Professor Robert Winston, the pioneer of fertility treatment and IVF, said: "I will say this categorically. You cannot change your sex. Your sex is there in every cell in the body. You have chromosomal sex, genetic sex, hormonal sex, all sorts of different types. Psychological sex and brain sex. We have unfortunately got very confused about this. Regrettably, we have got into this argument." The scientist who developed the medical treatments that allow infertile couples to have children knows a thing or two about hormones, genetics, and the differences between male and female bodies.
Some people - some of them scientists, some of them philosophers - argue that sex is not binary, but a "continuum" or "spectrum". Lots of people cite Anne Fausto-Sterling, a Professor of Biology and Gender Studies. She falsely claimed that 1.7% of the population is intersex, and that therefore sex was not binary.
But she started with the idea, and then tried to find evidence to justify it. She included conditions which clinicians do not recognize as intersex, such as Klinefelter syndrome and Turner syndrome. These are conditions where someone is still obviously male or female, but they have an extra chromosome, or one of their chromosomes is not functional. Thsese are malfunctions that cause unforunate health conditions. They are not extra sexes, or intermediate states between male and female. She included late-onset adrenal hyperplasia, where a lack of a certain enzyme causes hormonal problems. In fact, her criteria were so wooly that a woman who lost an ovary or who had a hysterectomy would be counted as intersex!
In reality, conditions in which chromosomal sex is inconsistent with phenotypic sex, or in which the phenotype is not classifiable as either male or female, occur in 0.018% of the population. A hundred times lower.
If you test the DNA of any random indivudal trans person, there's a 99.92% chance that they are not "intersex" or have some chromosomal abnormality. Over the past 30 years there has been loads of studies, and if there was a tendancy for trans people to have biological abnormalities compared to non trans people, that would have been discovered and been big news! People that identify as the opposite sex are no more or less likely to have one of these rare medical conditions. You could test one thousand trans women and the liklihood is that every single one would have male chromosomes, XY. In fact genuine intersex people have gotten angry and told trans people to stop bringing them into their arguments, because they have nothing in common.
The fact that there are a very small number of people that do have such abnormalities who are not trans does not mean there is a spectrum or continuum of sex. Even if the false claims were true, and it was the much bigger number of 1.7%. As professor Kathleen Stock points out, “hard cases are not a special fact about the categories of male and female,” and “difficulty about borderline cases is absolutely standard for biological categories”. Kathleen Stock is a philosopher, not a biologist, but her statement was based on gathering research from biologists, and it is a correct biological point.
There are people that are born without arms or legs, or with 6 fingers, or who have their heart on the right side of the body. Yet we can confidently say that human beings have 5 fingers on each hand, and that the heart is on the left side of the body. Human beings do not have a "spectrum" or a "continuum" of arms and legs - normally humans have 2 arms, 2 legs, and occasionally there's an abnormality.
Unlike sex, it is actually reasonable to argue that "race" (white, black, native american, etc) is a spectrum or a continuum. If someone has a black parent or a white parent, are they black, white or mixed race? Barack Obama identies as black despite having a white parent. Yet a white person identifing as black or asian is considered unacceptable.
And in fact, it is reasonable to argue that "species" (homo sapiens) is a spectrum. Long ago, all lions, tigers, panthers etc evolved from the "Proailurus". But as evolutionary biologist Professor Richard Dawkins likes to point out, animals don't just suddenly evolve from one specides into another overnight, there must be a continuous sequence of intermediates. So in the fossil record you might find animals that have traits that are somewhere inbetween Proailurus and a Lion. So deciding exactly where Proailurus ended and the fist lion began is tricky, there are edge cases.
And yet it is possible for biologists and zoologists to define what a modern-day lion is. Lions and tigers can mate with each other, and can produce fertile offspring, and yet they are considered seperate species because they each have their own subset of genes that they share within their species. There are certain traits that all lions have, that tigers do not.
By comparison, in Kathleen Stock's words, "the sex division is one of the most stable and predictable there is". It is easier to be certain about sex than it is about species!
Or, in Richard Dawkins's words, race and species are spectrums, but "sex is a true binary".
www.newstatesman.com/ideas/2023/07/biological-sex-binary-debate-richard-dawkins
https://richarddawkins.com/articles/article/race-is-a-spectrum-sex-is-pretty-damn-binary