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 a “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.

Truth be told, 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. That all this arises from a single fertilised cell, which springs into life when a female ovum and male sperm unite, is nothing short of miraculous.

GENOTYPE

Sex is one of the most fascinating (and fundamental) aspects of human biology – and the most pertinent in the current so called “culture wars”. This curious function of our genetics produces two types of humans – males and females. The differences between males and  females take time to become apparent, because we all develop from a single fertilised cell (zygote), which looked identical to the old-fashioned microscopic eye of the 1950s. 

However, there is one fundamental difference from the outset: the male zygote contains X and Y sex chromosomes and the female zygote contains only X sex chromosomes. More precisely, in the absence of abnormalities, all human females – women and girls – will have two X chromosomes, and all human males – men and boys – will have X and Y. These different sex chromosome complements will drive sex differences in development of male and female zygotes from pre-implantation stage, down the appropriate routes, eventually resulting in adult male and female phenotypes and reproductive functions.

PHENOTYPE

Phenotype refers to observable characteristics, as opposed to the underlying genetic blueprint which gives rise to these characteristics. When we talk about male and female phenotypes in context of biological sex, we are referring to two different adult forms that emerge over time, as the person develops and matures.

Disordered sex development (DSD) doesn’t result in the normal opposite sex phenotype, nor does it constitute a sex change, or erasure of the underlying genetic – ie. chromosomal – sex. This holds true regardless of whether the person even has gonads with which to produce viable gametes, regardless of the appearance or degree of functioning of their reproductive system and certainly regardless of any achieved reproductive potential.

The same can be said for “transgender medicine” experiments, which seek to emulate DSD-type incongruities between genotype and phenotype, in otherwise normally sexed individuals. In both instances, humans came into being as either male or female, and every subsequent cellular division will carry that fundamental blueprint.

Therefore, to say that our genetic sex is indelible and that it asserts itself biologically throughout our lives in myriad ways, is as uncontroversial as medicine gets. When I say “uncontroversial” I mean that it is a fact that is observed everywhere around us. However, human sex differences are extremely controversial culturally, and they have given rise to many deeply controversial practices.

SEX AND GENDER

Human culture, which is greatly influenced by human sexual dimorphism, has had a male sex-bias throughout history, long before we had any idea about chromosomes or genes or indeed gametes (sperm and eggs). Men are typically bigger, stronger and more aggressive, and they have penises and testicles which enable them to impregnate a female. On the other hand, women 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 females subordinate in our society. This creates a social hierarchy called “gender”, in which males are dominant and enabled socially to exploit females. 

Men invented sex-role – gender – stereotypes of masculinity and femininity, 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 lives. 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 46 XY but they can have a genetic disorder resulting in atypical male karyotype such as 47 XXY or 47 XYY – are identified as male, while foetuses that only have X chromosomes – typically 46 XX but they can have genetic anomalies such as 45 X0 or 47 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 the 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. 

PSEUDOSCIENCE AND IDEOLOGY

In this highly ”gendered” environment, we now have “gender identity” ideology, which claims that humans can change sex by changing their physical appearance, and it points to disorders of sex development as “proof”. When confronted with reality of genetic sex in humans – which is present in every cell and this cannot be changed using any known medical intervention – this ideology declares genetic sex and sex chromosomes “irrelevant”, and reaches for any number of bizarre attempts to re-categorise people as male and female based on different set of parameters.

I watched 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 biology-denying activists have captured not just laws and policies but medical establishment itself, with a confusing web of incomprehensible and often mutually incompatible claims, while targeting those who debunk them with 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 been observing this loss of sense and integrity within the medical institutions for years now, as well as the concurrent and aggressive removal of the word “woman” from healthcare, loss of single-sex spaces and deepening of the sex disparity in research which has plagued medicine since its inception. 

