Dissociative Identity Disorder

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The most famous form of disordered plurality, Dissociative Identity Disorder (DID) is a condition listed in the DSM-V. It's the most severe dissociative disorder, wherein a diagnosis of DID makes any other diagnosis in that category null. DID is most known for its alters (or headmates), but it is much more complex than just having multiple individuals in one mind.

DID systems have amnesic barriers, or dividers between headmates blocking some or all memory sharing, often to keep trauma knowledge quarantined in certain headmates. This can manifest in headmates not knowing about each other as well, but not always (this is called an amnesiac system).

They can have any number of headmates, from small systems of two or three to large systems into the tens of thousands.

Related Terms
Dissociative Disorder Not Otherwise Specified (DDNOS) and Otherwise Specified Dissociative Disorder (OSDD) are other related conditions. Some systems, including disordered ones like in DID, fit the Theory of Structural Dissociation, but there is controversy surrounding that model. DID is often traumagenic, but not always; many DID systems are mixed-origin too. DID systems with dozens of fragments or over one hundred headmates are often referred to as "polyfragmented".

Some, but not all, DID & OSDD-1 systems believe that trauma is the only possible cause of plurality, and that multiples cannot exist outside of those diagnoses. They are referred to as sysmeds, a shortened version of "system medicalists".

DID is often comorbid with Complex PTSD.

History
Reports of amnesiac plurality have existed for centuries and been called by many names. In 1791, German doctor Eberhardt Gmelin dubbed a system of two alters as a case of "exchanged personality"; in 1887, the case of Felida X, "a one-way amnesic multiple personality" (A knows of B but B does not know of A); the Sybil case putting the condition into the public consciousness; etc. Each of these examples is fraught with questionable science and controversy. In the DSM-II (1968), "Hysterical Neurosis, Dissociative Type" was listed, which included symptoms of DID, which was renamed to Multiple Personality Disorder (MPD) in the next edition of the DSM in 1980. The DSM-IV (1994) officially changed the name to "dissociative identity disorder", due to a misclassification and included amnesia into the diagnostic criteria.