Mental disorders more common than we think.

• Position: Your Life

The prevalence of anxiety, depression, and substance dependency may be twice as high as the mental health community has been led to believe. It depends on how one goes about measuring. Psychologists Terrie Moffitt and Avshalom Caspi of Duke University, Durham, N.C., and colleagues from the United Kingdom and New Zealand used a long-term tracking study from birth to age 32 to reach the conclusion that people vastly underreport the amount of mental illness they have suffered when asked to recall their psychological history years after the fact.

Yet, such self-reporting from memory is the basis of much of what we know about the prevalence of anxiety, depression, and alcohol and marijuana dependence. Longitudinal studies--like the Dunedin Study in New Zealand--that track people over time are rare and expensive. "If you start with a group of children and follow them their whole lives, sooner or later almost everybody will experience one of these disorders," contends Moffitt, professor of psychology and neuroscience.

The Great Smoky Mountains Study, a similar effort based at Duke, has tracked American children from age nine to 13 into their late 20s and found similar patterns, asserts Jane Costello, professor of medical psychology. "I think we've got to get used to the idea that mental illness is actually very common. People are growing up impaired, untreated, and not functioning to their full capacity because we've ignored it."

The prevalence of mental illness has been debated hotly by policymakers and mental health providers for many years. The pharmaceutical and health insurance industries also have a stake in the debate. The best retrospective studies, the U.S. National Comorbidity Survey and the New Zealand Mental Health Survey, have found the incidence of depression from ages 18 to 32 at a rate of about 18%, but they have been...