Sex Differences between Men and Women

And why they matter in sport.

---

Because apparently it still needs saying, I am going to talk about sex differences and athletic differences, and why this matters for female sport. Some of what follows might sound patronising—indeed, much of the content is taught at primary school level—but the impact I have had in this arena has been building from basic biological principles, rather than after-the-fact firefighting to defend terrible policies.  

Note: A lot of the following is unreferenced. If you need references, you don’t care about sport.

Like almost all animals and many plants, male and female humans are built differently, because they do different things in reproduction. Males have testes that make sperm to be delivered by the penis. Females have ovaries that make eggs to be fertilised by sperm and grow, within her uterus, into babies.

In humans, like almost all animals and many plants, there are two sexes.

Our different reproductive systems develop, in the uterus, under the influence of genes and then hormones, but the role of hormones does not stop at moulding reproductive systems. At puberty, hormones cause males and females to develop sex-typical secondary characteristics that form a mature body prepared to reproduce. Shaped by evolution and sexual selection that, for example, favours fighting skills in male primates, these secondary characteristics also help those males and females secure or attract a mate with which to reproduce.

While both sexes share the experiences of reproductive maturation, growth spurts, radically-increased bone density, the onset of sexual desire, sprouting body hair, and the spotty skin of hormone changes, the most obvious differences in these secondary characteristics are that males—stereotypically hunters and fighters—are bigger and stronger. They are taller with longer bones, broad shoulders and narrow hips. They have far larger muscle mass, particularly in the upper body, and their muscles are built differently, they attach to bones more tightly and they receive movement signals differently. They have bigger hearts, a bigger lung surface, and can carry more oxygen around their bodies. Females—stereotypically choosy attractors—carry a lot more body fat, grow breasts and widen their hips, all the better to bear and nourish healthy offspring, and perhaps to advertise that fact.

There are perhaps thousands of physical differences between males and females, and they are not always as obvious as washboard abs. Females tend to have better peripheral vision than males, and maybe that makes us better at guarding wandering toddlers and spotting berries to gather. In contrast, males are twice as fast at accurately detecting the trajectory of a moving object; that is, how fast is it moving, in which direction is it moving, and where is it going to be 1 second from now? That’s a helpful skill when you’re trying to chuck a spear at a rabbit; it’s also helpful when you’re trying to intercept a rugby ball.

It is no surprise, when considering the physical nature of most sport, that physical differences between males and females lead to functional differences relevant for sport. Height and long fingers permit the easier palming of a basketball through a slam dunk, a move routine in every (male) NBA game but which very few female basketballers have executed. Compared with females, males can run 10% faster, jump 20% longer, throw their balls 50% further and lift 65% heavier. Broader shoulders and greater upper body mass mean males are nearly twice as strong as females through their back and shoulders; they can execute a punch-like motion over 2.5 times harder than females.

The evidence of this superior male athletic ability is written over elite, club and school records, and it is the reason female athletes need a protected sports category. Without them, female athletes cannot hope to win; in some sports, none would make the top 10000. Comparing athletes matched like-for-like in talent, training, nutrition and so forth, the male advantage in athletic competition appears insurmountable; some academics have concluded that females will never run or swim as fast as males. In fact, elite females cannot run as fast as schoolboys, who overtake elite female records from the age of 14 or 15 years old, underlining the significance of puberty as a developmental event.

Male puberty is the making of both the hunter-fighter and the sub-10 second 100m sprinter (a feat achieved by over 150 males but no females). What drives the development of male athletic advantage at puberty? Hormones: specifically, testosterone. This is the “make male” hormone, the “make height” and “make muscle” hormone. At puberty, males experience levels of testosterone up to 20 times greater than females, directing development during the ensuing teenage years of the male secondary sex characteristics that, in the context of sport, are translated to male athletic advantage.

