Medicaid Expansion and Frequent Mental Distress Among U.S. Adults: Staggered DiD Using BRFSS, 2011 to 2024

The Affordable Care Act (ACA) allowed states to expand Medicaid eligibility, potentially improving access to care and reducing psychological distress among lower-income adults. However, evidence on population-level mental health effects remains mixed, and conventional two-way fixed-effects difference-in-differences (DiD) estimates can be biased under staggered adoption. This study estimates the effect of Medicaid expansion on frequent mental distress (FMD) among non-elderly adults, with emphasis on lower-income adults. We used Behavioral Risk Factor Surveillance System (BRFSS) data from 2011 to 2024 and restricted the sample to adults aged 18 to 64 years. FMD was defined as ≥14 days of poor mental health in the past 30 days. Medicaid expansion was assigned by state implementation year. For transparent weighting and computational feasibility, we collapsed microdata to state-year prevalence using BRFSS survey weights and applied prespecified minimum cell-size and effective-sample screens (lower-income panel: unweighted N ≥ 150 and Kish nₑff ≥ 50; pooled and higher-income panels: nₑff ≥ 200). We estimated group-time average treatment effects using the Callaway-Sant'Anna staggered DiD framework (csdid), clustered standard errors at the state level, and used pair-balanced observations when panels were unbalanced. Income strata were defined using BRFSS household income categories (lower-income: inccat = 1; higher-income: inccat ∈ 7, 8) and are interpreted as economic-status proxies rather than Medicaid eligibility. In the lower-income state-year panel (353 state-years), Medicaid expansion was not associated with a statistically significant change in FMD (ATT -0. 0125; 95% CI -0. 0521 to 0. 0271; P =. 54). Estimates in the higher-income (656 state-years) and pooled (709 state-years) panels were similarly small and not statistically significant. Event-time estimates indicated modest heterogeneity at selected leads/lags without a consistent post-expansion pattern. Medicaid expansion was not associated with significant reductions in population-level frequent mental distress during 2011 to 2024. Any mental health effects appear modest and heterogeneous across cohorts and follow-up periods, suggesting that insurance expansion alone may be insufficient to shift population psychological distress without complementary behavioral health capacity and upstream social supports.