I have a confession to make – talking about sex on social media makes me feel physically sick. I see a comment that I know I have to respond to, and my stomach sinks. I start to feel queasy. My heart starts beating. I ask myself for a millionth time whether it would be better to just leave it, but having thought through the consequences of saying nothing, I know that in good conscience I cannot ignore it. So I brace myself. Here we go.

You might think that I’m some kind of a prude. Probably by today’s standards I am. These days, hypersexualised adults are inserting themselves in all spheres of life (including those reserved for children) and anyone who tries to evoke principles of safeguarding immediately gets monstered as some kind of “bigot” who is “spreading moral panic”. But that’s not the kind of sex I’m talking about. The conversations I am dreading concern biological sex in humans.

I didn’t use to feel this way. I’m a retired medical doctor so I find all aspects of human biology fascinating (except phlegm – sorry I just can’t deal with phlegm, not happily anyway but that’s a story for another time. Yours, an ironically afflicted asthmatic). I love learning about bodies, keeping up with new developments and I love just how deep our knowledge of human biology and pathology goes. I love the elegance with which nature ensured the myriad complex functions our bodies perform – from sexual reproduction and immunity to detoxification and above all consciousness (truly humbling when you think about it). To think that it managed to make all this develop from a single cell, which springs into life when a female ovum and male sperm unite, is nothing short of miraculous.

One of the most fascinating (and fundamental) aspects of human biology – and the most pertinent in the current so called “culture wars” – is sex. There are two basic categories of humans – males and females. Even though we all develop from a single fertilised cell (zygote), which looks identical to the “naked” microscopic eye, there is one fundamental difference: the male zygote contains X and Y sex chromosomes and the female zygote contains only X.

This is as uncontroversial as medicine gets. When I say “uncontroversial” I mean that it is a fact. However, human sex differences are extremely controversial and they have given rise to many deeply controversial practices.

To illustrate: our culture has had a male sex-bias throughout history, way before we had any idea about chromosomes or genes or indeed gametes (sperm and eggs). Males are typically bigger, stronger and more aggressive, and they have penises and testicles which enable them to impregnate a female. On the other hand, females are smaller, weaker and less aggressive, and they have vaginas, uteruses and ovaries that enable them to get pregnant and birth young. They also have breasts, which secrete milk that feeds the baby in the first months or years of its life. These unique and different reproductive functions, and bodies that develop to support them, are both fundamental to the continuation of the human species as well as a source of incredible vulnerability for human females, both women and girls.

Sexual coercion, or rape, is – feminists would say – a foundation of patriarchy. I don’t want to focus on that enormous and fundamental topic in this essay but I wanted to acknowledge it. Women and girls world-wide are used by men for the purposes of procreation and sexual pleasure. Human males also use rape to dominate and punish females (and occasionally other males too) and the mere threat of this has been enough to keep females subordinate in our society. This creates a sex-class hierarchy where males are dominant and enabled socially to exploit females.

Men invented sex-role stereotypes of masculinity and femininity (otherwise known as “gender”) to justify this hierarchy as “innate and natural” and to absolve themselves of guilt for harming so many women and girls in the course of their lifetimes. I do, to a degree, empathise with men and boys who are born and socialised into this system, but by and large it is women and girls who suffer disproportionately.

For example, ever since humans devised methods to detect which sex the foetus is, they have practiced sex-selective abortions. We have several methods to detect sex in this context, typically using ultrasound imaging to visualise genitals in utero, but if we want to make absolutely sure, we use genetic testing to determine foetal sex chromosomes. Foetuses that have the Y chromosome – typically XY but they can have a genetic disorder resulting in atypical male karyotype such as XXY, XYY – are identified as male, while foetuses that only have X chromosomes – typically XX but they can have genetic anomalies such as X0 or XXX – are identified as female. And I don’t mean “identified” in the sense of “gender identity” or “internal, personal sense of being male or female or neither”. I mean objectively identified as you would identify a species of flower in a forest, or a female cow as fundamentally different from a male bull.

In all our societies, since as far as human written records go, having a male baby was seen as preferable to having a female baby. This has changed somewhat in recent decades, but I don’t think it’s changed as much as we think it has.

Statistics from countries that practice sex-selective abortions (like India or China, especially during their “one child policy”) testify to tens of millions of girls “missing at birth”. This has resulted in an extreme sex ratio imbalance, with so many more men and boys than women and girls, that in some places this has led to horrific sexual assaults by gangs of sexually frustrated, entitled men against the fewer women and girls. Human sex ratio, if nature was allowed to take its course, would likely be quite even, or perhaps slightly more in favour of males, although some studies indicate that female bodies have their own internal wisdom and rationale by which they decide both which sperm is allowed to fertilise their egg as well as which embryo to carry to term, depending on the environmental factors.

