Pseudoscience has become a serious problem. From Covid conspiracy theories to climate change denialists, the spread of scientific misinformation threatens our health and the health of our planet. Now there’s a new pseudoscience as bogus as flat-earthism or creationism. But this time there’s something different: those who you might expect to fight against pseudoscience are turning a blind-eye — or in some cases spreading it. This is the phenomenon of sex denial: the rejection of one of the most basic facts of biology in the name of ideology.

I’ve spent much of my career fighting against pseudoscience. I worked with the legendary sceptic James Randi to debunk homeopathy; I’ve also battled climate denialists and anti-vaxxers. I know pseudoscience when I see it. Sex-denial is a classic of the genre, using all the same techniques to sow confusion and misinformation. Their target is the seemingly uncontroversial, indeed obvious, fact that humans can be female or male.

Here’s what the science says: there are only two human sexes. That’s because there are only two types of gamete (the sex cells — egg and sperm). Humans (like all mammals) can develop along one of two pathways: towards making eggs (female) and towards making sperm (male). If anyone ever finds a third sex it would be a discovery on a par with finding a new continent — with a guaranteed Nobel prize. Until you see those headlines, you can rest assured there are exactly two sexes.

Denying sex is dangerous as well as disingenuous

A small number of people have disorders or variations in their sex development (VSDs) meaning some aspect of their anatomy or genetic makeup may be atypical. But most people with VSDs are still clearly and unambiguously male or female. Indeed, most would consider it offensive to say that just because some part of your body is atypical that you are less of a male or less of a female. In a tiny subset it can be difficult to distinguish whether someone is male or female — sometimes called intersex conditions — but these likely account for less than 0.02 per cent of births. So, the overwhelming majority of people are unequivocally female or male, with their sex fixed from before they’re born to the moment they die. None of this is remotely new or controversial (at least in science).

Biological sex exists and it matters — most obviously because the existence of the human race depends on it. You can’t make a human baby without a male and a female — yet the sex-denialists hardly ever mention reproduction. Which is odd since that’s precisely why sex exists. Sex also matters for a host of other reasons. It influences your height, weight, strength and lifespan. It determines your likelihood of getting breast cancer or testicular cancer, heart attacks, mental illness, even your chance of dying from Covid-19. Denying sex is dangerous as well as disingenuous.

So what exactly do the sex-denialists claim? Like climate-deniers or flat-earthers, there’s no single alternative theory — rather a hodge-podge of different claims designed to confuse the public and push an ideological agenda. At the most extreme there are those who flat out deny the reality of sex. “It is not correct that there is such thing as biological sex”, says Prof Nicholas Matte at the University of Toronto. Dawn Butler, a British MP and the Labour Party’s Shadow Minister for Women and Equalities, said on national television: “A child is born without sex.” What is so extraordinary about this claim is that it is so obviously untrue. At least the flat-earthers have some degree of everyday experience on their side: it’s easy to forget we’re on a spinning ball of rock. But to deny something that everyone knows and experiences every day is bizarre — and of course not supported by any science.

To deny something that everyone knows and experiences is bizarre

Another approach is to accept that the sexes exist but imply they’re a human invention, like faiths or football teams. For instance, Chase Strangio of the ACLU says, “The notion of “biological sex” was developed for the exclusive purpose of being weaponised against people.” This is a classic pseudoscience confidence trick. Of course it’s true that all scientific concepts are in one sense human creations. Mammals, atoms, temperature and earthquakes are all concepts created by scientists. However, those concepts are useful precisely because they describe real aspects of the physical world. Surely no one would claim that these exist purely in our minds. Similarly, the reality of biological sex is a fundamental fact about all mammals that existed long before humans did — just as gravity existed long before Newton.

A third approach is to accept that sex exists but claim it’s so complicated that you really shouldn’t bother your pretty little head about it. A recent article in The Skeptic took this approach — drawing an analogy between the concept of sex and the concept of species. It’s true that there are cases where the borderline between species can get fuzzy — for instance hybrid polar and grizzly bears can exist with the delightful name of pizzly bears. But such rare cases don’t invalidate the concept of species — indeed biology would be impossible without it. The overwhelming majority of vertebrate animals are members of a single species — just as most humans are members of a single sex.

Whereas most popular science articles are trying to take a complex subject and make it seem simple, articles like these strive to take a simple concept and make it seem complex. The evidence is clear in one of the most unusual corrections I’ve ever seen. “This article was updated as it previously omitted a reference to primary sexual characteristics.” That’s right — an article all about the reality of biological sex “forgot” to mention the primary sexual characteristics. This is deliberate scientific obfuscation.

One can no more self-identify one’s sex than you can self-identify your height

So why would anyone want to deny something as important and obvious as sex? Perhaps it is the misguided belief that obscuring the reality of sex will help trans people. It is of course important to distinguish between sex and gender or gender identity (someone’s internal sense of who they are and the social roles they fulfil). There are people whose biological sex and gender identity do not match: trans people. I believe people should be free to self-identify as whatever gender they wish. However, one can no more self-identify one’s sex than you can self-identify your height.

This needn’t be a problem — we can celebrate that there are people who want to break out of the traditional roles and social expectations associated with their sex. But the new ideology says that a trans person doesn’t merely change their gender, they change their sex — even if they’ve had no surgery or hormone treatment. This means believing that someone can have a body identical to that of a typical male and yet in fact be female purely through the act of identifying as such. The only way to make that falsehood true is to demolish the very notion of biological sex.

Without the truth on their side, the sex denialists’ only option is to shut down discussion. Anyone who dares question the ideology faces insults, abuse and even violence. It’s an approach that has proven highly successful. Despite this being an issue of great public interest, very few scientists or science journalists have made any attempt to communicate what the science says. When I approached the Science Media Centre, which prides itself on being able to find scientists to talk on even the most controversial subjects, they said they were unable to provide a single expert. Places that once championed rationality and evidence like the Freethought Blog now explicitly ban those who dare present views on the existence of biological sex that they consider heretical.

There is no clearer demonstration that sex denialists are charlatans

When a biologist tweeted that stating biological facts is not bigotry, she was attacked by the very body you might expect to support her — The Royal Society of Biology — which labelled her comments as “transphobia”. Perhaps there was some detail of the science she got wrong — in which case you would expect this learned society to point out the error. But despite numerous attempts to find out what was incorrect about her statements, they have refused to answer. Even at its most censorious — the Catholic Church would tell blasphemers what their crime was. The modern witch-burners won’t even do that — they will rarely even discuss their claims with anyone who does not already share their beliefs.

Even one of the world’s best-known biologists isn’t safe. Prof. Richard Dawkins recently tweeted to ask whether there was a difference between self-identifying your race and self-identifying your sex/gender. This was the final straw for the American Humanist Association which duly stripped him of a 25-year-old lifetime award — something they’d only done once before when a recipient was accused of serious sexual harassment. Humanism is supposed to stand for rationality and freedom of thought, but for the AHA it seems heresy is still a crime punishable by excommunication. These are far from isolated examples. Many academics, particularly women, have faced threats and harassment merely for daring to talk about biological sex. There is no clearer demonstration that sex denialists are charlatans; their only weapons are creating fear and confusion. It’s time the rest of us stood up to them.