This article is from the Dissociation FAQ, by Discord (email@example.com) with numerous contributions by others.
Multiple Personality Disorder is defined as the existence within a person
of two or more distinct personalities or personality states, in which at
least 2 of these personalities "take control" of the functioning of the
body at given points. Each personality controls the body seperately, and
there is a memory loss for at least some personalities when others are in
control of the body.
Other personalities may have wildly different traits, belief systems,
relationships, names, and so forth. Some clinical studies have shown
that EEGs differ by personality. The personalities may themselves have
other psychological disorders, such as depression; these disorders may be
present in only one, some, or all of the personalities.
The degree of interaction and/or cooperation of the personalities varies
extremely; the degree of co-consciousness (the state of being able to
share memories of the various personalties' actions, and being able to
cooperate in the control of the body) also varies extremely.
Age of onset for MPD is usually (nearly always) in childhood. In nearly
all cases of MPD, there was childhood abuse or other severe childhood
trauma. MPD is noted in females more often than in males. The degree of
impairment ranges from minimal to extreme. No figures are available on
the prevalnce of MPD (and this is a hotly contested area).
Psychogenic Fugue and Psychogenic Amnesia, while having some of the
qualities of MPD, do not have the shifts in personality.
Schizophrenia sometimes includes fragmented thought and the perception of
voices in ones head, as well as a feeling of being controlled by another
entity; however, the shift in control does not appear as it does within
MPD, and schizophrenic patients generally report their voices as being
external in origin.
Borderline Personality Disorder is marked by instability in mood, action
and thoughts; however, these different, conflicting ideas, beliefs, and
goals are resident within a single personality.