Dissociative identity disorder (DID), as defined by the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders (DSM), is a psychiatric diagnosis that describes a condition in which a single 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. 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 due to drug use or medical condition. Earlier versions of the DSM named the condition multiple personality disorder (MPD), and the term is still used by the ICD-10.
There is a great deal of controversy surrounding the topic. There are many commonly disputed points about DID. These viewpoints critical of DID can be quite varied, with some taking the position that DID does not actually exist as a valid medical diagnosis, and others who think that DID may exist but is either always or usually an adverse side effect of therapy. DID diagnoses appear to be almost entirely confined to the North American continent, adding to the possibility that DID may not be a legitimate diagnosis.