Medicine still considers men as a human default. All teaching is primarily done using male examples, with females being trotted out mostly when we talk about female reproduction. Although there have been attempts to reduce this imbalance in recent years, the patriarchal backlash enacted through gender identity ideology has been so extreme that now, even in clinical literature, women are referred to as “(insert uniquely female body part/biological function) haver/body/person”, while men are still men. In addition, sex-denialism has further compromised medical research. Historically, medicine used female menstrual cycle, and it’s role as a confounding factor, as an excuse to use only, or mostly, male subjects in research. In the brief respite period, where medicine acknowledged its own sexism and inadequacy of this approach which harms women, we got a modest increase in female subjects being routinely included in medical research. This has now morphed into a mixed-sex “female” category, which also includes males who “identify as women”. The result is, for the lack of a better expression, a hot mess. A mess that yet again disadvantages females.

DISORDERS OF SEX DEVELOPMENT

I first realised that misconceptions about biological sex in humans have become a serious issue, 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 differentiate 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.”

One example often cited by proponents of this approach is the fact that 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 resulting in infertility and insufficient masculinisation, because both would have been perceived, and would have believed themselves, to be female. So what does it matter what chromosomes they have?

Medical management of DSDs – some of the rarest conditions in medicine – has provided the answer, by demonstrating the indelible nature of chromosomal sex in all sorts genotypic and phenotypic outcomes, and the fundamental ways in which this matters.

For example, certain male-specific DSDs impair the process of masculinisation in the womb, through a genetic resistance to testosterone. As a consequence, affected male babies are born appearing female at first glance, but due to their underlying male sex, they still have internal or partially descended testes. Other DSD males have conditions that result in non-functional “streak” gonads (sometimes testes-like, sometimes ovary-like), a vestigial uterus and Fallopian tubes. Both groups of patients will develop a male-typical body habitus thanks to the tall stature and greater lean muscle genes which are found on the Y chromosome, and they will have increased risk of their gonadal tissue turning malignant. Furthermore, whether because their bodies are resistant to testosterone, or are unable to produce sex hormones altogether due to the absence of 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. 

These males aren’t “female”. They are males who have a developmental disorder affecting their reproductive system, and this creates unique challenges and health needs. These patients don’t need false or ideologically twisted information about their conditions. They need specialist, multidisciplinary medical care, which will adequately address their unique health needs and improve their quality of life.

On the other hand, some biological (46 XX) females can end up with one or more aberrant genes that are normally found on the Y chromosome (such as an sry gene, but not necessarily, as a percentage of female DSD patients are sry negative). The outcome is masculinisation in the womb, resulting in ambiguous or even male-typical genitalia. However, because these female babies lack the Y chromosome, which is needed for normal male sex development, they will develop a female-typical body habitus (short, more delicate stature, less lean muscle mass), they tend to present with gynaecomastia and hypogonadism in puberty, and they are not fertile as males. They too often require sex hormone therapy, in this case testosterone to aid masculinisation and to maintain muscle mass and bone density in adulthood.

This condition, on surface, resembles 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. Kleinfelter’s patients are biologically male, they have male-typical body habitus and have a chance of being fertile as males (although this usually requires assisted reproductive techniques).

I should also mention the rarest of all DSDs, arising from fusion of multiple embryos, or errors in cell division early on in embryonic development, which result in multiple karyotypes (cell lines with different genetics) throughout the body. In cases where cell lines are “sex discordant” (ie. both male and female in one human being, such as 46 XY/45X0/46 XX karyotype) 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.

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. The take home message is that the “spectrum” we are seeing here is not a “spectrum of sexes”. It is a spectrum of severity of disorders that affect sex development within male and female sex categories.

MANUFACTURING SEX

Due to historical ignorance, enduring sex-inequalities, and human wariness of anyone who is perceived as “different”, people with DSDs have experienced discrimination. Unfortunately, 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 DSDs in mystery, developing practices to “assign” sex to DSD babies through combination of opposite sex markers on documentation and medical interventions designed to achieve opposite sex phenotype. As discussed above, biological males who fail to sufficiently masculinise due to a genetic resistance to testosterone, are raised and socialised as girls, and offered hormonal and surgical treatments help them to appear more typically female. By convention, they are referred to in medical literature as “46 XY women/girls/females”, and socially as well as for the purposes of sport, they are classified as women.

In the current social climate, presenting as the opposite sex socially is known to improve the quality of life of many DSD patients, whose conditions result in androgynous or opposite sex appearance. However, disorders of sex development have various degrees of ambiguity, and medical interventions a varying degree of success, so conventions developed over time to overlook inconsistencies. 