But male advantage over females is not limited to those physical and functional differences conferred by male morphology, shape and size. Most obviously, female athletes must typically deal with the effects of the menstrual cycle and the cyclical effects of hormones on training capacity and performance. The menstrual cycle is known to affect cardiovascular, respiratory, brain function, response to ergogenic aids, orthopedics, and metabolic parameters, and represents a barrier to athletic capacity not experienced by males. Further, injury susceptibility differs between males and females, with subsequent impacts on training time. Emerging research shows that compared with males, female rugby players appear more susceptible to concussive injuries, with more severe outcomes. This has been attributed to lower impact resistance in their neck muscles and more delicate brain structures. Studies of neuronal fragility in male and female nerve cells in a Petri dish have found that female neurons are more easily damaged by stretch injuries.

In most sports, female athletes need a protected category—one that excludes male advantage and treats female athletes as people in their own right—to have fair and equal opportunity. And protections for female athletes, in the form of permissible discrimination, are supported in UK equality law. The Equality Act 2010 s.195(1) allows sex discrimination in sex-affected sports—those where “strength, stamina and physique” influence outcome. Darts and chess may struggle to justify a protected female category on the basis of sex, although they may choose to hold female-only competition to encourage participation in these traditionally-masculine “pub” sports­. But running, rowing and rugby and more can­—should they choose to—exclude males from female sports; no further justification is needed beyond being a sex-affected sport.

---

Female sports categories exclude those who benefit from the boost of testosterone-driven male puberty and the acquisition of greater height, muscle mass, CV capacity and countless other differences than would be present had that male, with a time machine, been conceived as female. Weighing up inclusion of transgender women­—males who, according to Joanna Harper (more soon), have an “innate sense of gender, or gender identity, does not match their biological sex” and who have, in most cases, experienced male puberty—creates a headache for regulators.

Given the key role of testosterone in male development, transgender women seeking to reduce the functional or visual impact of their male physical characteristics may suppress testosterone and supplement estrogen (the hormone that underpins development of female secondary sex characteristics). Many commentators argue that this hormone regime is sufficient to qualify transgender women for female competition, regardless of the magnitude of effect it has on the body (or perhaps assuming it generates parity with female metrics). However, the possibility of transgender women retaining enduring advantage from testosterone-driven male puberty—that testosterone at 13 years old has baked in physical effects that cannot be undone by lowering testosterone at 30 years old—was acknowledged as far back as 1990 by World Athletics (then, the International Association of Athletics Federations), and then reconsidered by the International Olympic Committee (IOC) in 2003 at the now infamous “Stockholm meeting.”

In 2003, the IOC, guided by a single scientific study measuring thigh muscle size in transgender women suppressing testosterone, determined that testes removal at least two years before competing, legal status as female and hormones in line with female profiles—the result of testes removal and estrogen supplementation—would be enough to ensure fairness between females and transgender women. This, despite the suspicion of enduring male advantage and supporting evidence in the study showing transgender women, while losing thigh muscle mass (muscle atrophy being a known effect of low testosterone in males), retained significantly larger thigh muscles than females, even three years after initiating hormone treatment. The lead author’s remarkable interpretation of these data reveal the blossoming degradation of fairness for female athletes; he argued it is “arbitrary” where one draws the line between male and female competition.

The IOC updated their guidelines for inclusion of transgender women in female sports in late 2015. Crucial here was the presentation of a study by Joanna Harper (told you…), of the pre- and post-suppression performances of eight sub-elite transgender women runners (including herself) that calculated similar relative rankings in the female field after testosterone suppression as those achieved in the male field before testosterone suppression. In other words, a male with a particular running score in the male field achieved the same running score in the female field (what we now analyse in terms of “ranking advantage”). Although the attempt to measure actual performance changes in athletic transgender women was—and remains—the right approach to inform guidelines on inclusion, the study was poorly-executed, with the cohort often self-reporting their own times on different running courses over different distances, and with no control for training, injury, diet and so on.

