There has been extensive discussion and research on the associations among life stress, depression, and recurrence of the disorder. Although the results of a number of studies underscore the importance of severe life events for first lifetime episodes relative to recurrences, few investigators have pursued this issue by examining the prevalence of major life events over specific recurrences (e.g., first, second, third). Not only did the present study examine major life events in relation to such a differentiated history of depression, but further, it is one of the few investigations to also examine chronic stress to understand differences between first lifetime episodes of depression and recurrences with respect to life stress. Our results establish the independent association of both forms of severe stress with the onset of a depressive episode and suggest that each form of stress plays a different role with regard to the lifetime history of depression.
As predicted for acute life events, severe events are significantly more likely to precede a first lifetime episode than all recurrences combined. This finding is reinforced and expanded when recurrences are classified more specifically by episode number. The changing association between severe events and recurrence persists well past a first lifetime recurrence (see ). These data are consistent with the premise that severe events continue to be of etiological importance well after the first lifetime episode but that such events become a less common contributor to recurrence for participants with a greater history of depression. Indeed, if the etiologic effects of severe events remain germane for individuals with up to three lifetime episodes of depression, then severe events are of causal significance for the majority of people estimated to suffer from MDD in the general population (American Psychiatric Association, 1994
; Monroe & Harkness, 2005
). As other causal arrangements or pathways become more influential in the etiological picture with repeated recurrences, they may increasingly supplant, but not completely replace, the causal influence of major events. This process suggests that with repeated recurrences, there will be diminishing statistical power in traditional research designs for detecting the impact of the increasingly infrequent major events. Investigators will need to target the question of impact more specifically, examining the likelihood of recurrence given the occurrence of a major event as a function of depression history (see Monroe & Harkness, 2005
Although previous work has shown that chronic difficulties are associated with the onset of depression (Brown & Harris, 1978
), there has been a paucity of research on this topic, especially compared with the extensive research on major life events (Hammen, 2005
). Few studies have examined chronic difficulties in relation to first episode versus recurrence considerations (cf. Daley et al., 2000
). The present findings suggest that major difficulties are a relatively small factor in accounting for episode onset overall (given the relatively low incidence of severe difficulties in the full sample, 7.3%). Yet the pattern of these data is intriguing. On the one hand, these findings suggest that major difficulties are not of great importance for people with one or with just a few lifetime episodes. If this proves to be the case, it would help to direct and sharpen the focus on acute major events in relation to the majority of episodes of depression in the population at large. On the other hand, people with multiple episodes of depression are among the most vulnerable and impaired, requiring special attention to arrest the continued progression of pathology in their lives. Severe chronic difficulties may be a large factor in bringing about such especially frequent recurrences and may provide an important focus for intervention. In this light, understanding how severe difficulties originate and how they may contribute to the recurrence of depression would be useful research avenues to pursue.
These findings for major events and major difficulties together help address the question of what changes over successive recurrences with respect to life stress. The design of the present study does not permit a direct test of causal premises (i.e., currently nondepressed persons with and without a history of depression were not assessed in the present study). And as noted above, future work will need to address whether, in addition to the changing frequency of events as a function of depression history, there is a diminishing impact of such stressors for triggering depression. Nevertheless, it is in this context of understanding possible changes in impact of life stress over time that the present findings for severe events and difficulties are of particular interest. In contrast to the decreasing association demonstrated for severe life events, severe chronic difficulties were more common for individuals with many prior depressive episodes. (Indeed, not a single person with fewer than three lifetime episodes reported a severe difficulty.) Such results suggest one possible mechanism through which the role of life stress may change over repeated recurrences: The overall contribution of severe events may lessen, whereas the emerging contribution of severe difficulties may increase.
The greater prevalence of severe difficulties as lifetime history of depression increases is consonant, too, with the stress-generation hypothesis (Hammen, 1991
). For instance, it might be expected that with many lifetime episodes, adversities accrue and become more enduring. As these chronic stressors become major problems in themselves, they likely spawn additional adverse life events. Such processes involving the generation of both chronic and acute events would clearly increase the total stress in the lives of people who suffer from recurrent depression. The question then arises as to the potential of varied types and severities of stressors for precipitating recurrences of depression (i.e., many types of life stress may be undesirable and psychologically noxious but not necessarily capable of triggering clinical depression; Harkness et al., 1999
; Monroe & Simons, 1991
). Although the stress-sensitization model and stress-generation model can be viewed as providing alternative explanations of the increasing likelihood of recurrence with repeated depressive episodes, these two models can also be viewed as mutually reinforcing. That is, as repeated depressions lead to increasing stress (stress generation), and as lesser severities of stress acquire the capability of triggering recurrence (stress sensitization), these two processes combine to render the person with recurrent depression especially susceptible to another episode.
