Manda, recent research suggests that dissociative disorders of identity are caused not by 'splitting' as such, but by the lack of integration within a normally developing personality. Meaning - that everyone's personality is 'fragmented' to a degree at birth, and the normal course of development 'integrates' these fragmented parts into one cohesive concept of self. In dissociative disorders of of identity the child doesn't have enough of the experiences to integrate the personality, and it remains fragmented... with each part having its own sense of 'I'.
Read up about *structural dissociation* sometime - it's fascinating.