Multiple personality disorder (MPD) is a mental illness. It is classified as a dissociative disorder and has been renamed dissociative identity disorder (DID). MPD or DID is a condition in which two or more distinct identities or personality states alternate in controlling the patient's consciousness and behavior.

The precise nature of DID (MPD) is still a subject of debate. Some researchers think that DID may be a culture-specific syndrome found in western society, caused primarily by both childhood abuse and unspecified long-term societal changes. Unlike depression or anxiety disorders, which have been recognized for centuries, the earliest cases of persons reporting DID symptoms were not recorded until the 1790s. Most were considered medical oddities or curiosities until the late 1970s, when increasing numbers of cases were reported in the United States.

Psychiatrists are still debating whether DID was previously misdiagnosed and underreported, or whether...