There is a quote that I half-remember from my philosophy degree. It went something like: “The existence of the twilight does not mean we cannot distinguish the day from the night.” It is usually (and possibly apocryphally) ascribed to either Samuel Johnson or Edmund Burke, and describes the continuum fallacy, sometimes called the “fallacy of the beard”.

Most of the time we don’t mind simply saying that some cases are ambiguous. But science loves categories. For instance, species are defined as groups of organisms that can interbreed and produce fertile organisms. House mice, Mus musculus, can breed with other house mice and their offspring are fertile. They are a species. They cannot interbreed with the black rat, Rattus rattus, but rats can breed with each other. The black rat is a separate species.

But there are intermediate cases. There are species of finch on the Galapagos which were thought separate, but a hybrid intermediary has been discovered which can breed with both. Does that mean that, instead of two species, the existence of the intermediate means there is now one? Lions and tigers can interbreed, but only the female offspring are fertile. Are lions and tigers therefore the same species? How about dzo, the cross between cows and yaks? Again, the female is fertile, the male not. Do species therefore not exist? 

Or, consider the famous case of Pluto. There are asteroids, and there are planets. Planets are defined by the International Astronomical Union as celestial bodies which are in orbit around the sun, are big enough to have formed a near-spherical shape under their own mass, and which have “cleared the neighbourhood” — that is, swept its orbit clear of other objects. All those criteria are comfortably met by eight of the planets.

But Pluto, although spherical and in orbit around the sun, has not cleared its neighbourhood. Nor have the other bodies Ceres, Eris, Haumea, and Makemake. In 2006, a major astronomical conference declared by fiat that Pluto, therefore, was not a planet; they made up a new category, “dwarf planet”, and shovelled it in there with the other not-quite-big-enough spheres.

There is nothing set in stone about that definition of the word “planet”, or “species”. We could define them any other way. We could say that any sun-orbiting body large enough to have formed a sphere is a planet; then there would be 13, not eight, recognised planets. We could have defined “species” by morphology, by appearance, rather than by breeding compatibility. Definitions are human things, not Platonic forms. Still, almost all humans would agree that Jupiter is a planet, and the asteroid Eros is not; that black rats and house mice are separate species; and that the existence of some edge cases doesn’t change that. 

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Having said all that, I’m going to talk about Jo Swinson, leader of the Liberal Democrats.

Swinson, on Radio 4’s Today programme, was asked by Justin Webb whether she believes that “biological sex exists”. She replied: “Not on a binary, from what I’ve read. I’m not going to pretend that I’m an expert in the subject but I don’t think that things are as binary as are often presented.”

I should say that she was quite tentative, admitted to not being an expert, and said that it was not a scientific debate she was interested in but one about people facing prejudice and discrimination. She also said that most people do fall into the categories “man” or “woman”. I don’t want to accuse her of flying in both-footed, but I do want to look quite closely at what she’s saying.

With sex, just as with the concepts of “planet” or “species”, there do exist ambiguous cases. The intermediate cases in human sex are usually known as “intersex” people. There are various ways in which one can be intersex: the Intersex Society of North America (ISNA)’s FAQ site mentions genital ambiguity, such as girls with very large clitorises or boys with very small penises, or scrotums that have divided to form labia-like structures; external genitals that appear female, but with male-typical anatomy inside; chromosomal abnormalities, such as mosaicism, when some cells are XX and others XY, or having three sex chromosomes such as XXY; or having standard male XY chromosomes but a body that is insensitive to the androgenic hormones such as testosterone, which tell the embryo to start becoming male.

How common intersex is depends on which of those conditions you consider intersex; if it’s all of them, then perhaps one in 100 children. But it’s not that if you’re intersex you’re neither male nor female. One common intersex condition is hypospadias, a condition in which the male urethral opening is not at the tip of the glans but further back along the underside of the penis, which happens once in every few hundred births.

I think it’s fair to say that most people born with hypospadias would consider themselves unambiguously male. Most people born with complete androgen insensitivity will, to all intents and purposes, be female, and may never know that they have XY chromosomes, unless they get genetically tested (although they will be unable to get pregnant). 

