Dissociative Identity Disorder (DID) is also known by it's former label of Multiple Personality Disorder (MPD) in the American Psychiatric Association’s Diagnostic & Statistical Manual (DSM)
Dissociative Identity Disorder is the condition in which a person displays more than one unique identity or personality, each with its own pattern of behaviors.
The diagnosis of Dissociative Identity Disorder requires that at least two personalities routinely take control of a person's behavior. This is accompanied by an associated memory loss that goes beyond normal forgetfulness. Each personality is unaware, or unfamiliar with the others.
Additionally, for a valid DID diagnosis, the symptoms cannot be explained by substance abuse or another medical condition.
Earlier versions of the DSM named the condition Multiple personality disorder (MPD) and the term is still commonly used.
DID is a controversial diagnosis. There is argument about its validity, existence, causes, and epidemiology.
DID Characteristics - The DSM-IV Criteria
Diagnostic criteria for 300.14 Dissociative Identity Disorder (formerly known as Multiple Personality Disorder)
A. The presence of two or more distinct identities or personality states (each with its own relatively enduring pattern of perceiving, relating to, and thinking about the environment and self).
B. At least two of these identities or personality states recurrently take control of the person's behavior.
C. Inability to recall important personal information that is too extensive to be explained by ordinary forgetfulness.
D. The disturbance is not due to the direct physiological effects of a substance (e.g., blackouts or chaotic behavior during Alcohol Intoxication) or a general medical condition (e.g., complex partial seizures). Note: In children, the symptoms are not attributable to imaginary playmates or other fantasy play.
DID/MPD is a controversial diagnosis. There are a number of influential voices who do not recognize it as a legitimate diagnosis and are advocating for its removal from the DSM.