Apart from enabling sex-selective abortions, our knowledge that embryos are sexed from the moment of fertilisation (based on their sex chromosome complement) has resulted in some clinics offering sex-selective IVF too. Under the auspices of screening for genetic anomalies, clinicians are able to tell which embryo has a Y chromosome and which doesn’t, and by choosing embryos accordingly, they can guarantee the sex of the baby to the future parents. 

This is an entirely unremarkable state of affairs, considering how strong male sex-bias is in society, and that medicine has rather happily gone along with that. For more info on this much broader topic, I can recommend my book Born in the Right Body and Caroline Criado Perez’ Invisible Women, among a large body of work that addresses this issue. 

In this overall sexist environment, we now have an ideology that essentially denies sex. Or rather, we have a proliferation of activists who claim that humans can change sex by changing their physical appearance. When these activists are confronted with reality – that sex is determined at fertilisation by the presence or absence of the Y chromosome (because both human males and females have an X but only males have a Y) and this cannot be changed using any known medical intervention – they declare genetic sex and sex chromosomes “irrelevant”.

I have seen my medical colleagues laugh this off for years. “Surely these people are just ignorant, they are talking nonsense (not exactly rare on the internet when medical issues are concerned!) and besides – who cares?”

While accurate, such laissez-faire attitude has gotten us to a point where activists who are denying basic human biology have now captured not just laws and policies but medical establishment itself, and any doctor who tries to debunk pseudoscientific “sex denialist” ideology will very quickly experience disproportionate interpersonal and institutional discrimination and violence. 

I have seen doctors wade into these discussions in good faith, only to have activists report them to their governing bodies, which now have policies that conflate sex – being man, woman, boy, girl – with gender – emulating masculine or feminine stereotypes of appearance and behaviour. And god help anyone who doesn’t cower and apologise immediately. 

The general public has observed this loss of sense and integrity within the medical profession for years now, which has been accompanied by the removal of the word “woman” from healthcare, loss of single-sex spaces (which are particularly important to women and girls due to the male violence I briefly discussed above) and deepening of the sex disparity in research which has plagued medicine since its inception. 

Medicine considers men as a human default – all teaching is primarily done using male examples and females are trotted out mostly when we talk about female reproduction. Although there have been attempts to reduce this imbalance in recent years, the backlash has been so extreme that now women are mostly known as “(insert uniquely female body part/biological function) haver/body/person” while men are still men. 

In addition, this sex-denialism has further compromised medical research. Historically, doctors have held an attitude that the female menstrual cycle is just *so unpredictable* in terms of confounding factors, it would mess with their results, so they preferred to use male subjects only. In the brief respite period where medicine acknowledged it’s own sexism and laziness, we got a modest increase in female subjects being routinely included in medical research. This has now morphed into a “female” category, which also includes males who identify as “women”. So we ended up with a male category, and a mixed-sex category which purports to be female-only, and the results are, for the lack of a better expression, a hot mess. A mess that yet again disadvantages females.

I first realised that misconceptions about biological sex in humans have become a serious problem, when I encountered the following argument online:

“We all start out as female in the womb. It is only the development of gonads and subsequent secretion of sex hormones that differentiates a foetus into a male. Therefore, in case of abnormalities which result in complete or partial failure of this developmental pathway, the foetus develops into a female regardless of their sex chromosomes.”

While on the surface this argument (and variations thereof, which are routinely espoused by transactivists as well as some gender critical commentators) may seem simple enough to be correct, it is in fact wrong because biological sex is more than gonads, reproductive system or eventual external appearance.

Prior to the development of genetics and hi-tech imaging techniques, we might not have been able to scientifically explain (or in some cases even tell the difference) between an infertile female and a male with a disorder of sex development that results in infertility and insufficient masculinisation, because both would have been perceived to be female. But now that we know better, why do we allow such myths to persist?

I believe these myths persist because sexist belief that human females are non-masculine, “non-male” or “defective male” human beings still pervades our culture, despite advances in science and more thorough understanding of human biology.

