Duke University psychologists Terrie Moffitt and Avshalom Caspi and their collaborators from the UK and New Zealand say that it depends upon how one goes about measuring.
The researchers tracked more than 1,000 New Zealanders from birth to age 32, and came to the conclusion that people vastly underreported the amount of mental illness they had suffered, when asked to recall their history years after the fact.
However, said the researchers, 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.
According to Moffitt, longitudinal studies like the Dunedin Study in New Zealand, in which people are tracked 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," said Moffitt, the Knut Schmitt-Nielsen professor of psychology and neuroscience at Duke.
Jane Costello, a professor of Medical Psychology at Duke, points out that similar patterns have been found in the Great Smoky Mountains Study, a similar effort based at Duke, which has tracked 1,400 American children from age 9-13 into their late 20s.
"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," Costello said.
For years, policy makers and mental health providers have hotly debated the prevalence of mental illness.
Moffitt says that the pharmaceutical and health insurance industries also have a stake in the debate.
While the best retrospective studies have found the incidence of depression from ages 18 to 32 at a rate of about 18 percent, the researchers point out that the latest analysis from the Dunedin Study found 41 percent of that age range had experienced clinically significant depression.
They further reveal that the survey studies have reported a 6 to 17 percent lifetime rate of alcohol dependence between ages 18-32, compared to nearly 32 percent in the Dunedin Study.
"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.
The researcher further says that 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.
Moffitt believes that these findings may help reduce the stigma against mental illness and mental health care.
New Zealand, for example, has begun a new campaign of public service announcements featuring sports heroes saying they've experienced mental health issues.
Costello said: "If we're serious about this problem, we need to get serious about preventing it. We do know a lot more about prevention now."