After mandatory health insurance coverage was required in Massachusetts in 2006, the mortality rate dropped significantly. As the first experiment with universal coverage in the U.S., the New York Times reports that this could lend some research-based support to the controversial Affordable Care Act, better known as Obamacare.

The study, published in the Annals of Internal Medicine, looked at the death rate in Massachusetts from 2001 to 2005 to establish the rate before the reform, and from 2007 to 2010 to investigate how it had changed after the reform. The study focused on individuals between the ages of 20 and 64 in certain counties. They found that the mortality rate dropped approximately three percent after the reform.

The New York Times calculates that, if the same improvement was seen on a national level, it would mean 17,000 fewer deaths per year for Americans under the age of 65. However, Massachusetts does not make a good analogy for the rest of the country. Massachusetts is whiter and more affluent than most states, and has more doctors per capita and fewer uninsured people.

However, researchers note that the state’s health insurance law nevertheless amounted to the best natural experiment the country has had for testing the effects of a major insurance expansion on a large population. Other studies can only focus on cases of Medicaid or have to look at other nations. A study in Oregon on Medicaid, the insurance program for the poor, found that coverage improved mental health and financial security, but not physical health. The study was too small to gauge mortality effects.

Of course, the study does not prove causation. The authors identified other counties that were most similar to Massachusetts in demographics and levels of poverty and insurance, and then compared their mortality rates with that of Massachusetts. They found that the rate declined 2.9 percent in Massachusetts, but remained flat in counties outside the state.