National income inequality and suicide mortality across 170 countries, 2000–2021: within-country evidence and sensitivity to exposure uncertainty, with implications for health communication
Abstract
Introduction Income inequality is a distal population exposure, and ecological panel data cannot identify individual-level suicide mechanisms. We examined whether within-country changes in national disposable-income inequality were associated with changes in suicide mortality and assessed how the estimated association depended on exposure uncertainty, outcome measurement, sample composition, population weighting, and temporal identifying assumptions. Methods We linked World Health Organization (WHO) age-standardized suicide rates with Standardized World Income Inequality Database (SWIID) disposable-income Gini estimates for 170 countries from 2000 to 2021 (3,257 country-years). The primary specification was a fully adjusted, unweighted two-way fixed-effects model with country-clustered standard errors. All 100 completed SWIID version 9.92 series were pooled using Rubin’s rules and Barnard–Rubin degrees of freedom. Sensitivity analyses examined population weighting, temporal specifications, alternative outcomes, reporting and measurement-quality subsets, and country influence. We also conducted an observed-panel simulation and equivalence testing within ±0.010 log-rate units per Gini point. Results The posterior-mean plug-in estimate was b = −0.0157 (95% CI, −0.0286 to −0.0027; p = 0.018), whereas the SWIID multiple-imputation estimate was b = −0.00831 (95% CI, −0.01688 to 0.00026; p = 0.057; fraction of missing information = 0.173). This sensitivity bracket was not treated as formal bounds and straddled the post hoc smallest effect size of interest of |b| = 0.010. Population- and log-population-weighted estimates were −0.00861 (95% CI, −0.02166 to 0.00443; p = 0.193) and −0.00864 (95% CI, −0.01718 to −0.00010; p = 0.047), respectively. Country-trend, region-year, first-difference, and dynamic estimates were smaller and less precise. All 100 completed-data coefficients and all 170 leave-one-country-out posterior-mean coefficients were negative, while subsets with narrower WHO uncertainty intervals.