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.

(Read the full Science News article here.)