It has been a decade since scientists last fanned out across the country to assess the rates of mental illness. The latest census, just completed, indicates that a whopping 46 percent of Americans will suffer from a mental disorder during their lifetime. Tens of thousands of people answered questions about their deepest thoughts and behaviors for the study, the most extensive ever conducted.

In any given year, 18 percent of respondents suffered from a serious anxiety disorder, 10 percent from depression or bipolar illness, 9 percent from an impulse disorder, and 4 percent from alcohol or drug addiction. “This is depressing,” says Harvard University epidemiologist Ronald C. Kessler, who directed the huge study, which was published as a series of papers in the Archives of General Psychiatry.

The findings also speak volumes about treatment. Only 40 percent of those who researchers deemed would have qualified as mentally ill said they had received some kind of treatment, and often that was from someone other than a mental health provider. “We have to figure out how to improve the quality of the care these patients receive,” Kessler says.

The prevalence of problems is much greater than that reported 20 years ago, when the first survey of this scope was carried out. Those results prompted an overhaul of the Diagnostic and Statistical Manual (DSM), a massive tome of criteria that psychiatrists use to determine whether an individual's symptoms qualify as a clinical illness. The updated fourth edition, known as DSM-IV, has been the bible since, but it may need to be revised again in light of the new data.

In total, more than 15,000 Americans have participated in the two National Comorbidity Survey Replication studies, which Kessler also led in 1994. Perhaps the only good news is that most clinical cases are mild and that only a small proportion are severe. But most people said that the first signs of their illness appeared before age 18, arguing for more extensive treatment of young people. Kessler's small research army is now analyzing an additional 10,000 adolescents and performing separate studies of African-Americans, Hispanic-Americans and Asian-Americans to look for more specific trends.