To investigate whether job strain interacts with psychosocial factors outside of the workplace in relation to the risk of major depression and to examine the roles of psychosocial factors outside of the workplace in the relationship between job strain and the risk of major depression.

Data from the longitudinal cohort of the Canadian National Population Health Survey (NPHS) were used. Major depressive episode (MDE) in the past 12 months was assessed by the Composite International Diagnostic Interview-Short Form. Participants who were working and who were between the ages of 18 and 64 years old in 2000/2001 (n = 6,008) were followed to 2006/2007. MDE that occurred from 1994/1995 to 2000/2001 were excluded from the analysis.

High job strain, negative life events, chronic stress and childhood traumatic events were associated with the increased risk of MDE. There was no evidence that job strain interacted with psychosocial factors outside of the workplace in relation to the risk of MDE. The incidence proportion in participants who reported having exposed to none of the stressors, one type of stressor, two types of stressors and three or more types of stressors was 2.6, 4.3, 6.6 and 14.2%, respectively. The odds of developing MDE in participants who were exposed to three or four types of stressors was more than four times higher than the reference group.

MDE may be facilitated by simultaneous exposure to various stressors. There is a dose–response relationship between the risk of MDE and the number of stressors.