In any given year 26 percent of American adults suffer from mental disorders, based on guidelines in the official handbook for diagnosing mental illness, the DSM-IV. Only about a fifth of the cases are serious enough to cause a major disruption of everyday life, however, which has prompted some experts to call for more stringent diagnostic criteria. Others counter that tracking mild symptoms is important for preventing their escalation into more severe illness. The chart below lists many of the most prevalent mental illnesses in Americans older than 18 years, according to a 2005 survey by the National Institute of Mental Health. Nearly half of all people who have one illness also suffer from at least one more.