Abnormal is the new normal — at least for mental health. Only a small share of the population stays mentally healthy from age 11 to 38. Everyone else experiences a mental illness at some point, a new study finds.

“For many, an episode of mental disorder is like influenza, bronchitis, kidney stones, a broken bone or other [common] conditions,” says Jonathan Schaefer. He is a psychologist at Duke University. A coauthor of the study, he notes that “Sufferers experience impaired functioning. Many seek medical care, but most recover.”

The study looked at 988 people who lived in New Zealand. Only 171 — or about one in six people —experienced no anxiety disorders, depression or other mental ailments from late childhood to middle age. Of the rest, half experienced a mental disorder that lasted a short time. This was typically just a bout of depression, anxiety or substance abuse. The person then recovered.

The remaining 408 people — roughly two out of every five — experienced one or more mental disorders that lasted at least several years. Their diagnoses included more severe conditions. These may have included bipolar and psychotic disorders.

Schaefer and his colleagues shared their findings in the February Journal of Abnormal Psychology.

By the numbers…

The researchers analyzed data on people born between April 1972 and March 1973 in Dunedin, New Zealand. Each person’s general health and behavior was assessed 13 times from birth to age 38. Mental health was assessed eight times from age 11 onward.

Previous studies had linked several traits with a lower chance of developing mental disorders. These included growing up in an unusually well-off family and enjoying really good physical health. Scoring very high on intelligence tests also has been linked to good mental health. Surprisingly, however, the New Zealanders who stayed mentally healthy scored no better of those qualities than anyone else.

Instead, people with good mental health tended to have personality traits that gave them some sort of advantage. Those traits started emerging in childhood, the surveys showed. These people rarely expressed strongly negative emotions. They also tended to have lots of friends and very good self-control. Those with lasting mental health also had relatively few family members with mental disorders, compared with their peers.

There were some benefits in adulthood for those who always had good mental health. These people had, on average, more education, better jobs and higher-quality relationships. They also reported more satisfaction with their lives than the others did. But lasting mental health doesn’t guarantee an exceptional sense of well-being, Schaefer points out. Nearly one-in-four people never diagnosed with mental illness scored below the entire group’s average score for life satisfaction.

Less surprising was the 83 percent overall rate of mental disorders. That matches recent estimates from four other long-term projects. Two had focused on Americans. One looked at people in Switzerland. The last was another study from New Zealand. These studies followed people for 12 to 30 years. And over that follow-up, between 61 percent and 85 percent of the participants reported having at least some mental disorders.

Such high rates also were reported in an earlier study, from 1962. It had surveyed a random mix of people living on the island of Manhattan in New York City. Many researchers had doubted that study’s findings, however. Why? It had relied on a diagnostic system that was less strict than the ones used to evaluate the people in New Zealand, explains William Eaton. In fact, he says, the Manhattan study now appears to have been on the right track. Eaton is an epidemiologist at the Johns Hopkins Bloomberg School of Public Health in Baltimore, Md.

There is often a stigma attached to mental illness. But if more people realize that most will eventually develop some mental disorder, at least briefly, that stigma might fall, Eaton suspects.

Ronald Kessler is also an epidemiologist. He works at Harvard Medical School in Boston, Mass. Kessler directs U.S. surveys of mental disorders. He suspects the numbers of people who experience a mental disorder may be even higher than what was reported in these studies.

Many people that seemed to have enduring mental health in these studies may not have. They may have developed brief mental disorders that got overlooked, he says. It might have been something such as a couple of weeks of serious depression after a romantic breakup.

Focusing on those rare cases of lasting mental health may not be the best idea, he says. “The more interesting thing is to compare people with persistent mental illness to those with temporary disorders.”

By Bruce Bower