Ignornace: Multiple Personality Disorder:

One disorder frequently used in novels and movies is Dissociative Identity Disorder, more frequently called Multiple Personality Disorder.  Typically those with this disorder in stories exhibit a break between reality and their perception of it, typically from some trauma that makes them create alternate personalities to protect themselves and thus the second or other personalities grow and become fully fledged, driving the individual to homicidal actions.  While sometimes those with DID do have other mental disorders which are violent, it is not in and of itself necessarily the cause of said violence and psychiatrists are in agreement on that point about DID...  But on little else.

Some psychiatrists think DID is indeed caused by trauma, usually childhood sexual trauma, but others think people are born with a gene for it that causes the break later in life, and still others think that it is actually caused by the hypnotic state that some psychiatrists put their patients into.  There exists no medically universal agreed definition on what causes DID, its progression, its affect on the mind when separating it from other mental disorders, nor even its symptoms in general.  It has been noted that it was rarely diagnosed before 1980, but overall, DID, among many other disorders, is rare and difficult to study.  One thing most psychiatrists agree upon though, is that it is quite different from schizophrenia when a patient does not have both.  The fragmented identities do not talk to one another and while they may be aware of each other, they do not push one another into taking actions contrary to that identity's nature.  This, among problems with other disorders, is frequently portrayed in films and in novels as being contrary and having the 'personalities' fight with one another either to attack the protagonists or to try to stop their other personalities form achieving something horrid.  Writers, please do not fall prey to this rampant stupidity and research any disorder you intend to include in your works before implementing them.  The one you think it is may not be what you think it is but still exist, or be completely fictional, which is fine, but then you should invent a new name for this new 'rare' disorder.