One transgender woman in the Harper study performed far better in the female field than she had in male competition, the result of high intensity training, something one might expect in the athletic population at which derived guidelines are aimed, and providing a glimmer of evidence that testosterone suppression in transgender women does not inevitably damage athletic performance. Harper removed this athlete from her final calculations, apparently an “anomaly.” Harper has been clear that her 2015 study results are limited to sub-elite running (although her lab-based doctoral research is continuing with athletes in other disciplines) and that some amount of male advantage—she often cites height and strength—are retained. In her book, Sporting Gender, she recalls being open about this with the IOC in 2015.

Even if the flaws are ignored, Harper’s data set showed that running performance did not inevitably suffer and Harper herself acknowledged retained advantage. Other experts present raised questions about muscle memory and skeletal proportions. The dude from 2003 updated his previous advice from a 2-year requirement for testosterone suppression to just 1 year, despite his data set published in 2004 showing significant advantage retained at three years after initiation of testosterone suppression. And since 2003, there had been a handful of further studies showing that transgender women suppressing testosterone for at least one year typically lose some mass and strength but retain larger muscles and greater strength than females.

Yet still, in 2015, the IOC issued less stringent guidelines, deciding to jettison the surgical requirement, swap legal female status for a sworn declaration regarding gender identity, and require 1 year of ‘low testosterone’ at a maximum threshold far exceeding typical female levels. So why, despite these scientific data further confirming a 25-year-old hypothesis-now-evidenced that male advantage like larger muscles and greater strength persists in this timeframe, did the IOC reduce the stringency of their physical criteria for entry of transgender women into female sport? To permit entry to a ringfenced category those with disqualifying physical advantages would appear category-defeating. The IOC—and consequently, most sports federations who somewhat lazily defer without scrutiny to the IOC on policy decisions—seem to think differently, although scientific evidence for why they think differently is difficult to find.

---

With an eye on the 2015 guidelines that we inferred the IOC thought secured fair competition for females, Dr Tommy Lundberg (Karolinska Institute, Sweden) and I reviewed what happens to the bodies of transgender women (and other males) when they suppress testosterone. We looked for studies of lean body mass, muscle size and strength in transwomen suppressing testosterone in accordance with those IOC guidelines, and we found a scientific consensus. That is, over a dozen or so papers studying transwomen suppressing testosterone, their skeleton doesn’t change and they lose a little bit of muscle mass and strength. About 5% loss. Compared with nearly twice as much baseline strength advantage in the upper body, 50% in the quads, these small changes don’t meaningfully eat in to strength, which we know is a key metric for male athletic advantage, which we know makes one ineligible for female sports. The female category is set up to exclude male advantage.   

In fact, some transwomen don’t lose any strength at all, and there is a large body of evidence showing that training offsets losses when testosterone is suppressed in males for medical reasons. And if testosterone suppression in adulthood cannot remove male athletic advantage, particularly if someone is in an athletic training programme, then self-identification into a sporting category by “gender identity” collapses sex-segregated sport altogether.

So back to the legal picture (at least, in the UK), the Equality Act 2010 s.195(2) allows for discrimination in sex-affected sports on the basis of the protected characteristic of gender reassignment. That is, transgender women who are biologically male but in possession of a gender recognition certificate (GRC) and, thus, legally female can be excluded if it is necessary to secure fairness and/or safety for female athletes who do not have the protected characteristic of gender reassignment (biological females). The justification for exclusion of transgender women with a GRC—who are legal females with the protected characteristic of gender reassignment—is that male puberty has cemented male advantage—a transgender woman is bigger and stronger than if they had experienced female puberty—and you cannot unboil an egg by turning off the testosterone heat.

(How the law may apply to transgender women who are legally female and have not, via puberty blocking, experienced male puberty is unknown. There is a data gap in the scientific literature, and the physiological outcomes for these biological males requires further study. Recently available is a study showing that: “Trans girls grow tall: adult height is unaffected by GnRH analogue and estradiol treatment.” In this study, transgender girls who had received puberty blockers from around 13 years of age, then cross-sex hormones at 16 years of age, acquired an average adult height far larger than the population female average and close to the population male average. Adult height may turn out to be largely refractory to hormone regimes of any kind.)