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Still, it’s certainly true that some people with intersex conditions would not feel themselves to be the same gender that they were assigned, upon inspection of their genitals, at birth. And it’s also true that, until recently, many intersex babies underwent unnecessary and dangerous surgery because surgeons were too conservative in mind to accept that some people didn’t fit neatly into either sex category. Alice Dreger’s wonderful, horrifying book Galileo’s Middle Finger tells the story of her own activism — along with Bo Laurent, the founder of ISNA — towards ending that practice, and allowing intersex babies to grow up and decide which category they wanted to be in.

Still: that doesn’t mean that sex doesn’t exist “on a binary”. If you did a graph, plotting human beings by height, you will see two clear peaks on that graph — average male height and average female height. If you plot weight, it would be even clearer. Foot size, hip-to-waist ratio. There would be significant overlap between men and women, but there would be very clear differences.

And those are not really sexual characteristics. If you were to plot, say, volume of mammary tissue, or number of ovaries, or number of sperm cells produced, you would see a far, far clearer picture of two enormous spikes with almost no overlap at all. Sex is about as binary as any biological or natural classification gets. It’s certainly a lot more binary than planetary status, and we’re happy to teach kids that Mercury is a planet but Pluto isn’t. There is a twilight, but it’s tiny compared to the size of day and night.

Weirdly, though, “sex is not a binary” seems to be a common claim these days. You get articles about it written in the New York Times, for instance. And you get leaders of relatively major UK political parties saying it in interviews on Radio 4. But while it is possible to define the words “sex” and “binary” in such a way as to make it true, most native English speakers would find those definitions very weird and counterintuitive. 

What’s even weirder is that it’s so unnecessary. For one thing, being trans and being intersex are entirely different things. By the nature of intersex condtions, a larger minority than usual of intersex children end up saying that the sex they were assigned at birth is not the one they are comfortable living as – if your genitals are ambiguous, it’s easier for the paediatrician to make the wrong guess. And some intersex conditions don’t appear until puberty; things like congenital adrenal hyperplasia, in which female-born children develop male sexual characteristics.

But most trans people are not intersex. They are physically male or female, but they have an internal feeling which does not match their external characteristics. I’m going to quote ISNA’s FAQs again, on the difference:

“People who have intersex conditions have anatomy that is not considered typically male or female. Most people with intersex conditions come to medical attention because doctors or parents notice something unusual about their bodies. In contrast, people who are transgendered have an internal experience of gender identity that is different from most people.”

The existence of the “twilight” between the categories of day and night does not matter in the case of most transgender people. The argument is not over whether transgender people’s bodies are ambiguous, but whether someone’s description of their internal sense of gender identity should override their physical appearance and history in terms of how society defines them. In fact, I think quite a lot of intersex campaigners actively want to be distinguished from transgender people, because they think it muddies the issues; ISNA at least doesn’t want to abolish the gender binary.

For the record: I am a coward. I have consistently avoided writing about transgender issues; partly, that is, because I don’t think I have any great insight into them, and partly because I think a disproportionate amount of coverage is given to them already, but mainly it’s because I don’t want the online grief. It is a topic which will definitely get me shouted at, so I have avoided it. On this occasion, though, I wanted to get involved, because the Lib Dem leader talking about sex not being a binary struck me as a big deal. 

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Jo Swinson wants — as she said — to help people who are facing extreme prejudice and discrimination, which transgender people clearly do face. You can do that, without making these at best strange and at worst flat wrong statements about biological sex. You can make the arguments about the Gender Recognition Act, about self-ID for trans people, without tying them to biological claims.

That is also the sensible thing to do, because if you tie a moral argument to a factual claim, then if that factual claim turns out to be wrong, your moral argument disappears. The important arguments are about whether self-ID should be sufficient for legal assignment of gender, or about whether safe spaces for women or women’s prisons should be limited to female-bodied people. Those arguments do not hinge on whether or not a small subset of people have sexually ambiguous bodies.

As I mentioned, I’m a coward, so I don’t want to get into those arguments here myself, although I will note that when Swinson said that women commit domestic violence as well as men and so keeping men out of rape shelters isn’t the whole answer, it’s saying something technically true but kind of asinine. Male suspects in domestic homicides outnumbered female ones by more than seven to one in England and Wales from 2016 to 2018. Men — male-bodied people — are simply much more violent than women, and it’s silly to pretend otherwise.

But those arguments are worth having, and important. We should have them openly, not tie them to a strange and frankly unnecessary sideshow about whether the existence of intersex people means that men and women aren’t real. Twilight is absolutely real, but so are day and night.