Rates of Common Mental Disorders Double Up

By Bruce Bower, Science News

Some mental disorders aren’t merely common—they’re the norm.

Depression, anxiety disorders, alcohol dependence and marijuana dependence affect roughly twice as many people as had previously been estimated, a new study finds. Nearly 60 percent of the population experiences at least one of these mental disorders by age 32, say study directors and psychologists Terrie Moffitt and Avshalom Caspi, both of Duke University in Durham, N.C.

That figure probably gets higher by the time people reach middle age, Moffitt suggests, as additional people develop at least one of these four ailments for the first time.

In a paper published online September 1 and in an upcoming Psychological Medicine, Moffitt and Caspi present results from a study of more than 1,000 New Zealanders assessed for mental disorders 11 times between ages 3 and 32. This study took a prospective approach, following people as they aged, and assessed prevalence rates based on long-term data. Moffitt’s team focused most intensely on the period from age 18 to 32, when these disorders first start to appear. Earlier prevalence estimates for mental disorders in the United States and New Zealand relied on self-reports and therefore adults’ ability to remember and willingness to recount their own past emotional problems.

 “Like flu, if you follow a cohort of people born in the same year, as they age almost all of them will sooner or later have a serious bout of depression, anxiety or a substance abuse problem,” Moffitt says.

It comes as no surprise that, compared with one-time survey responses, the new prospective study identified considerably more people who have had mental disorders, comments epidemiologist Ronald Kessler of Harvard Medical School. But self-report responses remain valuable, he says. Evidence indicates that individuals who report past mental disorders in surveys display an increased likelihood of developing such ailments in the future. Kessler directs ongoing U.S. surveys of mental disorders based on self-reports.

Half of the people diagnosed in the new study had a mental disorder for a relatively short period or in a single episode. Moffitt nonetheless regards these cases as serious, since short-term symptoms often led to work problems, efforts to get mental-health care or suicide attempts.

Among 32-year-old New Zealanders, Moffitt and her colleagues find lifetime prevalence rates of 50 percent for anxiety disorders, 41 percent for depression, 32 percent for alcohol dependence and 18 percent for marijuana dependence. Participants who developed one of these disorders tended to experience others as well, including less-common ones such as eating disorders.

Self-report surveys in the United States (SN: 6/11/05, p. 372) and New Zealand have found lifetime prevalence rates for common mental disorders that are about half as large as those in the new investigation.

A long-term study of 1,400 North Carolina children tracked into young adulthood finds rates of mental disorders comparable to those reported by Moffitt’s team, according to Duke psychologist and study director Jane Costello. Those data have yet to be published.

Researchers generally agree that self-reports underestimate lifetime prevalence rates of mental ailments. Other investigations suggest that many adults forget periods of depression, and even hospitalizations for depression, from earlier in their lives.

Still, some researchers have charged that self-report surveys inflate prevalence rates by assigning mental ailments to many people with mild symptoms of no real clinical concern.

As work intensifies to develop a new diagnostic manual of mental disorders by 2012, Moffitt says the new findings indicate that prevalence estimates for serious mental disorders have been too low, not too high. The upcoming manual, known as DSM-V, is used as the standard for classifying disorders in the United States and some other countries and is published by the American Psychiatric Association.

Higher prevalence rates can be used to support either side of a long-running dispute over psychiatric diagnoses, Moffitt notes. Some researchers see a large, unmet need for mental-health care which leads them to support definitions of certain mental disorders as serious, though not recurring. Others want to narrow DSM definitions in order to avoid labeling temporary emotional woes as mental illnesses.