In sport, for example, we’ve had a long-standing debate about males with DSDs being allowed to compete in female sport. Those males who have no obvious signs of masculinisation due to their DSD, but have the advantage of taller stature and greater lean muscle mass, have been allowed to compete with female athletes without restriction. Over time, males with DSDs resulting in incomplete masculinisation were also allowed to compete in the female category, followed by males who have undergone physical castration and other interventions associated with “sex/gender/transgender change”.

And although various attempts were made, including testosterone suppression rules, to mitigate the male sporting advantage, all these males have been framed as “women”, “girls” and “female”.

Female athletes, meanwhile, kept objecting to the inclusion of males from the start of female sport, citing fairness as a reason, but 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”. In fact, all throughout this process, the medical facts known about females DSDs causing masculinisation changes in females were used to confuse both the authorities and viewers, about males competing as females as well as about the testosterone doping affecting female athletes. 

The logical end point of this approach happened in 2024, when the International Olympic Committee abandoned any pretence of fairness toward female athletes, deciding to use sex markers on passports as proof of eligibility, knowing full well that men in many countries can obtain passports with female sex markers by a simple written request.

The history of confusion about biological sex is a long one, and it has been exploited by the proponents of gender identity ideology, who posit that sex is not what we are but what we “feel” we should be, and that our bodies need to be altered medically to fit that feeling. It is therefore not accidental that the experimentation on people who “feel they should be” or “want to be” the opposite sex has run concurrently with experimentation on people with DSDs, or that the entire practice of transgenderism seeks to induce DSD-like phenotypic changes in physically healthy individuals. However, while the society acknowledges that experiments on DSD patients have caused far more harm than good, 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. Just like the medical scandal of old, 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 surgeries performed on their healthy 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 that can influence their development or alter their appearance. Some focus on innate or induced sex hormone profiles and judging how sex-stereotypical someone’s behaviour is, in order to determine their sex. 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. I have been following the discourse around biological sex, DSDs and trans for years and I am still surprised by the myriad ways people are able to misconstrue facts of biology, either out of ignorance, ideological primacy or for personal gain. That we now know so much about human biology, and are learning more every day, doesn’t automatically help matters. It only fuels fantasies of “upcoming discoveries” which “must be just around the corner or already happening, it’s just that with all these XY females birthing normal babies, it never occurs to anyone to karyotype them”.

Hopefully, science will stand the test of time, not in the least because it is far more fascinating and miraculous than any myth or a story.

PLAYING GOD

I fear that there is another reason, other than lack of knowledge or information, that will perpetuate ongoing confusion around sex.

“Isn’t nature marvellous?” they say. “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 body is an extremely carefully balanced system, and disturbing normal development and inducing abnormal changes to appearance and function produces a disease state. Regardless of how desired or pleasing to the eye these changes may seem, they don’t constitute “sex changes” or “identities”, but medical conditions which 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. The rationale being that the sooner sex development is arrested and redirected, the better these humans will “pass” as the opposite sex later on. 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 – normalised dangerous ideas, such as uterus implantation 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 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. 

I would like to ask anyone proposing to attempt these experiments: have you considered that requests for these experiments are motivated by male fantasies of miscarrying or having an abortion? What about the rights of mothers whose eggs will be used? And don’t even get me started on how they propose to acquire uteruses for these experiments… 

I hope by now it is obvious why speaking about sex accurately is important. We are not “sexless, bipotential beings”. Sex in humans is not a “spectrum”. Abnormalities – whether genetic or induced – do not change our sex. And misunderstandings around these concepts, coupled with sexism and prejudice, have already caused enough harm. Yet, just for saying this, I’ve been attacked, ostracised, threatened with doxxing, called an “extremist”, “chromosome and genital fetishist”, “phobic” and worse. 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 internet”. It isn’t “just internet”. In real life, female humans have lost all their rights and protections – including the word with which to describe themselves as separate from males – and we are in the middle of social and medical scandals related to the confusion about biological sex. As we march toward an ever-earlier manipulation of sexed bodies – which foreshadows a frankly dystopian future for women and children – I think have to brace myself and keep talking.