It’s a fame that derives not only from the fact that his medical metamorphosis was the first sex reassignment ever reported on a developmentally normal child but also from a stunning statistical long shot that lent a special significance to the case. He was born an identical twin, and his brother provided the experiment with a built-in matched control – a genetic clone who, with penis intact, was raised as a male. That the twins were reported to have grown into happy, well-adjusted children of opposite sex seemed unassailable proof of the primacy of rearing over biology in the differentiation of the sexes and was the basis for the rewriting of textbooks in a wide range of medical disciplines. Most seriously, the case set a precedent for sex reassignment as the standard treatment for thousands of newborns with similarly injured, or irregular, genitals. It also became a touchstone for the feminist movement in the 1970s, when it was cited as living proof that the gender gap is purely a result of cultural conditioning, not biology. For Dr. John Money, the medical psychologist who was the architect of the experiment, this case was to be the most publicly celebrated triumph of a 40-year career that recently earned him the accolade “one of the greatest sex researchers of the century.”

But as the mere existence of this young man in front of me would suggest, the experiment was a failure, a fact revealed in a March 1997 article in the Archives of Adolescent and Pediatric Medicine. Authors Milton Diamond, a biologist at the University of Hawaii, and Keith Sigmundson, a psychiatrist from Victoria, British Columbia, documented how the twin had struggled against his imposed girlhood from the start. The paper set off shock waves in medical circles around the world, generating furious debate about the ongoing practice of sex reassignment (a procedure more common than anyone might think). It also raised troubling questions about the way the case was reported in the first place, why it took almost 20 years for a follow-up to reveal the actual outcome and why that follow-up was conducted not by Dr. Money but by outside researchers. The answers to these questions, fascinating for what they suggest about the mysteries of sexual identity, also bring to light a 30-year rivalry between eminent sex researchers, a rivalry whose very bitterness not only dictated how this most unsettling of medical tragedies was exposed but also may, in fact, have been the impetus behind the experiment in the first place.

But what for medicine has been a highly public scandal involving some of the biggest names in the world of sex research has been for the young man sitting in front of me a purely private catastrophe. Apart from two short television appearances (his face obscured, his voice disguised), he has never spoken on the record to a journalist and has never before told his story in full. For this article, he granted more than 20 hours of candid interviews and signed confidentiality waivers giving me exclusive access to a voluminous array of legal documents, therapists’ notes, Child Guidance Clinic reports, IQ tests, medical records and psychological work-ups. He assisted me in obtaining interviews with his former therapists as well as with all of his family members, including his father, who, because of the painfulness of these events, had not spoken of them to anyone in more than 20 years.

The young man’s sole condition for talking to me was that I withhold some details of his identity. Accordingly, I will not reveal the city where he was born and raised and continues to live, and I have agreed to invent pseudonyms for his parents, whom I will call Frank and Linda Thiessen, and his sole sibling, the identical twin brother, whom I will call Kevin. The physicians in his hometown I will identify by initials. The young man himself I will call, variously, John and Joan, the pseudonyms given for him by Diamond and Sigmundson in the journal article describing the macabre double life he has been obliged to live. No other details have been changed.

“My parents feel very guilty, as if the whole thing was their fault,” John says. “But it wasn’t like that. They did what they did out of kindness, and love and desperation. When you’re desperate, you don’t necessarily do all the right things.”

The irony was that Frank and Linda Thiessen’s life together had begun with such special promise. A young couple of rural, religious backgrounds, they grew up on farms near each other and met when Linda was just 15, Frank 17. Linda, an exceptionally pretty brunette, had spent much of her teens fighting off guys who were too fresh. Frank, a tall, shy fair-haired man, was different. “I thought, ‘Well, he’s not all hands,’ "Linda recalls." ‘I can relax with him.’ ” Three years later, at ages 18 and 20, they married and moved to a nearby city. Linda remembers Frank’s joy soon after, upon learning that he was going to be the father of twins – and his euphoria when the brothers were born, on Aug. 22, 1965. “The nurse asked him, ‘Is it boys or girls?’ ” Linda recalls. “And he said, ‘I don’t know! I just know there’s two of ’em!’ ”

Shortly before the births, Frank had landed his highest-paying job ever, at a local unionized plant, and the couple now moved with their newborns into a sunny one-bedroom apartment on a quiet side street downtown. But when the twins were 7 months old, Linda noticed that their foreskins were closing, making it hard for them to urinate. Their pediatrician explained that the condition, called phimosis, was not rare and was easily remedied by circumcision. He referred them to a surgeon. The operations were scheduled for April 27, 1966, in the morning. Because Frank needed the family car to get to his job on the late shift, they brought the kids in the night before. “We weren’t worried,” Linda says. “We didn’t know we had anything to worry about.”

But early the next morning, they were jarred from sleep by a ringing phone. It was the hospital. “There’s been a slight accident,” a nurse told Linda. “The doctor needs to see you right away.”