Death rates in the U.S. have risen for the first time in 10 years, according to preliminary data from The Centers for Disease Control. The CDC research indicates that more people are dying from drug overdoses, suicide and Alzheimer’s disease, among other causes.

Experts expressed surprise. “It’s an uptick in mortality and that doesn’t usually happen, so it’s significant,” Robert Anderson, the chief of mortality statistics for the CDC’s National Center for Heath Statistics told The New York Times. “But the question is what does it mean? We really need more data to know.”

CDC researcher Andrew Fenelon agreed. “We are not accustomed to seeing death rates increase on a national scale,” he said. “We’ve seen increases in mortality for some groups, but it is quite rare to see it for the whole population.”

As the Times notes, “The finding seemed to fit the broader pattern of rising mortality among working class whites.” Those most affected were uneducated or under-educated whites.

In a 2015 Princeton study, researchers Anne Case and Angus Deaton (recent winner of the Nobel Prize in economics) discovered a marked difference between the white population and other population groups in terms of rising mortality rates. While other population groups continued to see declines in mortality rates, whites saw increases. According to Case and Deaton, “This increase for whites was largely accounted for by increasing death rates from drug and alcohol poisonings, suicide and chronic liver diseases and cirrhosis.”

In addition, the deaths among whites saw greatest increases for those with the least education, said Case and Deaton. All-cause mortality for whites with a high-school degree or less increased by 134 per 100,000 from 1999 to 2013, while death rates fell by 57 per 100,000 for those with a Bachelor’s degree.

Noah Smith, a professor of finance at Stony Brook University, says the rise is best explained not by poor health or increasing economic insecurity, but by the breakdown of the family:

The uneducated class became a floating low-skilled labor force, which decreased the marriageability of white working class men. That impaired family formation. A couple of decades later, the lack of family support started to take a big bite out of the emotional health of working class whites, causing them to turn to alcohol, drugs and suicide once they reached middle age.

The libertarian-leaning scholar Charles Murray’s recent book Coming Apart makes a similar argument, that the white working class “is no longer part of a virtuous silent majority” and does not live by the “founding virtues” needed for success in society. He says “marriage has become the fault line dividing America’s classes,” with high divorce rates now a signature feature of the white working class. As The New Republic explains in a piece on Murray’s work, whites higher up the socioeconomic ladder “divorce less, self-report happier marriages, raise fewer children as single parents, and remain employed.”