Given the potential importance of both stress generation and stress sensitization, it is useful to speculate about such matters from the present data. In this vein, it is interesting to consider that the increased incidence of severe difficulties in the highly recurrent group reflects the continued potential of major stress to initiate new episodes of depression. This interpretation reinforces the idea that it is change in the frequency, rather than in the impact, of severe events that accounts for the lower prevalence of severe events prior to recurrence as lifetime history of depression increases. Although such inferences are consistent with the present data, they are not based on strong tests of the hypotheses. Future work that includes life events and difficulties of differing severity levels is needed to provide more definitive evidence of the mechanisms through which severe events may be replaced in triggering the recurrence of depression and how the stress-generation and stress-sensitization models may play out with respect to one another over time (Hammen, 2005
; Monroe & Harkness, 2005
Comparisons with other studies are limited, given the lack of detailed information in other studies on major stress and the onset of depression over successive lifetime episodes. With regard to acute major events, Kendler, Thornton, and Gardner (2000)
found the odds ratio between a major event and depression onset to decrease progressively from a first lifetime episode over successive recurrences. They noted that the association continued to decrease up to nine episodes. Although Kendler et al.’s findings are important for documenting the general changes between major events and onset as a function of depression history, they do not illuminate the possible underlying mechanisms. As we have indicated, it is not clear whether such findings are accounted for by a change in frequency or by a change in impact of major events. The present results provide a useful complement to Kendler et al.’s data, revealing that the diminishing frequency of major events with increasing lifetime history of depression is an important factor for understanding the overall changing association between major stress and recurrence over time. These data again lend some, albeit limited, support to the stress sensitization premise. For example, if we had found a constant rate of severe events for people with progressively more lifetime episodes, it would suggest that Kendler et al.’s findings were due to a diminishing impact of major events over successive recurrences. Again, future work will need to target the frequency and impact considerations more directly (Monroe & Harkness, 2005
Only one other study has addressed the association of major chronic difficulties with regard to the distinction between first lifetime onsets and recurrences.Daley et al. (2000)
found history of previous depression to interact with chronic stress. For women without a history of previous depression, chronic stress significantly predicted a first lifetime onset of depression over a 5-year period. In contrast, for women with a history of previous depression, chronic difficulties did not predict a new recurrence. Although these data appear to be inconsistent with the present findings, it is important to consider differences between these two studies in their designs and methods. With respect to design, the present study focused on major stress in relation to a differentiated recurrence history of depressed persons. The most direct comparison of the present study with Daley et al.’s report involves the chronic stress means for women experiencing a first lifetime onset of depression (M
= 14.50, SD
= 2.01) versus women experiencing any recurrence of depression (M
= 14.06, SD
= 2.53). Thus, although chronic difficulties differentially predicted a first lifetime episode of depression in Daley et al.’s study, chronic difficulties did not appear to diminish appreciably for recurrences overall.8
Moreover, given that Daley et al.’s sample was composed of relatively young women (mean age = 18.29 years at study entry), it is unlikely that there were many people with an extensive history of recurrences, the very people who evidenced a higher incidence of severe difficulties in the present study. Thus, without more differentiated information about past history of depression, and with such a relatively young sample, it is difficult to derive firm conclusions regarding comparisons with the present study.
Appreciating the differences in the methods used in the two studies may also be informative for future work. Chronic difficulties in Daley et al.’s (2000)
study were represented by a total chronic stress score, summed across five life domains for the 3-month period prior to onset (and a matched period for controls). In the present study, chronic stress was represented by the presence or absence of a severe difficulty (which is defined by high threat and lasting a minimum duration of 2 years prior to depression onset; Brown & Harris, 1978
). Thus, the two studies differed in terms of severity and duration requirements for the chronic stress indicators. Given the 3-month time frame of Daley et al.’s investigation for defining chronic stress, it is likely that their measure included more recent and acute stress features than did our measure, which, in turn, could help to explain the differential association with first onset versus recurrence. At the present stage of knowledge, it is premature to judge which approach is more valid or useful. Clearly, more studies using similar high quality measures need to take into account the potential importance of chronic difficulties for depression onset and recurrence (Hammen, 2005
Secondary analyses on independent and possibly dependent events indicate that it was largely the latter category that was significant with regard to lifetime history of depression. This might be in part due to the greater frequency of possibly dependent events in general (and in the present sample, 7 independent severe events vs. 18 possibly dependent severe events). It is important to note, however, that further analysis suggests that, once independent severe events were controlled for statistically, the effects for possibly dependent events on history of depression remained significant and strong. It is also noteworthy that other research has found the independent (or “fateful”) events to be especially strong for predicting depression in general (i.e., for first onsets and recurrences; Shrout et al., 1989
). It may be that events that are in part related to the behavior of the person are especially pertinent for first lifetime episodes of depression. Finally, we also investigated the interaction between severe events and severe difficulties. There was no indication that either form of stress operated in a manner conditional upon the other. Consequently, it appears that each form of severe stress is significant and independent in its relations to depression history.
The strengths of the present study include a well-characterized sample that is informative for a wide range of reported prior depressions. The assessment of life stress was based on one of the most reliable and valid systems available and included rigorous assessment of both major life events and major ongoing difficulties. Study inclusion criteria ensured a sample of individuals with a relatively recent and clear onset of a depressive episode (i.e., participants were relatively free of symptoms and had no depressive episode for 6 months prior to the index episode, thereby ensuring the sample was not confounded with chronic depression). These results provide important statistical and descriptive information for better understanding the relation between severe events and difficulties with regard to the onset of depression over successive recurrences. To test causal processes more directly, future researchers will need to include currently nondepressed persons, especially nondepressed persons with varied histories of total lifetime depressive episodes.
In addition, it is important to note that the present work, and the vast majority of the work in the literature, is based on a between-subjects analysis of life stress and recurrence. That is, associations for life stress in relation to differing histories of MDD are based on different groups of individuals with different histories of MDD. The design of the present study does not permit one to rule out the possibility that something about the different groups, and not necessarily changes in relation to stress over successive recurrences, is of relevance with regard to stress. Ideally, future research will also include a longitudinal within-subject component to test the changing association between life stress and recurrence over repeated episodes for the same individuals (see Kendler et al., 2000
). To justify the intensive research demands that accompany such longitudinal research, however, a firm foundation of supportive empirical evidence needs to be assembled. The present findings represent an important contribution toward this goal.