Dissociative Identity Disorder: Introduction

Dissociative identity disorder (DID) is a rare condition in which two or more distinct identities, or personality  states, are present in—and alternately take control of—an individual. The person also experiences memory loss that is too extensive to be explained by ordinary forgetfulness.

DID was called multiple personality disorder up until 1994 when the name was changed to reflect a better understanding of the condition—namely, that it is characterised by fragmentation or splintering of identity, rather than by proliferation or growth of separate personalities. The symptoms of DID cannot be explained away as the direct psychological effects of a substance or of a general medical condition.

DID reflects a failure to integrate various aspects of identity, memory, and consciousness into a single multidimensional self. Usually, a primary identity carries the individual's given name and is passive, dependent, guilty, and depressed. When in control, each personality state, or alter, may be experienced as if it has a distinct history, self-image, and identity. The alters' characteristics—including name, reported age and gender, vocabulary, general knowledge, and predominant mood—contrast with those of the primary identity. Certain circumstances or stressors can cause a particular alter to emerge. The various identities may deny knowledge of one another, be critical of one another, or appear to be in open conflict. 

Symptoms
The Diagnostic and Statistical Manuel of Mental Disorders 5th Edition (DSM-5) Diagnostic Criteria:
 
Criterion A
Disruption of identity characterized by or more distinct personality states, which may be described in some cultures as an experience of possession. The disruption in identity involves marked discontinuity in sense of self and sense of agency, accompanied by related alterations in affect, behaviour, consciousness, memory, perception, cognition, and/or sensory-motor functioning. These signs and symptoms may be observed by others or reported by the individual.
 
Criterion B
Recurrent gaps in the recall of everyday events, important personal information, and/or traumatic events that are inconsistent with ordinary forgetting.
 
Criterion C
The symptoms cause clinically significant distress or impairment in social, occupational, or other important areas of functioning.
 
Criterion D
The disturbance is not a normal part of a broadly accepted cultural or religious practice.
 
Criterion E
The symptoms are not attributable to the physiological effects of a substance (e.g. - blackouts or chaotic behaviour during alcohol intoxication) or another medical condition (e.g. - complex partial seizures).
 
Reference:
Psychology Today, 
https://www.psychologytoday.com/intl/conditions/dissociative-identity-disorder-multiple-personality-disorder

Dissociative Identity Disorder Research, American Psychiatric Association, 
https://did-research.org/did/basics/dsm-5/

Long Reads:
Bethany Brand, PhD., American Psychological Association, 
https://www.apa.org/news/podcasts/speaking-of-psychology/dissociative-identity-disorder
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Trauma Dissociation, 
http://traumadissociation.com/dissociativeidentitydisorder