http://www.nimh.nih.gov/publicat/numbers.cfm
National Institute of Mental Health, National Institutes of Health
Mental disorders are common in the United States and internationally. An estimated 22.1 percent of Americans ages 18 and older—about 1 in 5 adults—suffer from a diagnosable mental disorder in a given year. In addition, 4 of the 10 leading causes of disability in the United States and other developed countries are mental disorders-major depression, bipolar disorder, schizophrenia, and obsessive-compulsive disorder. Many people suffer from more than one mental disorder at a given time.