Actually Amanda, that is what people USED to believe about dissociation and DID. Current scientific theory has moved on from that and that old information is no longer part of the recommended treatment approach for DID.
Current theory is that in normal childhood development the personality is not integrated until around age 6 or 7, and abuse and /or trauma that occurs before that time does not create a split, but instead causes the failure of personality to integrate. In current scientific theory about Dissociative Identity Disorder there is not a core or birth personality that splits off to protect itself - there is one whole personality that fails to integrate its normal separate parts. With time and repetition of abuse, and without restorative integrative experiences these separate parts continue to separate and develop indepedently of one another. But the crucial difference is there is no 'core'. All parts are equally important aspects of the original developing self.
I eagerly await the day the new theories, ideas and corresponding treatment practices are widespread throughout the world. They are currently being used in Europe and Australisia, but for some reason the US lags behind and insists on clinging to the old beliefs. Why would that be?