摘要:Objectives. We assessed the impact of changes in dimensions of the psychosocial work environment on risk of depression in a longitudinal cohort of Canadian workers who were free of depression when work conditions were initially reported. Methods. Using a sample (n = 3735) from the Canadian National Population Health Survey, we examined the effects of changes in job control, psychological demands, and social support over a 2-year period on subsequent depression. We adjusted models for a number of covariates, including personal history of depression. Results. Respondents with increased psychological demands were more likely to have depression over the following 2 years (odds ratio = 2.36; 95% confidence interval = 1.14, 4.88). This risk remained statistically significant after adjustment for age, gender, marital status, presence of children, level of education, chronic health conditions, subclinical depression when work conditions were initially assessed, family history of depression, and personal history of depression. Conclusions. These results demonstrate that changes in psychological demands have a stronger influence than changes in job control on the onset of depression, highlighting the importance of not assuming an interaction between these 2 components of job strain when assessing health outcomes. Major depression is 1 of the top 3 causes of disability burden in high-income countries. 1 The burden of depression could be reduced by identifying predictors of the disease that are amenable to change and then intervening accordingly. Job strain has been identified as one such predictor. 2 Job strain refers to a situation where job control (people's ability to make decisions and use their skills at work) is low and the job's psychological demands (the pace and mental intensity of work) are high. 3 Although cross-sectional and longitudinal research has demonstrated that job strain and its components are related to an increased risk of depression, 2,4,5 this does not constitute evidence that changing these conditions would result in changes in the risk of depression (for better or worse). Yet, from both an organizational and public policy perspective, evidence that changes in psychosocial working conditions are associated with subsequent increased (or decreased) risk of depression is important if the potential mental health effects are to be accounted for when making decisions that will affect the psychosocial work environment. Ideally, this evidence would be generated from trials (preferably randomized). In lieu of trial data, longitudinal survey data where job strain is measured at 2 or more time points can be used to explore the impact that naturally occurring changes in job strain have on the risk of depression. Four longitudinal studies have examined whether naturally occurring changes in job strain were associated with changes in the risk of both depression and psychological distress. 6–9 The findings of all 4 studies suggested that beneficial changes (reductions) in job strain resulted in a lower risk of depression; however, the effect sizes were small in 3 of the 4 studies, and statistical significance was achieved in only 1 study. 8 In addition, each of these studies suffered from important methodological limitations in their design, which may have biased the findings reported. None of the previous studies took into account the day-to-day variability in job strain scores when assigning respondents to exposure groups. In each study, a respondent was classified as “exposed” to a change in job strain if his or her job strain score crossed a particular threshold between time points, regardless of the actual size of the change in the score between baseline and follow-up. As a result, some respondents were classified as “exposed” when the change was no greater than the day-to-day variability observed for the score, whereas other respondents were classified as “unexposed” when the change was greater than this variability. These classifications could dilute the “exposed” group, making it more similar to the “unexposed” group with respect to risk of depression (and vice versa). In other words, the use of thresholds in previous studies may have resulted in nondifferential misclassification across exposure groups that, in turn, may have produced an underestimate of the true effect of changing job strain on the risk of depression. 10–12 Most previous studies focusing on change in job strain have not considered the potentially different impacts of changes in the underlying components of job strain (job control and psychological demands). In addition to job control and psychological demands, it is also important to examine changes in social support because previous work has linked this psychosocial characteristic of work to an increased risk of depression. 7,9,13,14 Studies that focused on job strain (but not changes in job strain) have found that these components pose different risks for depression, with high psychological demands more strongly associated with the risk for depression than low job control. 15–17 Accordingly, a change in job demands may have a larger effect on depression than a change in job control. If this is the case, investigations focused only on changes in job strain (without consideration of beneficial or adverse changes in the underlying components) would result in an underestimate of the true potential impact of efforts to reduce job strain as a strategy for preventing depression. Only 1 of the 4 studies explored this possibility, reporting that changes in psychological demands had a stronger effect on the risk of depression than changes in job control, although neither change produced a statistically significant effect. 9 Because of data availability, each of the previous change studies had different time lags between when the change in job strain occurred and the measurement of subsequent depression (or depressive symptoms). Stansfeld et al. 9 allowed 2 to 7 years to elapse between the change in job strain and the measurement of depression, whereas Wang et al. 8 allowed 1 to 10 years. Research examining the effect of work conditions on mental health has demonstrated that different time lags can result in different study findings, with the strongest relationships between work and mental health found over 1- to 2-year periods. 18–20 As a result, 4 previous studies in this area of research have likely underestimated the effect of the change by collapsing short and long lag periods. The study design by de Lange et al. allowed for the risk of depression at year 1 to be attributed to changes in job strain that took place after depression onset. 6 Bourbonnais et al. 7 took their follow-up measurement of job strain at the same time that they assessed depression, which may have resulted in a spurious, or elevated, correlation between the 2 measures, because of the likely impact that depression has on self-reported job strain assessments. 9 Finally, each of the previous change studies failed to adjust for personal history of depression, which is a potentially important confounder. Personal history of depression is the strongest predictor of a future episode of depression. 21 Because it is plausible that a personal history of depression could also increase the risk for negative changes (increases) in job strain, this history may confound the relationship between changes in job strain and the risk of depression. Some authors have suggested that respondents with a previous history of depression should be removed from analyses focused on the impact of change in psychosocial working conditions on the risk of depression. 13 Although removal of respondents who have a potential confounder is one method to deal with confounding, this approach has the disadvantage of creating a hypothetical sample of the working population (i.e., only those workers without any previous history of depression). 22,23 The data used in our analysis (from the National Population Health Survey) demonstrate that up to 20% of the working population who are not currently depressed have a personal history of depression. Therefore, excluding these respondents from the analysis creates a sample of workers that is no longer representative of the working population. From a population health perspective, a more reasonable approach to dealing with confounding is to include personal history of depression as a covariate in multivariate analyses. We have designed our study to overcome the limitations of previous work by (1) focusing on psychosocial work environment changes that are greater than those expected as a result of day-to-day variability; (2) examining the separate impacts of job control, psychological demands, and social support in addition to job strain; (3) assessing change in work environment only among respondents not currently depressed; and (4) adjusting for the potential confounding effects of personal history and family history of depression. By overcoming these limitations, we aimed to generate more accurate estimates of the effect of changes in the psychosocial work environment on the risk of depression.