So he’s been treated with testosterone for a year and a half now and because this is all being controlled by a clinic at Stanford, he’s psychologically tested every few months. And so interestingly, I mean, even though this is now sex hormones being given to an adult, his brain is clearly changing dramatically so his ability in space… spatial ability is increasing. It’s… males do this better than females, where his verbal abilities, females do this better than males, is decreasing with time.

So it got me interested in this whole question of sex hormones and brain plasticity, and so the history here is quite remarkable. It was believed until the early 1970s that the brain of a male and a female is the same in a mammal, like us, and that the reason our behaviour is so different is that, you know, the external genitals are different and social influences affect your behaviour. So the first clue that that wasn’t right was in 1971 where someone in Britain discovered that the female rat brain is quite different, at least in one area, from the male rat brain and now… this has now been shown in many animals and it’s been shown in humans and there are many differences between the male and female brain.

And then in 1990 I believe Dick Swaab in the Netherlands showed that homosexual men, their brain in this one particular area, in one part of the brain has twice as many nerve cells as males that are not homosexual, that are heterosexual. And then a year later someone else in the States found a different nucleus where in homosexuals it’s different, it’s like a female, so that’s rather remarkable. And then in 1995 the same guy who made the first discovery about homosexual differences in the brain looked at transsexuals, male to female transsexuals, and showed that yet a different region of the brain is different in these males. They, in this region, look like females.

So it’s quite a remarkable thing and the… the second thing that’s I think quite interesting is that there’s a very big difference between your sexual identity and your sexual orientation. So Ben Barres is a transsexual; from as long as he can remember he never felt like a girl, even though he looks entirely female… always felt male. He never felt comfortably in his body and that’s true of all transsexuals, but their sexual orientation can be heterosexual, homosexual, bisexual, asexual. So Ben is now homosexual because always has been attracted to men and still is attracted to men. So your sexual orientation is quite different from your sexual identity, and it turns out it’s different parts of the brain that are doing it.

So how does all this happen, and how much of it is genetic, and how much of it is hormonal, and how much of it is behavioural? So it turns out that a hell of a lot of it is hormonal and it sort of works like this: we are born… earlier… not born. A fertilised egg as it develops can become either a female or a male. It depends on one gene sitting on one chromosome, the Y chromosome. So the male has an X and a Y, the female has two Xs. On the Y chromosome there’s one gene. If you have that, you develop as a male. You put that one gene into a female embryo and, bang, it becomes a male.

What that gene does is turn the gonad that’s developing into a testicle rather than… into a testis rather than an ovary, and the testis makes testosterone, the male hormone. And during a critical window of development that hormone makes your brain male, and all your sexual behaviour later is dependent on that sex hormone influence on how your brain develops. Quite remarkable.

Remarkably, it turns out that the male hormone is changed biochemically into the female hormone, oestrogen, in those cells in the brain that are going to be affected. So in the end it’s the female hormone that does it, but it comes from the male hormone, so it’s quite a remarkable thing. So if you take a rodent, a developing rodent and give it testos… a female rodent, genetically female rodent and give it testosterone, its behaviour will be in most ways male when it develops into a sexually active animal.

So these hormonal influences on your brain almost certainly determine your sexual identity, your sexual orientation, and if you ask Barres about mother and pregnancy it turns out that his mother was treated with a synthetic oestrogen, a female hormone, during the first three months of pregnancy. Now, in the 40s, 50s and 60s this was quite a common thing to do if there was bleeding in the first three months. It was thought that this would prevent any abortion that that might be leading to.  It turns out that’s not true, but it was a common way of treating it.

If you follow the female children produced of those pregnancies where the mother received oestrogen… synthetic oestrogen, it turns out that the incidence of lesbianism is very much greater in that population, and Barres’ mother was treated with this kind of drug and almost certainly that is why he’s a transsexual.