Why is multiple personality (aka dissociative identity)

so common in all forms of literature?

from the Bible's “My name is Legion; for we are many” (portrayed as demon possession)

 http://www.ewtn.com/vbible/search.asp?abbr=mark&ch=5&bv1=1&ev1=20

to the derivatives of Legion in popular culture

http://en.wikipedia.org/wiki/Legion_in_popular_culture

to popular literature in various media

http://en.wikipedia.org/wiki/Dissociative_identity_disorder_in_popular_culture 

Because normal multiple personality is relatively common, especially among the people who do the writing:

http://www.aywi.org/writers/2011/11/multiple-personality-disorder-among-authors/

http://jamigold.com/2012/01/multiple-personality-disorder-no-im-a-writer/ 

http://www.msauret.com/multiple-personality-disorder-autho/

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