Wilbur’s main drug of choice, Sodium Pentothal, is sometimes called “truth serum” for its ability to suppress higher brain functions, but is in fact known to produce unreliable confessions and bizarre fantasies. It’s also addictive, and Mason, who often lay for hours with the needle parked in her arm as Wilbur asked flagrantly leading questions, became hooked. She was, then, not only emotionally and financially dependent on her therapist but chemically dependent as well.

Nevertheless, Mason did at one point attempt to jump off Wilbur’s train, writing her doctor a long letter confessing that all the multiple-personality stuff — the lost time, the named “alters” and the grotesque tortures supposedly inflicted on Mason as a child by her supposedly psychotic mother — had all been made up. Wilbur briskly dismissed this as a “major defensive maneuver” designed to derail the “hard work” of therapy lying ahead. The pitiably vulnerable Mason soon caved.

For her part, Schreiber, who was brought in by Wilbur to tell Mason’s story for a general audience, applied to the job the skills of a former radio dramatist turned writer of dubious “as told to” articles. Nathan links this development in Schreiber’s career to a moment when general-interest magazines, faced with the new competition of television, demanded melodramatically confessional “human interest” narratives from their freelancers. (She also documents a connection between Schreiber and Terry Morris, a “pioneer” of this genre who freely admitted to taking “considerable license with the facts that are given to me.” Her son is the political strategist Dick Morris.)

Freakish as Mason’s condition was, it didn’t quite make for a thrilling narrative, and Schreiber became uncomfortably aware that a lot of the stories elicited from the “alters” by Wilbur — including a ridiculous tale of a flight to Amsterdam to aid a refugee from the Nazis — were simply impossible. Nevertheless, she plowed ahead, massaging any facts that proved insufficiently sensational, pleading the cause of “emotional truth” (the same term used by fibbing memoirist James Frey). After the book became a hit, she feuded with her two collaborators and launched a campaign to nominate herself for the Nobel Prize.

Similarly, Stewart Stern, the screenwriter hired to adapt the book, was initially skeptical of many of its claims. It took only a couple of days in Wilbur’s company, however, before he got with the program. Soon, he was writing to her, “I found myself wishing more and more that I too could have been — could be — your patient.” In the cause of “emotional truth” Stewart was obliged to insert “fiction after fiction to smooth out the parts of Flora’s book that seemed poorly motivated or downright illogical.” Throughout “Sybil Exposed,” Nathan traces the winding path from truth to falsehood, with each participant adding more fabrications to the story as it passes through his or her hands.

Mason, like so many patients diagnosed with multiple personality disorder (now rechristened “dissociative identity disorder,” in part to shake the bad rep of MPD), improved markedly under certain conditions — namely, the absence of her therapist. For several years after her therapy concluded, she lived happily as an art teacher at a community college, even owning her own house. But the publication of “Sybil” destroyed that life; Schreiber, who had invented so much of her biography, had so thinly disguised other details that many acquaintances recognized her. Too self-conscious to endure this exposure, Mason fled back to Wilbur and lived out the rest of her life as a sort of beloved retainer, cooking her doctor breakfast and dinner every day and nursing her on her deathbed.

Wilbur, on the other hand, thrived, presiding over the explosion of MPD diagnoses as one of the foremost experts on the condition. She played a key role in promoting the belief that conspiracies of fiendish, sadistic adults were secretly perpetrating murder, child rape and mutilation, human sacrifice, and cannibalism across the country and that repressed memories of such atrocities lay at the root of most MPDs. Innocent people were convicted of these crimes on the basis of testimony elicited from highly suggestible small children and hypnotized adults. Families were sundered by therapists who convinced their patients that they’d suffered similar ordeals despite having no conscious memory of it. This opened the door to years of expensive and ineffective therapy.

Surprisingly, Nathan is not unsympathetic to Wilbur. “She wanted to help, and she did the best she knew how,” she writes of the doctor, noting her tireless devotion to her patients and her particular willingness to support women who dreamed of achievement beyond the domestic sphere. But, as Nathan also points out, the care Wilbur provided frequently incapacitated her patients, leaving them even less able to fulfill themselves. MPD became a sort of illness-as-metaphor that women used to describe their conflicting desires in an age of increased opportunity. The result was to cast conflict and desire as disabilities, instead of what they are — the very fuel of life itself.