OK, so “sex change” is not a approved term these days, and anyway doesn’t really happen as such, but that doesn’t stop people wishing. I know when I was a kid I used to go to bed dreaming that when I hit puberty I’d grow breasts and start to menstruate. It didn’t happen. I guess that trans boys go to sleep at night dreaming that when they hit puberty they’ll grow penises. And the interesting thing is that some of them get lucky. It really does happen.

Currently BBC2 is running a short series of medical documentaries fronted by Michael Mosley and titled, Countdown to Life. They are all about the weird and wonderful things that can go on during embryo development in the womb. Doctors tend to call these things “developmental disorders”. I prefer to call them expressions of natural human diversity. Some of the conditions Mosley talks about in the series are trans- and intersex-related.

Depending on exactly how things go as you are growing from an egg into a person, interesting things can happen that mark you out from the mass of humanity. You might become left-handed. You might become an albino. You might grow six digits on your hands and feet instead of five. Or you might develop an intersex condition.

You may have heard me talk before about Androgen Insensitivity Syndrome. This is an intersex condition in which the body is unable to process testosterone. The child has XY chromosomes, but the Y has no effect because testosterone can’t do the job of masculinizing the body. These people are born looking perfectly female, grow up as girls, and go through puberty as girls. They grow breasts, but probably don’t menstruate. Often they don’t discover the truth about their biology until they have treatment for infertility and discover that they have no wombs.

Last night’s program introduced me to a different testosterone-related condition. It is known as Guevedoce, a term derived from a small Puerto Rican community where it is quite common. Children who have this condition lack the ability to make a special version of testosterone called dihydro-testosterone without which male genitalia do not develop in the womb. Consequently the children are assigned female at birth. But, like the people with AIS, these kids have XY chromosomes. The term Guevedoce translates as “penis at twelve”, because when puberty hits and a new surge of testosterone floods the body the developmental process gets kicked into gear and the kids, quite naturally, grow penises.

The really interesting thing about these two conditions is that most kids with AIS identify as girls, even before puberty, and tend to be distraught when their condition is discovered, especially if, as is generally the case they can’t have kids. In contrast, Guevedoces tend to identify as boys long before puberty reveals the truth about their biology, and they are delighted when they grow penises.

Here we have two very similar intersex conditions, one of which normally results in a gender identity at odds with the chromosomal sex, and one of which normally results in a gender identity congruent with the chromosomal sex. That’s pretty impressive circumstantial evidence that gender identity is a biological thing.

Mosley agrees. Later in the program he features a trans girl from California and explains, as I already knew, that the embryonic process that results in gender differentiation of the brain is separate from, and occurs at a different time to, the process that results in gender differentiation of the body. He sounded convinced that a biological explanation for being trans will be found.

Of course it isn’t that simple. It may well be that the process that causes someone with XY chromosomes to be trans is different from that which causes people with XX chromosomes to be trans. People who are non-binary may turn out to have a mild form of one or other of these conditions, or they may be something else entirely. In any case it shouldn’t matter. Trans people very obviously exist, and treatments are very obviously highly effective. We shouldn’t need a biological explanation to treat trans people as ordinary, sane human beings.

However, the more science like this we discover, the more obvious it becomes that those people who try to claim that “science” proves that trans people can’t exist make no more sense than those people who say that the Bible proves that trans people can’t exist. I think we’ve reached the point where we have to lump them in with evolution deniers and flat earthers.