Schreiber collaborated on the book with Dr. Cornelia Wilbur, the psychiatrist who asks, “What about Mama?” — and with Wilbur’s patient, whose name Schreiber changed to Sybil Dorsett. Schreiber worked from records of Sybil’s therapy, including thousands of pages of patient diaries and transcripts of tape-recorded therapy sessions. Before she died in the late 1980s, Schreiber stipulated that the material be archived at a library. For a decade after Schreiber’s death, Sybil’s identity remained unknown. To protect her privacy, librarians sealed her records. In 1998, two researchers discovered that her real name was Shirley Mason. In trying to track her down, they learned that she was dead, and the librarians at John Jay decided to unseal the Schreiber papers.

The same year that her identity was revealed, Robert Rieber, a psychologist at John Jay, presented a paper at the American Psychological Association in which he accused Mason’s doctor of a “fraudulent construction of a multiple personality,” based on tape-recordings that Schreiber had given him. “It is clear from Wilbur’s own words that she was not exploring the truth but rather planting the truth as she wanted it to be,” Rieber wrote.

It wasn’t the first indication that there might be problems with Mason’s diagnosis. As far back as 1994, Herbert Spiegel, an acclaimed psychiatrist and hypnotherapist, began telling reporters that he occasionally treated Shirley Mason when her regular psychiatrist went out of town. During those sessions, Spiegel recalled, Mason asked him if he wanted her to switch to other personalities. When he questioned her about where she got that idea, she told him that her regular doctor wanted her to exhibit alter selves.

And yet, in the popular imagination, Sybil and her fractured self remained powerfully tied to the idea of M.P.D. and the childhood traumas it was said to stem from. “Mamma was a bad mamma,” Wilbur declares in the transcripts. “I can help you remember.” But countless other records suggest that the outrages Sybil recalled never happened. If Sybil wasn’t really remembering, then what exactly was Wilbur helping her to do?

When Mason made her first visit to Dr. Wilbur’s Park Avenue office, in late 1954, it had been nine years since she’d first gone to her for help. At that time, Mason was an art student in the Midwest seeking treatment from Wilbur, a young psychiatrist in Omaha, for blackouts and disturbing behaviors that included disappearing for hours when her parents took her around town on errands. Mason talked to Wilbur about her lifelong ailments — anorexia, nervousness, anemia and feelings of worthlessness — and about growing up as the only child of Seventh-Day Adventists in the small town of Dodge Center, Minn. Oddly, Wilbur also talked about her own life, including her talent for treating hysterics and her interest in people suffering from the strangest type of hysteria of all: multiple personalities. Mason developed a crush on her psychiatrist, who seemed to understand her like no one else. She completed only a handful of psychotherapy sessions, but they gave her the strength she needed to finish college and eventually move to New York. She’d had relapses since, but now her former doctor was in the city! A half-dozen sessions, she thought, might keep her nerves from ever acting up again.