Background This study examined the prospective association of workplace social capital (WSC) with major depressive episode (MDE) among Japanese employees.
Methods A 3-year prospective cohort study was conducted among 1058 employees from a private think-tank company who participated in a baseline survey; after excluding those with MDE in the past 12 months, 929 were followed up. WSC at baseline was measured using a 3-item scale. MDE was assessed at baseline and at follow-up every year, by using a web-based, self-administered version of the WHO Composite International Diagnostic Interview (WHO-CIDI) 3.0 depression section, based on the Diagnostic and Statistical Manual of Mental Disorders Fourth Edition: Text Revision (DSM-IV-TR)/DSM-5 criteria. Cox discrete-time hazards analyses were used to estimate HRs and 95% CIs, adjusting for covariates.
Results A group with middle-level WSC scores had the lowest risk of MDE after being fully adjusted (HR 0.34, 95% CI 0.14 to 0.84, p=0.02). The relationship between WSC and MDE was U-shaped, although a non-linear model fit better than a linear model, with only marginally statistical significance (p=0.06). Dichotomised and continuous variables of WSC scores were significantly and negatively associated with MDE (p=0.03 and p<0.01, respectively).
Conclusions The current study replicated a previous finding from Finland that WSC was a protective factor of the onset of MDE in Japan. The slightly U-shaped relationship, that is, the group with high WSC having a small elevated risk of MDE, may reflect a dark side of WSC in a country with collectivity-oriented and hierarchy-oriented culture, such as Japan.
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Contributors ASa, KI, NK, AI, ASh, AT, MT and TT conceived and designed the experiments. AI and TT performed the experiments. ASa, KI and NK analysed the data. ASa, KI and NK wrote the paper.
Funding This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas (Research in a Proposed Research Area) 2009–2013 (No. 4102-21119001) from the Ministry of Education, Culture, Sports, Science and Technology, Japan (http://www.mext.go.jp/a_menu/shinkou/hojyo/main5_a5.htm), the KAKENHI grant numbers 26253042 and 26860433 from the Japan Society for the Promotion of Science (https://www.jsps.go.jp/j-grantsinaid/), and the Health and Labor Sciences Research Grant 2015–2017 (H27-Rodo-Ippan-004) from the Ministry of Health, Labour and Welfare, Japan. (http://www.mhlw.go.jp/stf/seisakunitsuite/bunya/hokabunya/kenkyujigyou/index.html).
Competing interests None declared.
Patient consent Obtained.
Ethics approval The study procedure was approved by the Research Ethics Committee of the Graduate School of Medicine and Faculty of Medicine, the University of Tokyo (No. 2772).
Provenance and peer review Not commissioned; externally peer reviewed.
Data sharing statement Since the data are still in process to transfer to a data archiving organisation, the ad hoc committee chaired by AT is taking care of this role. Data are from the occupational cohort study on social class and health conducted in Japan (Japanese Study of Health, Occupation, and Psychosocial Factors Related Equity: J-HOPE) whose authors may be contacted at firstname.lastname@example.org.
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