The End of Women’s Sports?

Author: Jamie O’Brien

When we think about people playing sports what is it we want to see? Sportsmanship? Good will between athletes? Fair and level playing fields? Unfortunately, in recent years, a new issue has arisen that compromises what we consider fair and level playing fields: transgender individuals in sports. As a society we have begun to more openly accept individuals of transgender and non-binary gender identity into every level of society. This includes things like local, national and international athletic competition. To many however, the issue is not as simple as allowing individuals to participate in the league or competition that conforms to their gender identity. Biological differences between the male and female sexes lead to an unfair advantage in favor of individuals with male biological characteristics. Several cis-gender female athletes have claimed the practice of allowing transgender women to compete with them is unfair and damages the idea of a level playing field. I am not here to argue of the place of transgender individuals in Western society, but to address the unfair aspects of male to female transgender athletes in female athletic competition.

Men vs. Women

In childhood and pre-pubescent individuals, there are no significant differences in male and female athletic performance (2). At the onset of puberty however, the differences between the two sexes become much more prevalent due to the “biological instructions for growth” which are encoded in an individuals’ genetics. There are three extremely relevant areas where adult males and females differ biologically that directly impact athletic performance: muscular development, oxygen carrying capability, and bone composition (2). Through many different mechanisms it has been observed that by in large, males have larger muscles which can produce a greater amount of force than females (2). In competition this equates to faster speed when sprinting, higher jumps, harder hits, and the ability to put more force into any given movement for males relative to females. Additionally, males have approximately 12% higher levels of circulating hemoglobin, the molecule responsible for carrying oxygen from the lungs to the body (2). Males’ ability to give oxygen to exercising muscles is significantly greater than females, meaning that in competitions which require long periods of exercise, males are more efficient in terms of getting oxygen to the body and maintaining high levels of performance. Finally, males have longer, stronger, and thicker bones than females, and the pelvic widening which women undergo during puberty to aid in childbirth is detrimental to physical performance (2). Males therefore have a larger skeleton with longer bones which can exert more force and tolerate stronger muscular contractions than females. It’s also been observed that stress fractures from running are more common in females and this is thought to be due to their less dense bones (1). We may also look at athletic competition results to see differences between males and females. Among  elite athletes, it has been found that males have an advantage in every event of swimming and athletics (track and field). These differences vary based on sport, for swimming it’s about an 8.9% difference between males and female times, in running a 10.7% difference, in jumping it is 17.5% difference in performance, and for weightlifting it’s 36.8% difference (4). This may not sound like a particularly large gap, someone may say “well if a woman was 9% of a better swimmer she could beat men.” However, I would draw attention to 2 points. For one, there is a principle we call the law of diminishing returns, which states that as you get better at a sport, it becomes much harder to progress in that sport. For a beginner, it’s possible to be 10% better, but for a world class athlete: it can take a decade just to get 1% better. Additionally, upon review of the 2008, 2012, and 2016 Olympic times across sex, it was found that the performance differences between winning first place, winning a medal, and making the final are all less than a 1% difference across swimming and track and field (2). So, the differences at elite levels of competition are actually quite large for all of these events. There are clearly sex based advantages that males experience over females, that is why we separate divisions of sport based on sex, because you would never see a female athlete trump a male athlete in most sports.

The Effect of Testosterone on Performance

What drives these massive differences we see between the sexes? To reiterate, the large differences we see in male and female athletic performance only become clear after puberty, they don’t occur overnight. During puberty, the body coordinates large changes in the manufacture and secretion of several hormones which affect growth and development. The hormone most associated with the male sex is testosterone: a hormone which acts in many different places in the body including stimulating growth in the bones, muscles, and hemoglobin levels of the body (10). During puberty and afterwards, the average male will have 10X the amount of testosterone the average female has. While this is only one variable that drives differences between males and females, testosterone levels have a clear implication in sports performance. Anyone who has followed professional sports has likely heard the phrase “steroid abuse”. The word steroid refers to a specific class of hormones- the most relevant in a sport context being testosterone. For decades, athletes have used testosterone or compounds that mimic testosterone to gain a biological advantage over their competition. That is why such compounds are banned in sport, they represent an unfair advantage that increases performance and is not naturally achievable by their competitors. Perhaps one area to look at in particular is the effect of increased testosterone in cis-gender female athletic performance. In one study cis-gender female subjects were given 10 mg of testosterone skin cream daily for a period of several weeks. Their serum levels of testosterone increased from 0.9nmol/L on average to 4.3 nmol/L (a level which is not naturally observed in females). It was also observed running performance was increased significantly by 8.5% (5). Additionally, files recovered from the Stasi (East German secret police) obtained after fall of the Soviet Union show evidence of a state sponsored doping program involving the administration of testosterone derived compounds to female athletes to increase athletic performance. This program was overseen by physicians who considered higher than possible levels of testosterone to be a performance advantage (3). The data and the research is clear, an individual with testosterone levels beyond what is naturally possible has a clear advantage over their competition.

Which category of competition do transgender women fall into?

