single motherhood).[11–14]
Although such research certainly supports the idea of a link between these chronic stressors and distress, the focus is typically on life circumstances in a single domain. A limited focus does not provide a broad picture of chronic stress across multiple areas of a person’s life.
Also, research has not sufficiently evaluated the independent or joint effects of chronic and acute stress. An important exception was Brown and Harris’s
demonstration that likelihood of depression is increased with an acute event that “matches” an ongoing difficulty. A limited number of other studies have examined both types of stressors, and have suggested that the effects of acute stress–depression associations may be modified by chronic stress
or that the effects of the two types of stress may differ.[16–18]
Given the relative paucity of the research investigating both chronic and acute stress in the same investigation, several issues warrant further investigation. One goal of this study is to evaluate empirically the association between chronic stress and major depression, in addition to acute stress and depression. It is possible that research that focuses solely on acute stressors has underestimated or misattributed the link between stress and depression. Thus, an unresolved issue is whether and to what extent both chronic and acute stress have independent associations with depression such that our understanding of the stress–depression link is incomplete if both are not evaluated.
Furthermore, research on possible functional relationships between chronic and acute stress is needed. For instance, higher levels of chronic stress may portend higher levels of acute life events (such as a chronic health problem causing loss of a job or chronic marital difficulties giving rise to a bitter divorce). Turner and Turner
reported on predictors of stressors in a large community sample, and found results consistent with the idea that chronic difficulties and contextual factors contribute to the occurrence of acute stressors. The effects of chronic and acute stress may be independent of each other; both may have an impact, whereas neither moderates the other’s effects. An alternative is that chronic stress increases the depressive consequences of acute life events, a form of “sensitization” in which individuals who are challenged by high levels of chronic stress might be more likely to become depressed after an acute stressor.
A further model suggests that chronic stress decreases the effects of acute life events, a “steeling” or protective effect. McGonagle and Kessler
and Cairney et al.
found some support for the idea that higher chronic stress appeared to reduce the effects of acute stress on depression, arguing that “saturation” results from ongoing adversity such that acute negative events have less impact.
This study examines the impact of both chronic stress and acute stress to address the issues described. As noted, chronic stress in prior studies has been variously defined as a specific ongoing environmental condition (such as marital discord, caretaker burden) or as a stressor with enduring impact (defined by Brown and Harris
as 4 weeks, by Dougherty et al.
as 6 months, and by McGonagle and Kessler
as 12 months). Consistent with recommendations by Mazure,
we assess chronic stress as typical, ongoing conditions over at least a 6-month period across multiple domains of role functioning typical of adults. The goal is to capture the ongoing conditions of life in the salient aspects of individuals’ environments to provide a reasonably comprehensive profile of experiences. As Mazure notes, unless stressors are sampled from a broad array of domains, limited assessments will underestimate the true role of stress in the prediction of depression.
This study employs a large community sample of adult women, in which participants were initially selected to represent a wide array of histories (or no history) of depression. This study addresses the following questions:
- Are chronic and acute stress independently associated with onset of major depression?
- What are the functional relationships between levels of chronic and acute stress?
- Are higher levels of chronic stress associated with the occurrence of more acute stressors? Further, is chronic stress a risk factor for stress generation, the occurrence of acute events that are at least partly caused by the person?
- Does chronic stress moderate the effects of acute stress on the probability of depression. More specifically, is there a “sensitization” effect (the association between acute stress and depression becomes stronger with increases in chronic stress) or a “steeling” effect (the association between acute stress and depression decreases as women experience greater chronic stress)?