The causal effect of trade unions on workers’ health: A parametric g-formula approach using longitudinal data from the Panel Study of Income Dynamics (PSID)

Background Trade unions are increasingly recognized as public health actors. They both protect workers’ health by ensuring workplace health and safety and, indirectly, by providing economic advantages to their members. However, previous research has not addressed the direct and indirect causal pathways between trade unions and health and only focused on membership, omitting the important role that the presence of trade union has within the workplace.

We used two decades of nationally representative longitudinal data from the United States (PSID 2001-2021; ≈26,000 individuals; ≈117,000 person-years), used Exploratory Causal Discovery to detect causal pathways and applied parametric g-formula modelling to account for time-varying confounding. Findings Sustained union membership was associated with lower psychological distress (Mean Ratio – MR 0.944; 95% CI 0.916–0.974), with a persistent direct effect after adjusting for income, working hours, and housing (MR 0.937; 95% CI 0.907–0.968). Always workplace union presence showed an association (MR 0.972; 95% CI 0.945–0.999) that attenuated after adjusting for mediators. Improvements in self-reported health were primarily mediated by income and working-hours pathways (always union membership total adjust MR 0.991; 95% CI 0.983–1.000; always union presence total adjust: MR 0.990; 95% CI 0.982– 0.998). Interpretation By examining both union membership and workplace union presence, this study captures the individual and institutional dimensions of unionization that have not been simultaneously addressed in prior research. It highlights the significant role trade unions play in improving workers’ health and the threat the erosion of collective bargaining in the US might pose for population health. Competing Interest Statement The authors have declared no competing interest. Clinical Protocols https://doi.org/10.17605/OSF.IO/QDXTF Funding Statement This study was funded by: The Belgian National Fund for Scientific Research (FNRS), Incentive Grant for Scientific Research (MIS), NegHealth, 40021242, beneficiary: Jacques Wels The Belgian National Fund for Scientific Research (FNRS), Research Associate (CQ), 40010931, beneficiary: Jacques Wels The UK Research and Innovation (UKRI) Guarantee funding for Horizon Europe ERC grant, UHealth, UKRI1426, beneficiary: Jacques Wels Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: This study only used existing, de-identified, publicly available data from the Panel Study of Income Dynamics (PSID) and did not require ethical approval. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes Data Availability The Panel Study of Income Dynamics (PSID) are freely available at: https://simba.isr.umich.edu/data/data.aspx