For more than four decades, mental-health professionals (especially in the United States) have referred to one book for standardized definitions of mental disorders. The Diagnostic and Statistical Manual of Mental Disorders, now in a revised version of its fourth edition (the DSM-IV-TR), is published by the American Psychiatric Association.

Health-care providers do not define mental disorders to "label" people. DSM-IV-TR gives doctors, nurses, social workers and counselors a common language to describe what they see in patients. When a psychiatrist in California describes a patient as having "generalized anxiety disorder," another psychiatrist in New York will have a good idea of how that term is being used. Definitions are also helpful for research. If scientists in Atlanta, Boston, Chicago and Denver are all studying patients with schizophrenia, the DSM-IV-TR gives them common definitions to use.

No system of diagnosis is perfect. Research continues on the terms and definitions currently used. The DSM-IV-TR will require changes and updates, leading to a fifth edition of the book scheduled for publication in 2013.