There are some measures by sporting organizations in place which attempt to make the competition between transgender women and cis-gender women fair. For example, both the International Olympic Committee and the World Athletics Association have mandatory maximum testosterone levels that transgender female athlete must fall below for an entire year before they are allowed to compete with cis-gender women. The IOC has stated a transgender athlete in the female division must display testosterone levels below 10nmol/L for 12 months leading up to competition, the World Athletic Association states they must be below 5nmol/L for the same period of time leading to competition (8). It seems like a fair proposition, to limit the amount of testosterone a transgender woman can have as to not give her an unfair advantage over cis-gender competitors. However, upon examination of the reference ranges for most cis-gender women obtained from other scientific literature, these mandatory maximums for transgender women are incredibly high. In one article examining testosterone average across 16 studies of over 4000 women the overall average is only 1.68 nmol/L with a standard deviation of 0.87 (2). For a cis-gender woman to reach 5 nmol/L of testosterone, she would have to be more than 3 standard deviations of the mean, which statistically is incredibly unlikely if it is even possible. According to the IOC, a transgender athlete must be below 10 nmol/L, that is almost 10 standard deviations away from the mean. The statistic  likelihood of this happening in a cis-gender woman is about 1 in 10^24 odds, it would hardly be an exaggeration to deem such a thing impossible (9). Transgender athletes can have far higher testosterone levels than any cis-gender athletes are capable of achieving naturally in the largest sporting organization in the world. In smaller organizations there are few if any regulations on mandatory maximums of testosterone levels for transgender women. Administration of hormones prescribed to transgender individuals as part of a hormone therapy likely plays a role in maintaining a denser skeleton. Estrogen in particular is a common hormone prescribed to transgender individuals because it has several effects that make an individual appear more feminine, in this way it helps their appearance better conform to their gender identity. It has been known as an important factor in preserving dense bones for decades (7). If a transgender woman has gone through puberty her skeleton’s density and strength is more similar to a male than a female. Although her testosterone levels will fall when she begins hormone replacement therapy, the bones won’t become less dense because the estrogen she is taking will continue to preserve the density of the bones. Bone density doesn’t change, but what about hemoglobin levels and muscle size? Transgender women display hemoglobin levels comparable to cis-gender women in the span of three years after beginning hormone replacement therapy, so in that regard the playing field is level (6). It is also true that transgender females experience losses in strength, lean body mass, and muscle size when they transition relative to when they identified as cis-gender males. Their strength, lean body mass, and muscle size however, even 3 years after beginning hormone replacement therapy are still much higher what is measured in cis-gender female athletes, indicating potentially permanent performance advantages for transgender women (6).

Why does this matter?

The current system of athletic organization accommodation of transgender athletes is insufficient. While it may seem that I am singling out transgender athletes as being different from their gender identity, it is not my intention. On the contrary, I don’t wish to see transgender athletes further marginalized and seen as cheaters or exceptions to the rule in a sport context. I refuse however, to ignore the biological evidence surrounding this issue. It is very complicated and not something that exercise researchers, pharmacologists, sociologists, psychologists, and many more will have to collaborate on to make the playing field as level as possible.


  1. Collado-Boira, E., Baliño, P., Boldo-Roda, A., Martínez-Navarro, I., Hernando, B., Recacha-Ponce, P., Hernando, C., & Muriach, M. (2021). Influence of Female Sex Hormones on Ultra-Running Performance and Post-Race Recovery: Role of Testosterone. International journal of environmental research and public health18(19), 10403.
  2. Handelsman, D. J., Hirschberg, A. L., & Bermon, S. (2018). Circulating Testosterone as the Hormonal Basis of Sex Differences in Athletic Performance. Endocrine reviews39(5), 803–829.
  3. Franke, W. W., & Berendonk, B. (1997). Hormonal doping and androgenization of athletes: a secret program of the German Democratic Republic government. Clinical chemistry43(7), 1262–1279.
  4. Thibault, V., Guillaume, M., Berthelot, G., Helou, N. E., Schaal, K., Quinquis, L., Nassif, H., Tafflet, M., Escolano, S., Hermine, O., & Toussaint, J. F. (2010). Women and Men in Sport Performance: The Gender Gap has not Evolved since 1983. Journal of sports science & medicine9(2), 214–223.
  5. Hirschberg, A. L., Elings Knutsson, J., Helge, T., Godhe, M., Ekblom, M., Bermon, S., & Ekblom, B. (2020). Effects of moderately increased testosterone concentration on physical performance in young women: a double blind, randomised, placebo controlled study. British journal of sports medicine54(10), 599–604.
  6. Harper, J., O’Donnell, E., Khorashad, S.B., McDermott, H., Witcomb, G.L. (2021). How does hormone transition in transgender women change body composition, muscle strength, and haemoglobin? Systematic review with a focus on the implications for sport participation. British Journal of Sports Medicine, 55 (15), 865-872.
  7. Cauley, J. A. (2015). Estrogen and bone health in men and women. Steroids99(Pt A), 11–15.
  8. Transcript of the International Olympic Meeting on Sex Reassignment and Hyperandrogenism November 2015.
  9. Sandberg, A. (2009). Ten sigma: numerics and finance. Andart. Available from:
  10. Bain J. (2007). The many faces of testosterone. Clinical interventions in aging2(4), 567–576.