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DID (Dissociative Identity Disorder) and MPD (Multiple Personality Disorder) are psychiatric diagnoses used to describe plurality, most frequently trauma-based systems. DID is the current term; MPD was outmoded a few decades ago.
DID is said to be caused when severe trauma results in a failure to integrate memories, thoughts, actions and beliefs into a core personality. The trauma associated with the memories and thoughts eventually develop into a separate identity with it's own pattern of behavior and personality. Due to avoidance of trauma, dissociation
Some plurals may associate with the term and identify with it, however others may not or not believe that trauma is the cause of their system.