Males who have disorders of sex development that result in them failing to masculinise due to their bodies being resistant to testosterone, still have internal or partially descended testes. Other males have DSDs that result in non-functional “streak” gonads (sometimes testes-like, sometimes ovary-like), a vestigial uterus and Fallopian tubes. Both types of patients have male-typical body habitus (tall stature and greater lean muscle mass), and they have increased risk of their gonads turning malignant. Furthermore, whether because their bodies are resistant to testosterone, or are unable to produce sex hormones altogether due to the absence or preventative removal of functioning gonads, sex hormone supplementation is needed for optimum health. Traditionally, feminising hormones such as oestradiol were used for this purpose, but studies show that in some cases, testosterone can have an equally beneficial effect. So males with DSDs and androgynous or feminine appearance aren’t females. They are males who have a developmental disorder affecting their reproductive system, and this can have a more wide-ranging effects too. These patients don’t need false information about their medical conditions. They need specialist, multidisciplinary medical care, which will adequately address their unique health needs and improve their quality of life.

Ditto for biological (46 XX) females who end up with one or more aberrant genes that are normally found on the Y chromosome (such as an sry gene). This masculinises them in utero, resulting in a development of a penis and testicles. However, because they lack the Y chromosome, they have female-typical body habitus (short, more delicate stature, less lean muscle mass), they often present with gynaecomastia and hypogonadism in puberty and they are not fertile as males. These patients often need testosterone therapy in order to sufficiently masculinise, and to maintain muscle mass and bone density in adulthood. This condition may on surface resemble a male-specific DSD called Kleinfelter syndrome, which occurs as a consequence of 47 XXY karyotype. Although some clinical features and treatments are similar, there are significant differences between these two conditions, not in the least that Kleinfelter’s patients are biologically male, have male-typical body habitus and have a chance of being fertile as males (although this usually requires assisted reproductive techniques).

Disorders of sex development are numerous, varied and they come in different degrees of severity. The above examples are by no means an exhaustive list of these conditions, and we can count that going forward, there will be more new and unique cases. But these conditions are sex-specific. So the “spectrum” that exists is not a “spectrum of sex”. It is a spectrum of severity of developmental disorders that affect sex development within male and female sex categories. I should also mention DSDs that result from fusion of multiple embryos, or errors in cell division early on in embryonic development, which result in multiple cell lines throughout the body. Cases where these cell lines are “sex discordant” (ie. both male and female in one human being) are extremely rare. Fertility in such people is even rarer. Importantly, we can understand what happened – and offer appropriate medical care, information and prognosis to these patients – precisely because we can identify biological sex of individual cell lines. However, take home message is that for the most part, disorders of sex development occur in people who have developed from a single embryo and who have only one cell line throughout their bodies. This cell line has sex chromosomes which determine their biological sex.

You can imagine, though, how much people with DSDs struggled in the past. Partly due to our ignorance about human biology, partly due to sex-inequalities, and partly due to human wariness of anyone who is perceived as “different” (whether they are disabled, not pleasing to the eye or not “typical”), these people have experienced discrimination, name-calling and even medical abuse.

However, instead of embarking on a long-standing public health campaign to educate the population about biological sex and disorders of sex development, clinicians and authorities shrouded these conditions in mystery, and developed conventions to “assign” sex to these patients, depending on the likely outcome of their development and medical treatment. For example – as discussed above – biological males who fail to sufficiently masculinise due to a genetic resistance to testosterone, are raised and socialised as girls, and hormonal and surgical treatments help them to appear more typically female. In literature they are often referred to as “XY females” and socially as well as for the purposes of sport, they are classified as women. 

In the current circumstances, presenting as the opposite sex likely improves the quality of life of DSD patients whose conditions result in androgynous or opposite sex appearance. However, because the disorders of sex development have various degrees of ambiguity, this doesn’t work for everyone. So conventions developed to “overlook” inconsistencies. 

In sport, for example, we’ve had a raging debate for decades, about males with DSDs being allowed to compete in female sport. Those who have no obvious signs of masculinisation but have the advantage of taller stature, greater lean muscle mass etc. have been allowed to compete with female athletes without restriction. This allowed other males with DSDs resulting in incomplete (ie. partial) masculinisation to compete in a female category too, and various rules, including testosterone suppression, have been devised to mitigate their male sporting advantage. The studies that looked into this issue conflated female sex with incomplete masculinisation in DSD males, and concluded that having a Y chromosome was an “acceptable variable that contributes to athletic success in elite female athletes”.

So the history of confusion about biological sex, which stems from the existence of abnormalities of sex development, is a long one, and because it is embedded in so many of our cultural practices, it has been exploited by the proponents of the ideology which posits that sex is what we “feel” we are and that our bodies need to be altered to fit that feeling. The fact that some people’s sex development is naturally impaired, and that their physical appearance (which is a combination of their development and medical treatments) determines which sex they are perceived as, has been used to argue that normally sexed adults can change their sex with help of medical intervention.

