In the early 1950s, Mason was a substitute teacher and a student at Columbia University. She had a history of blackouts and emotional breakdowns, and finally entered psychotherapy with Wilbur, a Freudian psychiatrist. Their sessions together are the basis of the book.
Mason later moved to Lexington, Kentucky where she taught art classes and ran an art gallery out of her home for many years. She died of breast cancer on February 26, 1998. A nonfiction book and film by psychiatric historian Peter Swales are planned about Mason and her relationship with Wilbur.
In the narrative, "Sybil", a name Schreiber gave Mason to protect her privacy, is a patient with severe issues of social anxiety and memory loss. After extended therapy Wilbur discovers that Sybil has 16 separate personalities. She first uses sodium amytal, then hypnosis interviews to encourage Sybil's various selves to communicate and reveal information about her life. The book begins with a list of Sybil's "alters," together with the year in which each appeared to have dissociated from the central personality. The names of these selves were also changed to ensure privacy.
Wilbur decided that Shirley Mason's multiple personalities resulted from childhood abuse at the hands of her mother, who was apparently schizophrenic. While the mother's instability was readily confirmed by Mason's contemporaries, the veracity of specific incidents of physical and sexual abuse related in the book remain controversial. Mason's therapy records have never been released, and both she and Wilbur are deceased.
According to people who knew Mason's family in Dodge Center, her mother was in fact given to bizarre behavior and controlled Mason very strictly. Her grandmother was kind to her, but unable to do anything about the treatment she received at her mother's hands. Her father often seemed blind or indifferent towards the many injuries inflicted upon Sybil by her mother. The family were Seventh-day Adventist, a religion that was apparently regarded with some suspicion by Dodge Center residents because of its superficial resemblance to Judaism.
The book's narrative describes Sybil's selves gradually becoming co-conscious, able to communicate and share responsibilities, and having musical compositions and art published under their various names. Wilbur attempts to integrate Sybil's various selves, first convincing them via hypnosis that they are all the same age, then encouraging them to merge together. At the book's end, a new, optimistic self called "The Blonde" emerges, preceding Sybil's final integration into a single, whole individual with full knowledge of her past and present life.
Rieber's study challenged the "Multiple Personality" diagnosis, insisting that Mason was "extremely suggestible hysteric" and claimed Wilbur had manipulated her in order to secure a book deal. The supposition that she did not have Multiple Personality Disorder was earlier supported by Dr. Herbert Spiegel. Dr. Spiegel saw Mason for several sessions while Wilbur was on vacation. His interpretation of her statements at that time caused him to believe that Wilbur was manipulating her into behaving as a multiple when she was not one. He has also stated that he suspects Wilbur of having publicized Mason's case for financial gain. However, the idea of the iatrogenesis of multiple personailty disorder has been challenged by many researchers.
However, an expert on multiple personalities, Dr. Richard Gottlieb, associate clinical professor of psychiatry at the Albert Einstein College of Medicine in New York, stated that Rieber's report does not shed light on whether Sybil's personalities were created in therapy. He also stated that Rieber's report failed to show if the book was a conscious misrepresentation. A review of Rieber's book Bifurcation of the Self by Mark Lawrence states that Rieber repeatedly distorted the evidence and left out a number of important facts about Sybil's case, in order to advance his case against the validity of Sybil's diagnosis.
Dr. Leah Dickstein, who worked with Mason in her later years, stands by her assessment of Mason as a multiple. Professionals and staff at Wilbur's Lexington, Kentucky clinic confirm that Mason was multiple, and that she attempted to integrate several times, without success. Periodically, according to Wilbur and other professionals who worked with her, Mason would deny that she was multiple. Dr. Leah Dickstein, whose mentor was Dr. Wilbur also stated that she was in touch with Mason for many years after Wilbur died. She remembers Mason telling her that "every word in the book is true." She stated that Wilbur had "no need to make this up.
Mason's case remains controversial. Supposedly, Wilbur's personal papers were to be unsealed as of 2005, but their contents have not been made available to the general public. There is a series of recorded tapes that were made by Wilbur of her sessions with Mason. Some of these tapes were accessed, with the permission of Mason and Wilbur, by the screenwriter Stewart Stern, who adapted the book for the film Sybil. In any case, these are not Mason's therapy records, provided Wilbur even kept any; she was reportedly careless about this area of her work. Due to privacy laws, it is very unlikely that Mason's therapy records will ever be released to the public.