What's more, the poorest white adults saw decreases in positive feelings, well-being, and life satisfaction between the two time periods, along with substantial increases in negative moods. Meanwhile, the richest saw little change in negative moods and some improvement in well-being since the '90s.

And though it's middle-aged white Americans who have suffered the greatest increases in premature death in recent years, the mental-health changes in this study were evident in all the age groups in the survey, not just people in their 50s or 60s.

"Our results paint a picture of substantial social stratification in psychological health among American adults," the authors write. In other words, the rich are getting happier and the poor are getting sadder.

The study is purely correlational, so the authors can only speculate as to why this is happening. They posit that it could include "increasing income inequality and wage stagnation for the working class; long-term deterioration in employment opportunities that have led to intergenerational decline in economic security; reduction in stable marriages … increasing work-family strain; and weakening interactions within communities and associated social isolation." In other words, it could be the very kind of cumulative distress Case and Deaton referred to last year.

Together, these forces can be enough to make someone want to reach for opioids or alcohol to numb their psychic pain. And ultimately, if declining life expectancy is any indication, some do.

Olga Khazan is a staff writer at The Atlantic.