Do Trans Women Belong In Sports?

It isn’t controversial to state that males are, on average, physically stronger than females, which is one of the many arguments for why sex categories in sport ought to be maintained. On average, males are, pound for pound, stronger than females (with significant overlap). But does this generalization hold true for trans athletes as well? 

If we were to merely go by statements of some sporting organizations like FINA (the world governing body for swimming), then we might be inclined to believe so. But there's reason to suspect these organizations, though well-intentioned, are operating on outdated notions of gender. ‘Femaleness’ and ‘maleness’ are not immutable properties. Organisms change over their lifespans due to maturation, external factors, and medical intervention. Biology has to allow for this mutability.

We describe humans as having four limbs, but you’re no longer four-limbed if an accident claims one of your hands. Similarly, when you were to alter your reproductive system and sexed physiology the way trans people do, you alter those dimensions of your overall sex. Depending on the field of study, an organism’s sex is defined based on a subset of several characteristics (relating to reproductive capacity) that are relevant to that field of study. For biologists who study the reproduction on a species level, sex is defined on the basis of gametes (gametic/biological sex). For geneticists researching how sex differences arise, sex is defined on the basis of genetics/chromosomes (genetic sex). In social contexts (and also for animal behaviorists), sex is usually defined based on phenotype and/or outward presentation. Obviously, gametes, gonads, and chromosomes, by themselves, do not play a direct role in sporting ability. It is instead the downstream effects of some of these traits. An individual’s current hormonal balance (which defines one’s hormonal sex) more directly impacts sporting ability. 

There has been some debate as to whether pre-pubescent girls are athletically identical to pre-pubescent boys. While research suggests that prepubescent boys tend to have around 10 percent more lean body mass compared to prepubescent girls (individual exceptions exist and this difference may not be the same in athletic populations), the same research tends to find minimal to no differences in other characteristics relating to athletic performance such as average speed, bone characteristics, pulmonary structure/function, etc. Not all studies demonstrate sex differences in total body fat prior to puberty. As for the 10% difference in lean body mass, no studies calculate how much this difference might affect overall athletic performance in this age group, where skill, coordination, and training play a far more important role.

Much of the peer-reviewed literature of competitive age-grade sports only finds a paltry difference ranging between 0 to 5 percent in performance depending on the sport. Most peer-reviewed literature routinely characterizes this difference as minimal or nonexistent, since it doesn’t provide a consistent or overwhelming advantage in well-structured youth sports environments. It's also minor compared to the total spread in performance among same-gendered youth athletes.

Some have implied this minute difference is driven primarily by innate physiology. But it could just as easily be attributed to better opportunities for boys to participate in sports and more societal encouragement of athleticism in boys. Female athletes are offered significantly fewer opportunities than male athletes, regardless of age. There are presently no studies quantifying how much of the difference in athletic performance between boys and girls before puberty is caused by social factors versus innate physiology. Seemingly backing the innate physiology angle, however, is a study showing that girls as young as four exhibit slower reaction times than boys, but this result doesn't show the whole picture, as “a previous study showed no significant differences between boys and girls in [reaction time] for age groups 6-12 years.”

Most importantly, none of this can be used to justify excluding prepubescent trans girls from girls’ sport, for the simple reason that they’re not cis boys, regardless of what anti-trans mouthpieces argue. Prior to taking pubertal suppressants and hormones, trans girls seem to exhibit different physical attributes compared to cis boys, including body fat percentage, where they’re closer to cis girls (likely due to the psychological effects of gender dysphoria). Even if we wrongly assume both categories are identical, no evidence suggests the potentially minimal to nonexistent differences in athletic performance carry forward after puberty blockers and hormone therapy, which are known to affect characteristics like lean body mass.

Thus, there’s no good reason to sort prepubescent trans children according to their gender assigned at birth, nor is there a legitimate reason to prevent trans women who medically transitioned at the onset of puberty from participating in sport. Public and institutional opinion seem to have come down against trans women who took hormones after undergoing a ‘male’ puberty, on the basis that they must be stronger and larger than cis women.

There's a misconception that muscle tissue in males is not just bigger, but also stronger than muscle tissue in females. This is not the case. When expressed per unit of muscle cross-sectional area, differences in strength between the sexes effectively vanish. If a ‘male’ muscle and ‘female’ muscle are the same size, they exert the same amount of torque on the joints. 

Nearly all sex-specific differences in athletic performance come down to fat-free mass. In strength sports like weightlifting, males can carry a lot more muscle within their weight limits than female athletes without health risks. In average men and women, a significant portion of the strength difference arises from men being taller than women on average. Adjusting for height and weight still leaves a strength disparity of around five to fifteen percent. But adjusting for fat-free mass eliminates this disparity, since men on average can carry a lot more muscle on their frame at a given body weight compared to women of the same height and weight. 

