The causal effects of transition into poverty on mental health in the UK working-age population
Abstract Background Addressing the impact of poverty through income and welfare policies is likely important for public mental health; however, few studies assess potential effect size using causally-informed methodologies. To provide meaningful information for policymakers, we aimed to estimate the average treatment effect of transitioning into poverty on mental health. Methods We used data for working-age adults (aged 25-64) from nine waves of the nationally representative UK Household Longitudinal Survey (2009-2019, n = 39,553 obs=155,963). Exposure was transition into poverty (household equiv. income <60% median). Outcome was score ≥4 on General Health Questionnaire-12, indicating likely common mental disorder (CMD). To minimise the influence of reverse causation and time-varying confounders we used a marginal structural modelling (MSM) approach to create inverse probability of treatment weights. We performed secondary analysis stratifying by sex, and calculated population attributable fractions for each model. Results Good balance of confounders was achieved between exposure groups. Experiencing new poverty was associated with increased odds of CMD (adjusted OR 1.33, 95% CI 1.24-1.42, p < 0.001) with a 4.8% (3.6-6.0) absolute difference in prevalence: 24.0% vs 19.2%. There was a difference in relative effect by sex, with OR 1.40 (1.25-1.56, p < 0.001) for men vs OR 1.28 (1.18-1.38, p < 0.001) for women. However, there was no marked difference in absolute effect by sex: 4.9% (3.1-6.8) for men vs 4.5% (3.0-6.1) for women. For all analyses traditional logistic regression using the same confounders underestimated the effect in comparison with MSM. Conclusions Moving below the poverty line increased odds of CMD by 32.7% after accounting for confounding and reverse causality, with a greater relative impact on men. Our causal estimates suggest transition into poverty currently accounts for 10.7% of the burden of CMD in the UK working-age population - 8.9% for women and 13.1% for men. Key messages Reductions in household income worsen mental health, and traditional analytical approaches may be underestimating this causal relationship. Applying causal methodologies to observational data can provide exposure-outcome estimates less susceptible to common biases, which may be of more use to policymakers.