Further, one could argue that failure to exclude males from female categories is in itself discrimination against females, most obviously at the individual level where a female athlete loses a place, a prize, some pride or, in the worse case scenario, their physical integrity when competing with or against transgender women. But also, there is indirect discrimination at the policy level where, compared with male sport, female sport is disproportionately affected by trans inclusion policies. A long and distressing history of Eastern Block doping has shown us that females, even pumped full of synthetic testosterone, growth hormone and goodness knows what else, simply do not challenge their male peers. Transmen are not a threat to male competitors, while a single transwoman can displace many female competitors.   

Now, in 2022, some governing bodies like World Rugby (and associated domestic federations like the English and Irish Rugby Union federations), USA Powerlifting and FINA (the international aquatic sports federation) have conducted—and made public their consultation process and data—their own analysis, and banned males who suppress testosterone post-puberty from competing in the female category. One can only hope that other governing bodies are as well-informed about the scientific data as these pioneers.

---

In a widely-reported roundtable discussion with journalists held during Tokyo 2020 in which the first “out” transgender woman Olympian competed in the super heavyweight category of female weightlifting, IOC officials appear to have acknowledged that key athletic advantages acquired by transgender women during male puberty were not removed by the then-current IOC criteria mandating testosterone suppression to 10 nanomoles per litre (nmol/l) for at least 12 months before (and during) participation in female sports categories and that those 2015 participation guidelines were outdated. 

In light of this, and with the interim publication of an extensive consultation by the UK Sports Councils which found that one cannot reconcile fairness and inclusion—sports federations must choose and justify that choice—I was expecting the new IOC guidelines, arriving in late 2021, to be stringent. They, as everyone expected, devolved policy-making to individual sports federations, and also purported to provide a framework under which those sports federations can develop tailored rules. Remarkably, the new 2021 guidelines claim that there should be “no presumption of advantage” for males suppressing testosterone in female sports, which is a position that undermines the very existence of any category.

Conceptually, all athletes have access to the package of talent, strategy, training and dedication that sporting competition seeks to reward. For example, both males and females, old and young, and able-bodied or impaired, can possess the ‘speed gene’ that alters muscle fibre type distribution, thought to favour the explosive power important in sports like sprinting. This same pool of athletes can be coached in equally good strategies and equally effective psychology during gameplay, can train equally hard, can eat equally well, can wear the same shoes, and so on. The gross physiological differences in different types of body—the advantages of being male, being able-bodied and being at peak age and physical maturity—transcend the differences in athletes that result from talent, strategy, training and dedication.

And this is not arcane knowledge. We all know that 40-year-olds have physical advantage over 80-year-olds, even if any given 40-year-old is slower than Usain Bolt will be in 2060. Like for like—equal talent, training and so on—we know which physical parameters confer advantage. They are age, able-bodiedness, sometimes weight, maybe height, and of course sex. Abandoning categories provides sports for 25 year old, able-bodied males, to the exclusion of everyone else.

Even more remarkably, IOC Medical and Scientific Director Dr Richard Budgett claimed that testosterone made no difference to sporting performance, and nobody in the sports science world—and at the World Anti-Doping Agency—is quite sure what to make of the abject denial of stuff we have known about—and exploited to great success in generations of Eastern Bloc females—for decades. 

Recent events in US and British sports have highlighted the inadequacies of policies that require testosterone suppression. This situation cannot continue. We have seen in swimming and cycling a movement from female athletes and advocates—who have been studiously ignored so far—asserting their right to a protected category, one that excludes males, no matter how much sympathy we can feel for those males with identity issues and dysphoria, and who are following the rules set by their governing bodies.

Those rules are wrong and female athletes are speaking up about fairness and science and in a way that cannot, and must never, be ignored.