It is not accidental that experimentation on normally sexed adults who “feel they should be” or “want to be” the opposite sex has run concurrently with medicine experimenting on people with DSDs. However, while the society acknowledged that experiments on DSD children have caused great harm through impairing their natural development, in the case of normally sexed people who seek to “change sex” the rationale has developed that the earlier we start to medically modify their bodies the better.

So just like the medical scandal where DSD children had surgical, hormonal and psychological interventions to make them resemble the opposite sex, we are now faced with a medical scandal of convincing children via schools and the media that they can “choose to be male or female” and doctors are being encouraged to block their normal sex development through a misuse of puberty blockers, cross sex hormones and cosmetic surgeries performed on their sex organs. 

Proponents of this ideology see nothing wrong with what they are doing, because they misguidedly believe that all humans could go either way, depending on various factors that can influence their development. Some focus on hormones, others go more deeply (but not quite deeply enough) and they cherry pick genes that significantly alter the appearance of genitals and the reproductive tract. In either case, they are refusing to see the bigger picture, where different sex chromosome complements – XX for female and XY for male (and if we want to encompass atypical karyotypes then it is the presence of the Y chromosome and the number of copies of each sex chromosome) – drive sexual differentiation from the moment of conception, and have wide-ranging and indelible effects on all tissues in our bodies.

I fear, however, that there is a further consequence to this lack of understanding of human sexual differentiation.

Asserting that embryos are “sexless” until gonads develop and sex hormones start to influence development of reproductive structures (either normally and in accordance with the sex chromosomes OR abnormally and at odds with the sex chromosomes) serves to absolve those who support “sex change” experiments of responsibility. 

“Isn’t nature marvellous?” they cry. “Isn’t medicine? We are beings with infinite (sex) potential! This means that sex really IS a spectrum between male and female, because either in utero, or later, we can make people look like one or the other, regardless of what nature gave them. That means it is us, humans, our minds, our science, our will and our imagination that determines our sex, not some pesky chromosomes – or genitals for that matter – which are so outdated. Get your nose out of your medical textbook grandpa and read this marvellous article written by someone with no medical expertise (but great enthusiasm for internet fame or fondness for science fiction!), who is telling you that sex has been redefined! Look at this colourful diagram some guy mocked up in photoshop and stop being a bigot.”

What they fail to appreciate is that human sex development is absolutely fundamental and predictable, depending on the sex chromosome complement. When something goes wrong (either in utero or later in life), it produces disease states. Regardless of how pleasing to the eye these consequences may be, they aren’t “sex changes” but conditions that carry risks and often require life-long medical care. 

We have already seen these disease states induced in healthy children who some adults thought “might come to identify as the opposite sex in adulthood”. This was done because the thinking is, the sooner sex development is arrested and redirected, the better these humans will “pass” as the opposite sex. 

Now contemplate a new way for unethical doctors to offer “sex selection” to prospective parents. Not sex-selective abortion or even sex-selective IVF, but hormonal and genetic manipulation of their baby’s sex development while the baby is still in the womb.

Or consider how this “lego” approach to biological sex – adding and subtracting sexed body features and parts – brought us to a point where healthcare institutions are proposing to try and implant uteruses in normally sexed males, whose bodies do not have the fundamental capacity to accommodate a uterus, or a growing pregnancy for that matter. All the microscopic surgery in the world – even if it succeeds to attach a female uterus to some structures in the male abdomen – cannot accommodate a shift in intra-abdominal organs, blood volume, immunity, pelvic bones and a host of other changes pregnant women naturally go through. And what about organ and foetus rejection? Uterus transplants in women have been most successful if done between identical twins. Anything else would necessitate immunosuppressant drugs which would harm the baby. 

So, I have to ask colleagues who are proposing to attempt these procedures: is this all about a male fantasy of miscarrying or having an abortion? What about the rights of mothers whose eggs will be used for these experiments? And don’t even get me started on how they propose to acquire uteruses for these experiments… 

So this is why I speak up whenever I see myths about human sex. We are not “sexless, bipotential beings”. Sex in humans is not a “spectrum”. Abnormalities – whether genetic or induced – do not change our sex. 

However, just for saying this, I have been attacked, ostracised, threatened with doxxing, called an “extremist”, “chromosome and genital fetishist”, “phobic” and worse. But, unlike so many of my esteemed medical colleagues, I can’t just shut up and keep out of it. I can’t laugh it off. I can’t maintain an irrational belief that “all will be well in the end” and that this is “just the internet”. It isn’t “just the internet”. If you haven’t noticed, there are several medical scandals related to the confusion about biological sex, and as we march toward an ever-earlier manipulation of sex development at the hands of people who systematically ignore the harms – which foreshadows a frankly dystopian exploitation of women and children – I have to brace myself and keep talking.