National income inequality and suicide mortality across 170 countries, 2000-2021: within-country evidence and sensitivity to exposure uncertainty, with implications for health communication.

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;