Dissociative Identity Disorder
One way to look at multiple personality overall (now being referred to by the community as plurality) is to look at how "professionals" in the field of psychology view the disordered version of the phenomenon. Currently, there are a couple diagnoses that are specific to plurality, one is called Dissociative Identity Disorder (DID).
In the DSM-III and DSM-III-R, there was Multiple Personality Disorder (MPD). With the advent of the DSM-IV, the diagnosis changed to Dissociative Identity Disorder, and is a dissociative disorder. Often people who are diagnosed with DID also have post-traumatic-stress-disorder (PTSD).
Many people who have multiple personalities object to medicalizing their plurality. Medicalization of something that is different from some unidentifiable "norm" is almost a tradition at this point — but it's possible that plurality is a neurotype and that multiples are expressing one end of a spectrum of behavior and self-structuring.
For educational purposes, the definition of Dissociative Identity Disorder is below (each current criteria has a link to an article on this website breaking down the criteria into layman's terms).
What is DID?
DID is a dissociative disorder that encompasses nearly all other dissociative phenomenon. A person with DID may experience depersonalization, derealization, dissociative amnesia, or other forms of forgetting that aren't usual. They have compartmentalized not only their memories and experiences, but also sets of all traits that define "identity" (such as perspectives, ethics, language use, physicality, preferences, reactivity, and so on) and it's thought to be an adaptive mechanism to survive adversity.
DID is generally recognized to also be a form of complex trauma, or involve complex trauma in some way. This means trauma to more than one bodily & mental health system for the (group) entity in question. So for example, complex trauma for people diagnosed with DID can involve damage to attachment & relations, physical or neurological trauma, endocrine system trauma, social trauma, mental health issues, and so on. So some may picture it as a trauma disorder, or a brain injury — but it might be much more a "both and" than an "either or" scenario; that's why it's called "complex trauma" after all.
How DID is diagnosed, like all other diagnostic criteria, has to account for how an unknowing client presents to a hopefully knowledgeable practitioner. The diagnostic criteria have been made cryptic on purpose, however, to prevent diagnosis (which is a matter of much controversy, and ought to be — stigma about DID is very entrenched in the psychiatric system, and they're the ones writing the diagnostic manuals)1
Diagnostic criteria for Dissociative Identity Disorder (DID) DSM-5 300.14 (F44.81)
A. Disruption of identity characterized by two or more distinct personality states New, which may be described in some cultures as an experience of possession. The disruption of marked discontinuity in sense of self and sense of agency, accompanied by related alterations in affect, behavior, consciousness, memory, perception, cognition, and/or sensory-motor functioning. These signs and symptoms may be observed by others or reported by the individual.
B. Recurrent gaps in the recall of everyday events New, important personal information, and/or traumatic events that are inconsistent with ordinary forgetting.
C. The symptoms cause clinically significant distress or impairment New in social, occupational, or other important areas of functioning.
D. The disturbance is not a normal part of a broadly accepted cultural or religious practice. New Note: In children, the symptoms are not better explained by imaginary playmates or other fantasy play.
E. The symptoms are not attributable to the physiological effects of a substance New (e.g., blackouts or chaotic behavior during alcohol intoxication) or another medical condition (e.g., complex partial seizures).
Reprinted without permission, but within the confines of fair use (negligible portion of an extensive text which does not compromise the integrity of the entire text, and well within the confines of educational purposes in a non-profit forum), from the Diagnostic and Statistical Manual of Mental Disorders: DSM-5, Fifth Edition, Text Revision (DSM-V). Copyright 2013, American Psychiatric Association.
DSM IV Criteria (for comparison)
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.
Reprinted without permission, but within the confines of fair use (negligible portion of an extensive text which does not compromise the integrity of the entire text, and well within the confines of educational purposes in a non-profit forum), from the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM-IV-TR). Copyright 2000 American Psychiatric Association.
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1 Misdirection away from DID diagnosis is very deliberate in the DSM, and the criteria are shady, including renaming the disorder, to prevent professionals from diagnosing people with it. For more on this DeliberateDiagnosisMisdirectionBiasplease read this article. ⇑