Testosterone is primarily responsible for the increase in muscle mass, and estradiol is the primary driver for the increase in fat mass. In other words, testosterone greatly benefits athletic performance. It increases the synthesis of muscle proteins, which leads to more muscle, including cardiac muscle. It contributes to an increase in height, bone formation, and stimulates red blood cell production, which correlates with oxygen carrying capacity. 

Though height plays a significant role in performance differences between males and females, it is a far weaker justification for sport segregation. Even in sports like basketball where height can be advantageous, we don't segregate based on height. This is because height doesn't meet our standard for intolerable unfairness. In fact, height can occasionally prove to be a disadvantage in sport, since taller people would have to engage in more work to lift the same load compared to a shorter person. 

Two studies, one by Timothy Roberts et al. in 2020 and a follow-up by Elvira Chiccarelli et al. in 2022 indicate that after about one year of hormone replacement therapy (HRT), athletic trans women are no longer advantaged compared to athletic cisgender women when controlled for height in measures of endurance, and after two years they are no longer advantaged in strength. There is some controversy, however. Some reviews of the literature use gross measures of lean body mass without considering height as a primary factor affecting the latter, naturally finding that the taller-on-average trans women (who underwent a testosterone-fueled puberty) have higher lean body mass compared to the shorter-on-average cis women. 

The 'advantage view' and the 'range view'

A divide persists over the question of whether height, with its effect on lean body mass, is enough to consider trans women advantaged. Proponents of the ‘advantage view’ argue against the so-called ‘range view,’ which they claim suggests that since trans women fall within the female range of performance metrics, they should be allowed to participate in women’s sports. The ‘advantage view’, on the other hand, argues that being taller than the average cisgender woman constitutes a categorical difference that should cause them to be seen as universally advantaged. Some proponents of this ‘advantage view’ go so far as to argue that the ‘range view’ is an argument against the existence of women’s sport; rather, it would prescribe a sports category based on some set of metrics as a substitute for women’s sport, such as tall sport and short sport. To put it simply, proponents of the ‘advantage view’ argue that ‘male advantage’ must be entirely removed in a trans woman if we are to allow her into women’s sport, even though the actual ‘advantage’ in question does not translate to differences in athletic performance.

In practice, the ‘advantage view’ asserts that, since HRT cannot reduce the increased height, bone size, lung size, etc., afforded by a testosterone-powered puberty, there is simply no way to fairly include any trans women who transitioned after puberty. In other words, height should not be controlled for in evaluating athletic performance for these trans athletes, suggesting that any height derived from ‘male’ puberty remains inherently unfair.

This entire framing of an ‘advantage view’ versus a ‘range view’ is flawed. It obscures the fact that an average difference in a physiological variable (height) does not translate to a difference in actual athletic performance. It's like saying a bigger trans woman who can do ten push-ups has an unfair advantage over a smaller cis woman who can also do ten push-ups, just because the trans woman is pushing more absolute weight. This standard of fairness is incoherent because it would mean the trans woman would have to perform significantly worse at push-ups for it to be considered fair to the smaller cis woman. Another analogy would be claiming that untrained Dutch women, as a category, have an unfair advantage over the category of Japanese female athletes due to their height, despite the obvious reality that this is not the case. 

Arguing that height ought to be considered an advantage in the case of trans women but not cis women begs the question—it presupposes that trans women are male, and that male and female are mutually exclusive, binary categories that would justify marking trans women as categorically separate from women. Built on these presuppositions is a double standard: even a minor, overlapping characteristic that’s testosterone-driven is invalidating in (what they deem to be) a male body, but is acceptable in (what they deem to be) a female body. 

Further, if someone is already considered ‘female,’ sporting organizations do accept some anomalies caused by ‘partial masculinization’ (such as XY chromosomes or increased testosterone levels during/after puberty due to genetic variations or adrenal disorders) provided that the individual’s circulating testosterone levels are within the ‘female range.’ In trans women, the effects of prolonged HRT resemble the aforementioned ‘partially masculinized’ physiology seen in some cis women, albeit with a different set of causes. If a cis girl grew unusually tall during puberty due to a condition like polycystic ovarian syndrome (PCOS) or a severe adrenal disorder that led to high androgen levels (which may delay the closure of growth plates, resulting in increased height), she would still be allowed to participate in sport, provided that her hormone levels are regulated in the present. To apply a different standard to a post-transition trans girl who was forced to undergo a natal puberty would be discriminatory. 

Interestingly, Lionel Messi underwent growth hormone therapy as an adolescent to make him taller. Yao Ming reportedly also received hormone treatment during puberty for the same purpose. Neither of them have been banned from sport.

