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)

BackgroundTrade 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. MethodsWe used two decades of nationally representative longitudinal data from the United States (PSID 2001-2021; {approx}26,000 individuals; {approx}117,000 person-years), used Exploratory Causal Discovery to detect causal pathways and applied parametric g-formula modelling to account for time-varying confounding. FindingsSustained 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). InterpretationBy 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.