NEW YORK » The U.S. death rate has been falling for decades, but researchers have detected one group in which the rates have been steadily ticking up — middle-aged white people. Suicides and deaths from drug overdose and alcohol abuse are being blamed.

Deaths rates for other races have continued to fall, as they have for whites 65 and older. But death rates for whites 35 to 44 have been level recently, they’re beginning to turn up for whites 55 to 64, and — most strikingly — death rates for whites ages 45 to 54 have risen by half a percent per year since 1998, said the authors, Anne Case and Angus Deaton of Princeton University.

The increase started in the late 1990s and probably is related to the increased availability around that time of certain prescription painkillers, they said.

“It certainly can’t be helping,” said Deaton, who last month was awarded a Nobel Prize in economics for unrelated work on consumer spending.

Their paper was published online Monday by the Proceedings of the National Academy of Sciences.

Federal researchers have reported — repeatedly — on worrisome increases in deaths from suicides and drug overdoses. And they have noted the bulk of those deaths have been white and middle-aged. So the Case and Deaton findings aren’t exactly surprising, said Robert Anderson, who oversees the Centers for Disease Control and Prevention branch that monitors death statistics.

But the Princeton pair brought a new lens to the government’s statistics, breaking down death numbers by age and race in a way the government has not highlighted, he added.

“White Americans who are middle-aged were really doing worse,” Case summarized. “And that’s not news we were hearing.”

There has not been a similar increase in middle-aged people living in other affluent countries, the researchers said.

White death rates still are not nearly as bad as black rates — not even for those 45 to 54. The rate is about 415deaths for every 100,000 white people in that age group. For blacks, it’s 582 per 100,000.

U.S. death rates have been on a general decline for more than century, thanks mainly to public health measures and advances in medical treatment. In recent decades, the improvement has been driven by declines indeath rates from heart disease and cancer — the nation’s two leading killers.

But from time to time, death rates for certain demographics have gone up. That’s generally happened in younger groups, who die in smaller numbers than the elderly and so have death rates that can be more easily swung. That happened with death rates for some age groups of white and black men during the height of the AIDS epidemic, for example, Anderson said.

Of the 2.6 million deaths in 2013, about 123,000 — less than 5 percent — were in white, non-Hispanic people ages 45 to 54.

But why the increase in this particular age set? And why only in white people? And why has it been inching up for them for 15 years?

The new study cited national health survey data showing increases over time in the proportion of middle-aged white people who said they suffered physical pain, trouble with daily activities, and poor mental health.

Those problems are not unique to white people. But studies have found white patients with pain are more likely to be prescribed opioid painkillers. And whites have been more likely to attempt suicide when faced with physical or mental hardships, for a range of possible reasons that include smaller networks of social support, say other experts.

Education is also a factor. The study found among whites with a college degree, the death rates were actually quite low. But for whites who achieved no more than a high school diploma, they were a whopping 736 per 100,000.

But again, why that one age group?

It’s not clear. But that is a particularly tough time in life to suffer a serious financial setback or a debilitating health problem, noted John Phillips, who oversees some of the National Institute on Aging’s funding of research into what affects aging and health. The institute funded the study.

“You’re supposed to be heading into your prime earning years, and far from being able to collect retirement benefits,” he said. A job loss or other long-lasting hardship can be very hard to cope with, he added.