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.

Duke University psychologists Terrie Moffitt and Avshalom Caspi and colleagues from the United Kingdom and New Zealand used a long-term tracking study of more than 1,000 New Zealanders from birth to age 32 to reach the conclusion that people vastly underreport the amount of mental illness they've suffered when asked to recall their history years after the fact.

But such self-reporting from memory is the basis of much of what we know about the prevalence of anxiety, depression, alcohol dependence and marijuana dependence. Longitudinal studies like the Dunedin Study in New Zealand that track people over time are rare and expensive, Moffitt said.

"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," said Moffitt, the Knut Schmitt-Nielsen professor of psychology and neuroscience at Duke.

The Great Smoky Mountains Study, a similar effort based at Duke, has tracked 1,400 American children from age 9-13 into their late 20s and found similar patterns, said Jane Costello, a professor medical psychology at Duke who runs the study.

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"I think we've got to get used to the idea that mental illness is actually very common," Costello said. "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 hotly debated by policy makers and mental health providers for many years. The pharmaceutical and health insurance industries also have a stake in the debate, Moffitt said.

The best retrospective studies, the US National Comorbidity Surveys (NCS) and the New Zealand Mental Health Survey, have found the incidence of depression from ages 18 to 32 at a rate of about 18 percent. But they have been roundly criticized by some for their rates being too high. The latest analysis from the Dunedin Study found 41 percent of that age range had experienced clinically significant depression.

Similarly, the survey studies have reported a 6 to 17 percent lifetime rate of alcohol dependence between ages 18-32, versus nearly 32 percent in the Dunedin Study.

Guidelines published by the American Psychiatric Association that set the bar for defining what is and isn't a treatable illness are currently being revised by a rewriting of the authoritative Diagnostic and Statistical Manual of Mental Disorders (DSM). But given the findings of these longitudinal studies, the stringency of the diagnostic criteria might need to be reconsidered, said Moffitt, who is on the committee writing the new DSM-Vstandards.

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"Researchers might begin to ask why so many people experience a disorder at least once during their lifetimes and what this means for the way we define mental health, deliver services and count the economic burdens of mental illness," Moffitt said.

On the one hand, it could be argued that the diagnostic standards have been set too low if so many people can be considered mentally ill. On the other hand, perhaps these findings argue for more and better mental health care because the disorders are more common than anyone had realized.

"There are two opposing camps, and I'm agnostic about that," Moffitt said.

At the very least, maybe these findings can help reduce the stigma against mental illness and mental health care, Moffitt added. New Zealand, for example, has begun a new campaign of public service announcements featuring sports heroes saying they've experienced mental health issues.

"If we're serious about this problem, we need to get serious about preventing it," Costello added. "We do know a lot more about prevention now."

Moffitt and Caspi's findings from the Dunedin Study appear online in the journal Psychological Medicine. Their work was supported by the New Zealand Health Research Council, the US National Institutes of Health and the UK Medical Research Council.