As discussed in my past article, treating ‘maleness’ and ‘male advantage’ as a discrete and innate trait possessed by cis men and trans women is far from accurate. The concept of ‘male advantage’ is, by its nature, reducible to various traits, and is not a fixed, male-exclusive attribute. Eliminating these traits would be sufficient in eradicating ‘maleness’ as an advantage in an individual. After all, ‘maleness’, as a biological category, relates to multiple properties (chromosomes, hormones, gonads, gametes, genitals, etc.).

What we know so far

More recent studies have investigated the relationship between HRT and athletic performance in more detail and addressing issues of selection bias.

A scientific review commissioned by the Canadian Centre for Ethics in Sport (CCES) found that most studies in this area between 2011 and 2021 failed to adequately adjust for factors such as height or lean body mass. The review also found that almost no studies examining the effects of testosterone suppression on trans women did so among trained athletes (they relied on information from sedentary adults instead), and that studies on the effects of testosterone on sport performance involved examination of individuals who use performance-enhancing drugs, rather than individuals on HRT: two dissimilar cohorts. 

The review concluded the following, given the limited evidence available: 

1. It only takes four months for testosterone suppression to undo the higher levels of red blood cell count (which affect oxygen-carrying capacity), hemoglobin (the holy grail for endurance athletes), and VO2max (the maximum rate of oxygen consumption attainable during physical exertion, currently the best measure for how much aerobic work one can do) experienced by cis men.
2. There is no basis for athletic advantage conferred by bone size or density beyond advantages gained by height. Elite athletes tend to be taller regardless of gender, and even in sports like basketball, height is not currently classified as an athletic advantage requiring regulation.
3. Non-athletic trans women experienced significant reduction in LBM, CSA, and strength loss within 12 months of hormonal suppression. 
4. When adjusting for height and fat mass, LBM, CSA, and strength after 12 months of testosterone suppression, trans women still retained statistically higher levels than sedentary cis women. However, this difference is well within the normal distribution of LBM, CSA, and strength for cis women. Importantly, LBM, CSA, and strength loss continues for trans women after the 12-month initial testosterone suppression.
5. The limited available evidence examining the effect of testosterone suppression as it directly affects trans women’s athletic performance showed no athletic advantage existing after one year of testosterone suppression.
6. Post gonad removal, many trans women experienced testosterone levels far below that of pre-menopausal cis women.

A 2022 cross-sectional study by Mobilia Alvares et al. showed that after 14 years of HRT, both muscle quality and VO2max levels in trans women were in the low-female range. The study also showed that, although higher in absolute terms, when VO2peak (highest value of VO2 attained upon an incremental or other high-intensity exercise test) was adjusted for weight, there were no statistically significant differences in the levels between trans women and cis women. Alvares et al. also measured expiratory volume in trans women, finding there to be no statistical significance when compared to cis women. The authors hypothesized this observation to be caused by estradiol acting as a potential bronchoconstrictor, reducing the caliber of the airways in the lungs of trans women. 

A 2024 cross-sectional study commissioned by the IOC (International Olympic Community) found that trans women athletes might even be at a disadvantage when compared to cis women athletes when it came to lung function and cardiovascular fitness.

A 2025 study on trans women volleyball players aligned with these findings, indicating that they performed similarly to cis women volleyball players, or were even disadvantaged.

Other metrics offered by those against inclusivity in sports are grip strength (supposedly a good proxy for athleticism), pelvic width, bone density, maximal pulmonary ventilation, blood volume, heart size, etc. Males on average are argued to have better grip strength, a narrower pelvic width, higher bone density, greater maximal pulmonary ventilation, and more. Since some trans women share these characteristics, the argument goes that this allows them to be advantaged in sport. 

