For reasons health experts can’t entirely explain, the death rate for middle-aged white Americans is rising, even as U.S. residents of other age groups, races, and ethnicities are falling — as are those of U.S. counterparts in other rich countries.

That finding was reported this week by two Princeton University economists, Angus Deaton, who won the 2015 Nobel Prize for Economic Sciences, and Anne Case, The New York Times reports.

Analyzing death records from the Centers for Disease Control and Prevention and other sources, they concluded that rising annual death rates among this group are not tied to big killers, like heart disease and diabetes but by an epidemic of suicides and substance abuse — specifically alcoholic liver disease and overdoses of heroin and prescription opioids.

The analysis also success lower education levels may be a factor. The researchers found the mortality rate for whites 45 to 54 years old — with no more than a high school education — increased by 134 deaths per 100,000 people from 1999 to 2014.

By contrast, the death rate for middle-aged blacks and Hispanics continued to decline during the same period, as did death rates for younger and older people of all races and ethnic groups.

“It is difficult to find modern settings with survival losses of this magnitude,” wrote two Dartmouth economists, Ellen Meara and Jonathan S. Skinner, in a commentary to the Deaton-Case analysis that was published in Proceedings of the National Academy of Sciences.

Taken together, the researchers concluded that suicides, drugs, and alcohol explained the overall increase in deaths.