Medicaid Expansion and Frequent Mental Distress in US Adults: Staggered Difference-in-Differences Evidence From BRFSS, 2011–2024

Background: The Affordable Care Act enabled states to expand Medicaid eligibility, potentially improving access to care and reducing psychological distress among low-income adults. Evidence on population-level mental health effects remains inconclusive, and conventional two-way fixed-effects difference-in-differences can be biased when adoption is staggered. Objective: To estimate the effect of Medicaid expansion on frequent mental distress (FMD) among non-elderly adults, with emphasis on low-income adults.Methods: We used Behavioral Risk Factor Surveillance System (BRFSS ) data from 2011 to 2024 and restricted the sample to adults aged 18–64 years. FMD was defined as reporting 14 or more days of poor mental health in the past 30 days. Medicaid expansion timing was assigned at the state level. For computational feasibility and transparent weighting, data were collapsed to the state-year level using BRFSS survey weights and screened for minimum unweighted cell size and effective sample size in the low-income panel. We estimated group-time average treatment effects using a staggered difference-in-differences framework implemented via csdid, clustering standard errors at the state level, and using pair-balanced observations when panels were unbalanced. We report pooled average effects and cohort-specific dynamic patterns. Results: In the low-income state-year panel (353 observations), Medicaid expansion was not associated with a statistically significant change in frequent mental distress (ATT −0.0125; 95% CI −0.0521 to 0.0271; p=0.54); estimates in the high-income and pooled panels were similarly small and not significant, while event-time estimates showed modest heterogeneity at selected leads/lags.Conclusions: Medicaid expansion was not associated with significant reductions in frequent mental distress at the population level during 2011–2024, though modest, time-varying effects may occur in specific expansion cohorts. Findings suggest that insurance expansion alone may be insufficient to meaningfully shift population psychological distress without complementary mental health and social support interventions.