When anyone has a baby the first question everyone asks is: "Is it a boy or a girl?"
Biologically it is sex hormones, physical appearance and the sex chromosomes - XX for a woman, XY for a man - which dictate whether someone is male or female.
But what happens if you bring up someone who was a boy as a girl?
There was a case just like this in the 1960s, a case which ended in tragedy.
Twins Bruce and Brian Reimer were born in Canada as two perfectly normal boys. But after seven months, both were having difficulty urinating.
Acting on advice, the parents, Janet and Ron, took the boys to the hospital for a circumcision.
The next morning, they received a devastating phone call - Bruce had been involved in an accident.
Doctors had used a cauterizing needle instead of a blade, and the electrical equipment had malfunctioned and the surge in current had completely burned off Bruce's penis.
"I could not comprehend what he was talking about," Janet Reimer remembered.
"I thought they were going to use a knife. I didn't know there was electricity involved."
Brian's operation was cancelled, and the Reimers took their twins home.
Months passed, and they had no idea what to do until one evening they met a man who would change their lives, and the lives of their twins, forever.
Dr John Money was a psychologist specialising in sex changes.
He believed that it wasn't so much biology that determines whether we are male or female, but how we are raised.
"We just happened to be watching TV," remembers Mrs Reimer.
"Dr Money was on there and he was very charismatic, he seemed highly intelligent and very confident of what he was saying."
Janet wrote to Dr Money, and within a few weeks she'd taken Bruce to see him in Baltimore.
For Dr Money the case provided the ideal experiment.
Here was a child he believed should be brought up as the opposite sex, who even brought his own control group with him - an identical twin.
If it worked this would provide irrefutable evidence that nurture could over-ride biology - and Dr Money genuinely believed that Bruce had a better chance of living a happy life as a woman than as a man without a penis.
And so, when Bruce was 17 months old, he became Brenda. Four months later, on 3 July 1967, the first surgical step was taken - with castration.
Dr Money stressed that, if they wanted the sex change to work, the parents must never let Brenda or her twin brother know that she had been born a boy.
From now on they had a daughter, and every year they would go and visit Dr Money who was keeping track of the twins' progress in what became known as the John/Joan case. Brenda's identity was kept a secret.
"The mother stated her daughter was much neater than her brother and, in contrast with him, disliked to be dirty," Dr Money recorded at one of these yearly meetings.
Although, in contrast, he also noted: "The girl had many tomboy traits, such as abundant physical energy, a high level of activity, stubbornness, and being often the dominant one in a girl's group."
By 1975, the children were nine years old, and Dr Money published a paper detailing his observations. The experiment, he said, had been a total success.
"No-one else knows that she is the child whose case they read of in the news media at the time of the accident.
"Her behaviour is so normally that of an active little girl, and so clearly different by contrast from the boyish ways of her twin brother, that it offers nothing to stimulate one's conjectures."
Yet by the time Brenda reached puberty at 13, she was feeling suicidal.
"I could see that Brenda wasn't happy as a girl," Janet recalled.
"She was very rebellious. She was very masculine, and I could not persuade her to do anything feminine. Brenda had almost no friends growing up. Everybody ridiculed her, called her cavewoman.
"She was a very lonely, lonely girl."
Faced with their daughter's sadness, Brenda's parents stopped taking her to see Dr Money.
Soon after, they did the one thing Dr Money had warned them against: they told her she had been born a boy.
Within weeks Brenda had chosen to become David.
He had re-constructive surgery and eventually he even married. He couldn't have children himself, but he loved being a stepfather to his wife's three children.
Yet what David did not know was that he had still been immortalised as 'John/Jane' in medical and academic papers about gender reassignment, and that the "success" of Dr Money's theory was affecting other patients with similar gender issues.
"He had no way of knowing that his case had found its way into a wide array of medical and psychological textbooks that were now establishing the protocols for how to treat hermaphrodites and people who lose their penis," said John Colapinto, a journalist for the New York Times who uncovered David's story.
"He could hardly believe that this was out there as a successful case and that it was affecting others like him."
Now well into his thirties, David had become depressed. He'd lost his job and he was separated from his wife.
In the spring of 2002 his brother died from a drug overdose.
Two years later on 4 May 2004, when David was 38, Janet and Ron had a visit from the police. David had committed suicide.
"They asked us to sit down and they said they had some bad news, that David was dead. I just cried."
Cases like "John/Joan" - where an accident had taken place - are very rare. But there are still decisions being made about whether to bring children up as male or female if they suffer from what is called Disorders of Sex Development.
"We now have well-functioning multi-disciplinary teams around the country so that the decision will be taken by a variety of professionals," explained Polly Carmichael from Great Ormond Street Hospital.
"The parents would be much more involved in terms of the decision making process.
In her experience, these decisions have been successful in helping children to grow up to lead a happy and fulfilling life.
"One of the wonderful thing about working with children and their families is that children are amazingly resilient.
"With support, I'm constantly amazed at what children are able to take on and manage."