1. Grip strength: This is just as much a measure of hand size (correlated with height) as it is muscular strength, since larger hands find it easier to grip the testing device. This test is used because it’s easy, not because it’s accurate and not because it’s a good proxy for athleticism. 
2. Pelvic width: Although some assume that a narrower pelvic width could affect performance, this is likely not the case. Studies that examine pelvic width cannot find any meaningful contributions of the latter to the gait differences or lifting mechanics between males and females, finding those differences to be a lot more associated with height and socio-cultural factors affecting how men and women typically move. It was previously believed that males had a lower incidence of knee injuries because they had a lower Q angle (the angle quadriceps pull on the lower leg) attributable to a narrower pelvis. But the relationship between pelvic width and Q angle is disputed, and studies indicate that an increased Q angle doesn’t automatically predispose female athletes to knee injuries, as this risk can be entirely removed with appropriate training.
3. Bone density: Although trans women seem to maintain their bone density post HRT, there are no studies that suggest bone density causally improves athletic performance. More importantly, black cis women, for instance, are known to have greater bone density than white cis men, erasing any potential for bone density to be considered a factor for unfairness in trans athletes.
4. Maximal Pulmonary Ventilation: The maximum amount of air that the lungs can contain. It should first be understood that the lung sizes between athletes and non-athletes are not dissimilar. Rather, it’s the respiratory muscles that confer breathing advantages to athletes as a result of endurance training. For most athletes, the limiting factor for performance is the heart’s ability to carry blood to the lungs and muscles and the blood’s ability to carry oxygen. It is true that the maximal pulmonary ventilation for males is, on average, greater than that of females, but a significant reason for that difference is that males are typically larger. Large people need larger lungs to break even, erasing any question of advantage on that front. Another reason maximal pulmonary ventilation is greater in males is because they, on average, have a higher VO2max level, owing to their greater concentration of fat-free mass and testosterone. However, even when VO2 is adjusted for fat-free mass, a significant difference remains, likely caused by differences in hemoglobin levels, which is known to adjust to cis female levels within about 4 months post commencement of HRT in trans women. 
5. Total blood volume: This is a product of the increased vascularization of fat-free mass, which HRT changes.
6. Fast-twitch muscle fibers: It has been long known that males generally have more fast-twitch muscle fibers (which benefit explosive power) and females have more slow-twitch muscle fibers (which benefit endurance). However, when looking at elite female weightlifters, this difference disappears, suggesting that this generalization may not hold true for athletic populations. 
7. Heart size and contractility: Heart size is heavily influenced by body size. Contractility is both acutely and chronically influenced by testosterone levels, which is affected by HRT.
8. Mitochondrial content: Heavily influenced by estrogen levels. 
9. Capillarization: Heavily influenced by testosterone and muscle mass. 
10. Differences in motor and visual spatial skills: Although males tend to show consistently higher levels of motor and visual spatial skills (thought to improve athletic performance) even prior to puberty, the conclusion that this confers an unfair advantage does not control for the well-known fact that girls are often encouraged to be less physically active and discouraged from sports. When comparisons are made specifically between elite youth athletes, the best girls keep up or even beat the best boys. Thus, this observed ‘sex difference’ is most likely not the result of biological differences, but of socialization. Thus, it can't be an intolerable advantage requiring segregation. Notably, lesbians are overrepresented in adult elite sports, but this doesn't mean being a lesbian confers a biological advantage over straight women.
11. Myonuclei: Muscle cells with multiple nuclei known as myonuclei are known to increase strength. Recent research suggests that if one were to stop strength-training for an extended period of time, although contractile protein will be lost—leading to weaker muscles—the myonuclei will remain dormant. Resuming strength training causes the myonuclei to wake up, allowing the muscle to regain its strength more quickly than without those myonuclei present. Testosterone is also known to lead to an increase in myonuclei. Therefore, a trans woman who underwent a natal puberty would have more myonuclei even after undergoing HRT, and thus potentially be able to get stronger at a faster rate compared to the average cis woman. While it is true that the amount of protein synthesis will be aided by the presence of more myonuclei in trans women, it will likely be significantly limited by the presence of hormones from HRT affecting the other muscle activities. In any case, more research is needed to arrive at firm conclusions. 

Where do we go from here?

It may seem constructive to pick apart each individual variation of these sundry biophysical variables between trans athletes and cis athletes. But it’s more practical to examine the total impact of HRT on an athlete’s performance in a holistic manner. There is no single biological marker that can be used as an effective proxy for the complex set of advantages and disadvantages that are attributed to an individual’s performance. Factors like height and skeletal structure do not automatically translate into superior performance across all sports. Each sport has different physical requirements—the perfect body for wrestling is not the perfect body for cycling. Grip strength, Maximal Pulmonary Ventilation, heart contractility, etc. are all limited in their reliability as documented above. Bodies are complex systems. Even a supposedly advantageous trait that persists after HRT might function very differently within a trans woman’s physiology compared to that of a cis man. The fact that the line separating males and females as categories is heavily blurred (even without considering trans people) adds to this. In other words, the most reliable method of assessing the impact of HRT on athletic ability is currently direct evaluation of performance, which makes it a better indicator of fairness in sport. 

We should also acknowledge that the question of trans women in sports has little to do with the cultural practice of sport itself. Rather, it’s merely a convenient way of normalizing transphobia: the thin end of the ‘wedge’, as it were. By claiming trans women aren’t ‘woman enough’ to participate in women’s sport, it opens the door to other avenues of unjust discrimination, which is all the more reason why this issue shouldn’t be conceded by trans people and their allies in the fight for gender equality.

Featured image is "Sport" by Flickr user Fr3ncs.