Over the past several years, a major debate has broken out among health policy watchers as to whether expanding insurance actually saves lives. About a year ago, I reported on a landmark study in Oregon that found no real improvement in physical health among those who had gained access to Medicaid.

But this week, another landmark study came out finding a significant reduction in mortality in Massachusetts after former Gov. Mitt Romney signed an expansion of insurance into law in 2006. Since that law served as the model for President Obama's health care law, it raises the question as to whether Obamacare's insurance expansion could save lives.

The Massachusetts study, published in the Annals of Internal Medicine, differed from the Oregon study in several key respects. In 2008, Oregon expanded its Medicaid program, but it didn’t have enough money to cover everybody. So the state set up a lottery system to determine who got covered. Because this created a pool of applicants for insurance with a similar demographic profile who were then separated into groups of people who gained Medicaid and went without it, it created a rare opportunity for a randomized controlled study.

The Massachusetts experiment didn’t benefit from the same randomized nature, but it also was able to have a much larger sample size — 10,000 newly insured in the Oregon study vs. 270,000 newly insured adults in the Massachusetts study. In addition, the Massachusetts study (authored by Benjamin Sommers, Sharon Long and Katherine Baicker, who also co-authored the Oregon study) looked at mortality over a longer period of time. Researchers looked at the change in mortality in Massachusetts in the four years before Romneycare (2001-05) and four years following its adoption (2007-10), and compared the findings to a control group of counties in other states with a similar demographic makeup.

In the end, the study found that “health reform in Massachusetts was associated with significant reductions in all-cause mortality and deaths from causes amenable to health care.” In number terms, mortality decreased by 2.9 percent relative to the control group. The study, the authors write, “implies that for approximately every 830 adults who gained insurance, there was 1 fewer death per year.”

As the authors note, Massachusetts fared better than most states in the aftermath of the recession, which also could be a factor — though they did their best to control for it.

The authors include several cautionary notes. The authors write that “we do not have individual-level insurance information and thus cannot directly link mortality changes to persons gaining insurance coverage” and that “defining mortality from causes amenable to health care is somewhat subjective.”

Additionally, the authors add: “Most important, our quasi-experimental approach cannot definitively demonstrate a causal relationship underlying the association between the Massachusetts reform and the state’s declining mortality relative to other states. It is possible that the postreform reduction in mortality in Massachusetts was due to other factors that differentially affected Massachusetts, such as the recession. ”

There is more uncertainty as to whether the results of the study can be applied more broadly to Obamacare. The authors note that the study does provide “suggestive evidence” that the national health care law will have an impact on mortality. “However,” the authors add, “it is critical to note the many dimensions in which Massachusetts differs from the rest of the nation, including lower mortality, higher income and baseline insurance coverage rates, fewer minorities, and the most per capita physicians in the country. The extent to which our results generalize to the United States as a whole is therefore unclear, which underscores the need to monitor closely the Affordable Care Act’s effect on coverage, access, and population health across all states.”

Thus, it will take many future studies of the implementation of Obamacare to determine whether the nation as a whole can achieve the type of reduction in mortality that the authors detected in Massachusetts. But, no doubt, the Massachusetts study offers the most compelling evidence to date that increasing health insurance coverage translates into fewer deaths.

It’s important to note, however, that this doesn’t answer the broader philosophical questions concerning whether the federal government has a role in guaranteeing access to health coverage or whether the goal of expanding insurance justifies infringing on personal liberty.

For more on the study, read Jonathan Cohn, who describes it as good news for Obamacare; Adrianna McIntyre, who writes, “The findings aren't bulletproof, but they're close”; Megan McArdle, who recalibrates her long-standing skepticism on the link between mortality and insurance; Ross Douthat, who seconds McArdle and also wonders whether he now has less of a slam dunk case for universal catastrophic insurance; Avik Roy, who argues the reduction in mortality can be explained because Massachusetts expanded coverage more through private insurance than through Medicaid; and Michael Cannon, who questions whether the variation in mortality can justify the massive cost of expanding insurance.