From Wikipedia

Multiple Personality Disorder

Dissociative identity disorder is a psychiatric diagnosis that describes a condition in which a person displays multiple distinct identities or personalities (known as alter egos or alters), each with its own pattern of perceiving and interacting with the environment.

In the International Statistical Classification of Diseases and Related Health Problems the name for this diagnosis is multiple personality disorder. In both systems of terminology, the diagnosis requires that at least two personalities routinely take control of the individual's behavior with an associated memory loss that goes beyond normal forgetfulness; in addition, symptoms cannot be the temporary effects of drug use or a general medical condition. DID is less common than other dissociative disorders, occurring in approximately 1% of dissociative cases, and is often comorbid with other disorders.

There is a great deal of controversy surrounding the topic of DID. The validity of DID as a medical diagnosis has been questioned, and some researchers have suggested that DID may exist primarily as an iatrogenic adverse effect of therapy. DID is diagnosed significantly more frequently in North America compared to other areas of the world.