I dated someone who had it- was beaten heavily during their childhood formative years, and told me that the separation of identities was a means of coping.

It seemed like an extreme emotional compartmentalization which left them with a fractured sense of self.

Some people might say that they could have just been pulling an elaborate hoax on me from a year and a half. But the amount of acting it would have required just to convince me of this without any real reward would have been a sign of even greater emotional problems than even the different personalities.