The study tallied deaths in Massachusetts from 2001 to 2010 and found that the mortality rate — the number of deaths per 100,000 people — fell by about 3 percent in the four years after the law went into effect. The decline was steepest in counties with the highest proportions of poor and previously uninsured people. In contrast, the mortality rate in a control group of counties similar to Massachusetts in other states was largely unchanged.

A national 3 percent decline in mortality among adults under 65 would mean about 17,000 fewer deaths a year.

“It’s big,” said Samuel Preston, a demographer at the University of Pennsylvania and an authority on life expectancy. Professor Preston, who was not involved in the study, called the study “careful and thoughtful,” and said it added to a growing body of evidence that people with health insurance could reap the ultimate benefit — longer life.