Death rates (measured as the number of deaths per 100,000 people in a given year) in the United States have been declining for decades, and researchers had expected to find more of the same.

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WASHINGTON — Gains in the American life span have slowed in recent years, according to a new report, with average annual death rates flattening for the first time since researchers started measuring them in the late 1960s.

Researchers from the American Cancer Society used federal mortality data to analyze trends in longevity from 1969 to 2013. Death rates (measured as the number of deaths per 100,000 people in a given year) in the United States have been declining for decades, an effect of improvements in health care, disease management and medical technology — and the researchers had expected to find more of the same.

Instead, they stumbled upon a disturbing development. The declines in death rates flattened in the most recent period, from 2010 to 2013, dropping by an average of just 0.4 percent annually, a rate so slight that it was not statistically significant. The rate had slowed in previous periods but never this substantially, researchers said.

Researchers said their analysis — which was published in JAMA on Tuesday and was the first of its kind using the most recent federal data — was intended to identify the broader trends, not explain them. But they offered one potential theory for what was driving the slowdown — a delayed effect of the obesity epidemic, which has plagued Americans of all ages since the 1980s.

Dr. Ahmedin Jemal, the head of surveillance and health-services research at the American Cancer Society and one of the report’s authors, cautioned that the slowdown had taken place over just four years, a very short period for the purposes of long-term mortality trends. He said it was too early to tell whether the finding marked the start of a trend. Even so, it caught researchers off guard. “I was surprised,” Jemal said. “We were expecting to see more declines.”

American life spans have lengthened dramatically since the 1960s, when advances in medical care and technology, particularly for heart disease and stroke, began to drive down death rates. But the pace of improvement has occasionally slowed, for example in the 1990s, when the HIV epidemic was raging. It picked back up in the 2000s, when new medicines to control risks like high cholesterol became widespread.

Over the 44-year period of study, death rates dropped by about 43 percent, the report found, and mortality rates for various ailments also fell: by 77 percent for stroke, by 68 percent for heart disease, by 18 percent for cancer and by 17 percent for diabetes. But the rate of decline in death rates had slowed in the most recent period for some obesity-related ailments — heart disease, stroke and diabetes — a pattern that some experts said supported the theory that obesity was a driver.

Declines for men slowed to one notch above statistical significance from 2009 to 2013, while the downward trend leveled off entirely for women in that period. Stagnation in mortality rates for American women has puzzled researchers for some time.

Researchers offered theories on what might be driving the flattening.

S. Jay Olshansky, a public-health professor at the University of Illinois at Chicago, pointed out that it took decades for smoking to show up in death rates, in the form of lung cancer and other ailments, and that obesity was expected to have a similarly delayed effect. The fact that women started smoking later than men has also been used to explain their stagnating mortality rates.

“The medical community seems to be under a fog that we can constantly and forever reduce death rates, and that’s simply not true,” said Olshansky, who published a study in 2012 showing that life spans for white women without a high-school diploma had declined, a rare event in developed countries.

“You need to look at the health status of the living,” not the mortality statistics of the dead, he said, adding that obesity is afflicting younger generations in a way that will eventually make the numbers worse.

Patterns identified

Researchers used a specialized program to find trends in the data that is common in statistical analysis. Computers identified distinct patterns in these periods: 1969 to 1978, 1979 to 2002, 2003 to 2010 and 2010 to 2013. The pace of decline in death rates slowed during the second of those periods, but not as much as during the most recent one.

David Cutler, an economics professor at Harvard who has worked extensively on life expectancy, said he was less persuaded by the obesity explanation, as many believe that epidemic has already started to affect mortality. An alternative theory, he said, is that much of the big gains from lifesaving medicines like statins for cholesterol, may already have happened.

“At some point everybody is taking a statin and you top out,” he said.

But John G. Haaga, the acting director of the Division of Behavioral and Social Research at the National Institute on Aging, said he believed the United States still had a lot of room for improvement.

Some pockets of the country, for example in the South, have all but stood in place for decades. And middle-age American men lower down the income ladder are no more likely to make it to old age than their fathers were, according to recent research.

“We’ve got a long way to go before we could catch up with Canada, Germany or France,” Haaga said.