“Deaths of despair,” such as suicides and drug overdoses, are on the rise in the U.S., and there’s no definitive explanation. A study out this week found a number of surprising statistics that deepen the mystery, primarily that rates of psychological distress are going down and outpatient mental health care use is up.

Mark Olfson, a professor of clinical psychiatry at Columbia University and an author of the study, said the study’s findings were not what he expected.

“We thought we’d find increasing rates of psychological distress in adults,” he told NBC News. “But the distress levels we found don’t explain rising ‘deaths of despair.’ ”

The study, published Wednesday in JAMA Psychiatry, used data from a nationally representative sample of nearly 140,000 people in three studies from 2004 to 2015. The data came from the Medical Expenditure Panel Survey, a set of large-scale surveys on the cost and use of health care conducted through in-person interviews by the U.S. Department of Health and Human Services.

“Almost a third of people who reported severe psychological distress are not getting mental health care.”

To quantify the mental health of people in the survey, researchers used participants’ answers to questions about their state of mind to score them on the Kessler scale, a points system widely used to standardize distress levels from depression and anxiety. The scale does not measure other mental illnesses, such as mania or substance abuse.

Researchers found that from 2004 to 2015, rates of self-reported severe psychological distress went down just over 1 percentage point, from nearly 5 percent to nearly 4 percent. In the same period, the percentage of adults receiving outpatient mental health care went up 4 percentage points, from just over 19 percent to 23 percent. This includes use of both talk therapy and psychiatric medications.

However, most of that increase in mental health care was in people who did not report severe distress. The group that reported a low level of distress, or even none at all, increased overall use of psychiatric medication at a higher rate than the group with severe distress.

“The people who need the care aren’t the ones getting it,” Olfson said. “Almost a third of people who reported severe psychological distress are not getting mental health care.”

The study found that the majority of people with low or no distress levels that are getting care are not getting it from a mental health professional, but rather from a primary care doctor who is prescribing psychiatric medication.

It’s possible that those taking medication and reporting low or no distress are feeling good because they are taking medication. But Olfson is skeptical of that. “It’s rare for medication to eliminate symptoms to that degree,” he said.

Adding to the mystery of the study’s findings is that the majority of the group that reported lower distress levels isn’t also getting mental health care.

“You’d think the declines in distress would map onto those getting treatment, but we didn’t find that,” he said. “So there are other things going on, such as lower rates of smoking, higher rates of aerobic exercise, or improving economic conditions following the great recession.”

Dr. David Ginsberg, clinical professor of psychiatry at NYU Langone Health, said he views the report as an indication that more research is needed rather than anything conclusive.

“The data is based off of self-reports and excludes serious psychological conditions,” he told NBC News. “The trends they found are intriguing, but it leaves a lot of questions.”

He said the way the health care system is set up prioritizes care for those who have access rather than need. “Many people with more serious mental illness don’t have commercial health insurance, or because of underlying conditions, they may need support from others to access care,” said Ginsberg.

The phenomenon of primary care doctors prescribing psychiatric medication didn’t surprise him, although it’s not ideal, he said. Again, he said, the health care system is a factor here. “Primary care doctors don’t have the time to spend on psychiatric issues that the specialists have,” he said. “There’s a lot of demands on prescribers these days, charting and administrative work, so specialists are better equipped to devote the needed time to issues like this.”

Earlier in 2018, the Centers for Disease Control and Prevention reported that suicide rates have gone up by 30 percent since 1999. A separate nongovernmental report found an 11 percent increase in overdose deaths from 2015 to 2016.

Olfson said the findings from his study suggest that there is no need to worry that mass despair across the country is causing these deaths.

“It’s not a groundswell of distress,” he said. “We need to make sure we are focusing treatment efforts on the right people and communities.