In 2015, a pair of economists received widespread attention for their study showing that since the late 1990s the death rate has been rising for middle-aged white Americans.

Now a new analysis by the same Princeton University team has identified which part of that population was driving that trend: People without college degrees.

White men and women in every age group between 25 and 64 who did not have college degrees saw their mortality rates increase between 1998 and 2015. Those with degrees saw their mortality rates decrease.

“There are two Americas,” said Anne Case, who conducted the research with her husband, Angus Deaton, a Nobel laureate. “There’s an America for people who have gotten a college degree, and an America for people who have not gotten a college degree.

“And if you had a checklist of well-being, the people without college degrees are getting worse and worse, and people with college degrees are doing very well.”

The analysis, published Thursday in the Brookings Papers on Economic Activity, found that the problem appears to be distinctly American.

In Europe, mortality rates for people with low levels of education are falling more rapidly than for those with more education.

In every age group the researchers examined, white Americans with a high school education or less now fare worse than blacks as a whole — a reversal of the situation in 1999.

Blacks and Latinos have seen steady improvement in mortality rates as whites without college degrees have been going in the other direction as a result of drug overdoses, alcoholism and suicide and a recent halt in a decline in deaths due to heart disease.

The nation’s opioid abuse epidemic played a major role.

But the authors said an underlying culprit is the widespread erosion of institutions that provided stability in American life for much of the 20th century: the manufacturing industry, the church, unions and stable marriage.

“A long-term process of decline” began around the time whites started entering the workforce in the 1970s, and “traditional structures of social and economic support slowly weakened,” the authors wrote.