Quasi-Experimental Study of...
The Effects of Medicaid Expansions on Mortality of Low-Income Adults
Reviewed
NBER (April 2025) posted a quasi-experimental study of the effects of state Medicaid expansions on the mortality of low-income adults. Despite the study abstract’s claims of a 2.5% reduction in mortality (covered in NY Times and elsewhere), the study found much smaller mortality effects that fell short of statistical significance in its main preregistered analysis.
We examine the causal effect of health insurance on mortality using the universe of low-income adults, a dataset of 37 million individuals identified by linking the 2010 Census to administrative tax data. Our methodology leverages state-level variation in the timing and adoption of Medicaid expansions under the Affordable Care Act (ACA) and earlier waivers and adheres to a preregistered analysis plan , a rarely used approach in observational studies in economics. We find that expansions increased Medicaid enrollment by 12 percentage points and reduced the mortality of the low-income adult population by 2.5 percent, suggesting a 21 percent reduction in the mortality hazard of new enrollees. Mortality reductions accrued not only to older age cohorts, but also to younger adults, who accounted for nearly half of life-years saved due to their longer remaining lifespans and large share of the low-income adult population. These expansions appear to be cost-effective, with direct budgetary costs of $5.4 million per life saved and $179,000 per life-year saved falling well below valuations commonly found in the literature. Our findings suggest that lack of health insurance explains about five to twenty percent of the mortality disparity between high- and low-income Americans. We contribute to a growing body of evidence that health insurance improves health and demonstrate that Medicaid’s life-saving effects extend across a broader swath of the low-income population than previously understood.
We examine the causal effect of health insurance on mortality using the universe of low-income adults, a dataset of 37 million individuals identified by linking the 2010 Census to administrative tax data. Our methodology leverages state-level variation in the timing and adoption of Medicaid expansions under the Affordable Care Act (ACA) and earlier waivers and adheres to a preregistered analysis plan, a rarely used approach in observational studies in economics. We find that expansions increased Medicaid enrollment by 12
11
percentage points and reduced the mortality of the low-income adult population by 2.5
1.3
percent, suggesting a 21
12
percent reduction in the mortality hazard of new enrollees.
The mortality reduction approached, but fell short of, statistical significance, and is therefore best viewed as tentative. We found larger mortality reductions among the subgroup of low-income adults without disabilities, and a possible mortality increase among the subgroup with disabilities. These subgroup findings should be considered preliminary until confirmed in future research, as they could have appeared by chance given the multiple subgroups examined. Additional findings for the subgroup of non-disabled adults include the following.
Mortality reductions accrued not only to older age cohorts, but also to younger adults, who accounted for nearly half of life-years saved due to their longer remaining lifespans and large share of the low-income adult population. These expansions appear to be cost-effective, with direct budgetary costs of $5.4 million per life saved and $179,000 per life-year saved falling well below valuations commonly found in the literature. Our findings suggest that lack of health insurance explains about five to twenty percent of the mortality disparity between high- and low-income Americans. We contribute to a growing body of evidence that health insurance improves health and
find
demonstrate that Medicaid’s life-saving effects
may
extend across a broader swath of the low-income population than previously understood.
No-Spin’s Study Overview
A quasi-experimental study of Medicaid expansions under the Affordable Care Act and earlier waivers found that the expansions reduced mortality of low-income adults by 1.3%. This finding is best viewed as tentative and not yet reliable because (i) the effect fell short of statistical significance; and (ii) the study design (non-RCT) can’t decisively rule out that factors other than Medicaid expansion caused the effect.
Study Design:
- The main preregistered study sample comprised 41.9 million adults ages 19-59 in 2010 with family income below 138% of the federal poverty line.
- The study compared mortality outcomes through April 2022 for (i) sample members in states and time periods with expanded Medicaid eligibility in or before 2014 (treatment group) to (ii) sample members in states and time periods without expanded Medicaid eligibility (comparison group).
- The study used statistical methods (“difference in differences regression”) to adjust for differences in characteristics between the two groups other than Medicaid expansion.
Findings:
- The study’s main, preregistered analysis found that the Medicaid expansions (i) increased Medicaid enrollment by a statistically significant 11 percentage points, and (ii) reduced mortality by 1.3 percent.
- The mortality effect approached, but fell short of, statistical significance (p ≤ 0.10) so is best viewed as tentative evidence of an effect.
- Additional analyses found a larger mortality reduction among the subgroup of low-income adults without disabilities, and a possible mortality increase among the subgroup with disabilities. Under established standards (FDA, IES), these subgroup findings are preliminary until confirmed in future studies, as they could have appeared by chance given the multiple subgroups examined.
Comment:
- Per established standards (WWC, FDA), the study’s quasi-experimental (non-RCT) design is inherently limited in the strength of evidence it can produce, as such designs “do not sufficiently rule out that something other than the intervention caused the observed effect” (WWC).
- The study received uncritical coverage in the New York Times, The Economist, NPR, and Time Magazine – coverage that cites the unreliable subgroup findings rather than main findings of the preregistered analysis, and doesn’t note the limitations of the study’s design.
- Importantly, an earlier high-quality RCT of Medicaid expansion found large reductions in catastrophic out-of-pocket health expenditures and sizable improvements in mental health, but no discernible effects on physical health, over two years. The RCT didn’t measure long-term mortality impacts.
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