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 n_eff ≥ 50; pooled and higher-income panels: n_eff ≥ 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: inc_cat = 1; higher-income: inc_cat ∈ {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;