A disorder in which a person's identity dissociates, or fragments, creating additional, distinct identities that exist independently of each other within the same person.

Persons with dissociative identity disorder (DID) adopt one or more distinct identities. Each identity or personality is distinct from the other(s) in specific ways. There are cases in which a person will have as many as 100 or more identities, while some people only exhibit the presence of one or two. ("Co-presence" is the term used to describe the situation that exists when two or more personalities are simultaneously present with or without knowledge of each other's existence or current presence.)

Prior to the publication of the Diagnostic and Statistical Manual of Mental Disorder-IV (DSM-IV), dissociative identity disorder was referred to as multiple personality disorder (MPD); the birth personality was referred to as the...