Sybil Isabel Dorset is the faux name used to protect the patient confidentiality of Shirley Ardell Mason, a woman who apparently suffered an extreme case of dissociative personality/identity disorder and whose case was documented in the book Sybil: The Tue and Extraordinary Story of a Woman Possessed by Sixteen Separate Personalities.
According to the book, written by Flora Rhetta Schrieber, the causative factors for Sybil's extreme mental breakdown are directly linked to Sybil's mother. She was also mentally unstable and supposedly abused Sybil from a very early age in such gruesome and morbid ways that the veracity of such claims is still questioned to this day.
It is said, however, that the controversial therapeutic practices performed on Sybil were experimental and invalid. Furthermore, until the death of Sybil at age 75 in the year 1998 (from breast cancer), the information that has been gathered about the entire therapy process shows that most of the information given back in the 70's was manipulated. In other words that the conditions of the interviews, the type of the therapy and the skill of the therapist in making much of not so much created the media sensation that became "Sybil".
Realistically, in order to learn what may cause something like Dissociative Identity Disorder, one must not refer to the book by Schrieber as reference. Not at all. Instead, go to the Diagnostics Statistics Manual of Mental Disorders in its latest edition (DSM IV) and look up the condition there. Dissociative Identity Disorder is under code 300.14 and is described as
a condition in which a person's identity dissociates, or fragments, creating additional, distinct identities that exist independently of each other within the same person.
However, it is also indicated that sexual abuse is certainly one prevalent factor in the background information of all patients who have suffered from this condition. Sybil Dorset presumably suffered terrible sexual abuse to the point of rupturing her uterus since the mom apparently stabbed her in her private area. She would also be hung by the neck until passing out, and suffered violent attacks by her mom that included throwing young Sybil down the stairs, and even storing her in a cabinet as a baby!
Yet, there is evidence of a leak that suggests that Wilbur (the therapist) and Sybil were in on a hoax and that they blew the entire story out of proportion. Nobody knows now how credible the events were but just for the sake of being careful, do not use the Schreiber book for anything other than (very sick and distasteful) "entertainment".