Our study documents that large expansions of Medicaid eligibility in three states were associated with a significant decrease in mortality during a 5-year follow-up period, as compared with neighboring states without Medicaid expansions. Mortality reductions were greatest among adults between the ages of 35 and 64 years, minorities, and residents of poor counties. These findings may influence states' decisions with respect to Medicaid expansion under the ACA.

Our study shows a mortality reduction associated with state Medicaid expansions to cover adults. Using state-level differences in Medicaid expansion as a natural experiment avoids the confounding between insurance and individual characteristics (e.g., poverty or health status) that plagues cross-sectional observational studies. These results build on previous findings that Medicaid coverage reduces mortality among infants and children3,4 and are consistent with preliminary results of a randomized, controlled trial of Medicaid in Oregon, which showed significant improvement in self-reported health during the first year (although objective measures of health are not yet available and 1-year mortality effects were not significant and were imprecisely estimated).14

We observed reductions in deaths from both internal and external causes. The relative mortality reduction was higher for external causes of death than for internal causes, though this difference was not significant. We hypothesized that internal causes would be more amenable to intervention through improved risk-factor management and medication adherence,30 though a study involving persons who were hospitalized after accidental injuries showed a reduction of nearly 40% in mortality among insured adults, as compared with uninsured adults, because of a greater intensity of care and longer lengths of stay.31

Our secondary analyses provide a plausible causal chain for reduced mortality that is consistent with previous research,32,33 with eligibility expansions associated with a 25% increase in Medicaid coverage, 15% lower rates of uninsurance, a 21% reduction in cost-related delays in care, and a 3% increase in self-reported excellent or very good health. However, it is not clear whether the magnitude of these changes is sufficient to account for the observed mortality reduction, and these associations do not prove causality.

Our estimate of a 6.1% reduction in the relative risk of death among adults is similar to the 8.5% and 5.1% population-level reductions in infant and child mortality, respectively, as estimated in analyses of Medicaid expansions in the 1980s.3,4 Our results correspond to 2840 deaths prevented per year in states with Medicaid expansions, in which 500,000 adults acquired coverage.15 This finding suggests that 176 additional adults would need to be covered by Medicaid in order to prevent 1 death per year.

A relative reduction of 6% in population mortality would be achieved if insurance reduced the individual risk of death by 30% and if the 1-year risk of death for new Medicaid enrollees was 1.9% (Table S4 in the Supplementary Appendix). This degree of risk reduction is consistent with the Institute of Medicine's estimate that health insurance may reduce adult mortality by 25%,34 though other researchers have estimated greater35 or much smaller36 effects of coverage. A baseline risk of death of 1.9% approximates the risk for a 50-year-old black man with diabetes37,38 or for all men between the ages of 35 and 49 years who are in self-reported poor health.39 The lower end of our confidence interval implies a relative reduction in the individual risk of death of 18%.

For Medicaid expansions to produce effects of this size, new enrollees must have had a higher-than-average risk of death that was responsive to medical care. We found that new Medicaid enrollees were older, disproportionately minorities, and twice as likely to be in fair or poor health as the general population, all of which suggest higher mortality,39 and these findings are consistent with previous expansions.40 Furthermore, Medicaid enrollment often occurs at the point of care for patients with acute illnesses — in emergency departments, doctors' offices, and hospitals41,42 — when the risk of death (and benefits of coverage) may be particularly high.

Our study has several limitations. We examined three expansion states, and the results are largely driven by the largest (New York), so our results may not be generalizable to other states. Common methods for estimating standard errors are imperfect when applied to a small number of states, although our findings were robust with the use of alternative methods. The mortality data set did not allow us to control for individual-level characteristics other than race, sex, and age (e.g., socioeconomic status or health status with respect to specific chronic diseases). We had to impute values for small subsamples after stratification according to county, race, sex, and age, although the results were robust with different imputation approaches.

Most important, our analysis is a nonrandomized design and cannot definitively show causality. Rates of insurance coverage and access to care increased in expansion states for both high-income persons and the elderly, even though the Medicaid eligibility expansions did not apply to them directly. Rates of death also declined among elderly adults, though the relative changes represented only one third of the mortality decline among adults between the ages of 20 and 64 years, leaving a significant mortality reduction among nonelderly adults that was independent of this trend. One possible explanation for these findings is that expanding coverage had positive spillover effects through increased funding to providers, particularly safety-net hospitals and clinics.43 Publicity about the expansion may also have encouraged uninsured higher-income and elderly persons to obtain insurance from other sources, including those over the age of 65 years who did not meet lifetime earnings requirements for Medicare.44

Alternatively, states may choose to expand Medicaid when their economies are thriving, and economic prosperity broadly improves coverage and access, which could produce a spurious association between eligibility expansions and health. However, our analysis of mortality was adjusted for a comprehensive list of economic measures that were specific to the county and year, and the results were not changed by these covariates. Similarly, states expanding Medicaid may simultaneously invest in public health or the health care workforce in other ways that could reduce mortality. However, we are unaware of any other contemporaneous large-scale changes in health policies in the states we studied. Moreover, the fact that mortality changes were largest in expected subpopulations offers some reassurance that we have isolated the effect of Medicaid expansions. Nonetheless, we cannot rule out other, concurrent trends that may have confounded our results.

In conclusion, our results offer new evidence that the expansion of Medicaid coverage may reduce mortality among adults, particularly those between the ages of 35 and 64 years, minorities, and those living in poorer areas. Ongoing research on the basis of randomized data13,45 will be invaluable in expanding on these findings. The Medicaid program is slated to expand coverage to millions of adults in 2014 under the ACA, though the recent Supreme Court ruling enables states to choose whether they will do so, and some states may instead consider program cuts. Policymakers should be aware that major changes in Medicaid — either expansions or reductions in coverage — may have significant effects on the health of vulnerable populations.