Dissociative identity disorder (DID), is also known as multiple personality disorder. This is a psychiatric diagnosis whose essential feature, according to the Diagnostic and Statistical Manual of Mental Disorders (DSM), "...is the presence of two or more distinct identities or personality states ... that recurrently take control of behaviour.
"The diagnosis requires that at least two personalities (one may be the host) routinely take control of the individual's behaviour 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. Memory loss will occur in those with DID when an alternate part of the personality becomes dominant. DID is less common than other dissociative disorders, occurring in approximately 1% of dissociative cases, and diagnosis is often difficult as there is considerable comorbidity with other conditions and many symptoms overlap with other types of mental illness. It is diagnosed significantly more frequently in North America than in the rest of the world.
Individuals diagnosed with DID frequently report severe physical and sexual abuse as a child.
The etiology of DID has been attributed to the experience of pathological levels of stress which disrupts normal functioning and forces some memories, thoughts and aspects of personality from consciousness, though an alternative explanation is that dissociated identities are the iatrogenic effect of certain psychotherapeutic practices or increased popular interest. The debate between the two positions is characterised by passionate disagreement.