We support gender equality and freedoms in cases in which ‘like equals like’. Such inclusion is central to a progressive society. However, inclusion could potentially conflict with fairness in cases concerning transgendered athletes in elite sport. Accepted science regarding male and female physiology suggests that transwomen have an advantage over their cisgendered counterparts. This advantage stems from relatively high testosterone levels and prior male physiology of transwomen. Conversely, transmen who wish to compete in the men's division may be disadvantaged in comparison with cismen. Hence, while inclusion supports transwomen and transmen competing in the division that matches their gender identity, this may not satisfy the principle of fairness. We reason that transwomen and cismen are not only advantaged, but unfairly advantaged, and propose that the gender binary in elite sport should be replaced with a nuanced algorithm that accounts for both physiological and social parameters. As the algorithm would be applied to all athletes, it would be both inclusive and fair.
As with most people, we support gender equality and freedoms. In cases in which ‘like equals like’, a broad understanding of fairness requires the gender identity of transgendered people to be accepted (for definitions of terms such as ‘gender’ and ‘transgender, refer to the glossary directly below). The principle of inclusion is important in all levels of sport [1,2], but fairness (where athletes must begin from roughly the same starting point) is also highly valued. In considering the inclusion of trans-athletes in elite sport, these two principles may conflict . Elite sport is not a case in which ‘like equals like’ due to the physiological differences between the two groups. The differences may indicate that elite trans- and cis-athletes should be treated differently [4,5]. Far from banning trans-athletes, we advocate replacing the gender binary in elite sport with a considered and nuanced categorisation to account for physiological and social parameters. This approach would respect differences between athletes and actively accommodate the needs of all .
Performance advantages and disadvantages
Strength, speed and recovery are integral to almost all athletic performance. The physiological parameters underpinning these are the musculoskeletal structure and the cardiovascular/respiratory systems. All of these parameters are enhanced in males either through Y-chromosome determinants or exposure to sex hormones. The major sex hormone in males is testosterone which is well recognised to provide increased bone length and surface area , muscle mass and strength , oxygen-absorbing and -carrying capacity , and cardiac output . These differences in physiology result in men having an advantage in sport and underpin the athletic performance of males, as highlighted by their domination of sporting world records . The overall physiological advantages are the major reason for gender separation within sport .
According to the International Olympic Committee (IOC) guidelines , transwomen may compete in the women's division if, inter alia, their testosterone levels have been under 10 nmol/L for at least 1 year. Testosterone concentration in health young males (18–40 years) ranges from 7.7 to 29.4 nmol/L. There is absolutely no overlap between this level and that of healthy pre-menopausal women (under 40 years), which range from 0 to 1.7 nmol/L . More importantly, the acceptable testosterone levels for transwomen (10 nmol/L) are more than five times higher than the upper level for healthy ciswomen. While women with polycystic ovary syndrome (PCOS) have slightly higher levels of testosterone reaching 4.8 nmol/L , these women still have less than half of the level of testosterone the IOC allows transwomen to have. Given the advantages of testosterone described above, this suggests that elite transwomen athletes may have a performance advantage over their cisfemale counterparts. Reducing transwomen's testosterone levels to under 10 nmol/L potentially reduces muscle mass and oxygen-carrying capacity [7,13–15]. However, hormone therapy will not remodel the male musculoskeletal or cardiovascular and respiratory physiological systems to that of an elite cisfemale athlete . This suggests that the IOC guidelines (2015) afford elite transwomen athletes a performance advantage.
The IOC guidelines (2015) allow transmen to compete without restriction . Transmen may be permitted a Therapeutic Use Exemption allowing them to take exogenous testosterone for transition purposes . This hormone therapy will increase muscle mass and strength and oxygen-carrying capacity above that of their former female physiology . However, as with transwomen, the hormone therapy taken by transmen will not alter sex-determined musculoskeletal or cardiovascular and respiratory physiological systems , suggesting that elite transmen athletes may have a performance disadvantage relative to elite cismen athletes.
There is minimal scientific research on the impact of hormone therapy on athletic performance during and after transitioning . Until such research is done, we rely on the science that emerges from the elite male/female physiology that points to strong sex differences in elite athletic performance. This suggests that the IOC guidelines (2015) create an uneven playing field disadvantaging ciswomen and transmen. While equals ought to be treated equally and begin from roughly the same starting point, differences exist between cis- and trans-athletes. The scientific evidence alone cannot justify treating these groups differently as the ethical arguments regarding fairness need to be considered.
Are the performance advantages/disadvantages unfair?
Not all advantages or disadvantages in sport are unfair, or as Devine says, some are ‘tolerably unfair’ . An example of a tolerable unfairness is the advantage tall people have in basketball. This is not considered unfair but part of the genetic lottery. In contrast, it would be an intolerable advantage if a heavyweight lifter competed in the bantamweight division. While it is not entirely clear what makes an advantage fair or unfair, we propose that the advantage transwomen have is unfair because high testosterone levels and prior male physiology are (a) not attainable by ciswomen and (b) provide an all-purpose benefit. Bianchi  argues that an advantage is unfair if no member of a group can attain that advantage. The endogenous testosterone levels of ciswomen do not reach that allowed by the IOC for transwomen (10 nmol/L); nor are ciswomen permitted to take exogenous testosterone . Moreover, ciswomen cannot attain the advantages of prior male physiology. While transmen may take testosterone , it would be difficult for them to reach the average testosterone level of an elite cisman athlete. Nor can transmen attain the advantage of having a lifetime of male physiology which is advantageous in almost all sports. Therefore, according to Bianchi's position, the IOC guidelines (2015) afford transwomen and cismen an unfair advantage.
As discussed in the previous section, high testosterone and male physiology are beneficial in almost all sports. In contrast, many other physical properties, such as height and weight, are only advantageous in some sports. This allows athletes, when quite young, to self-select into a sport that complements their physical attributes. For example, a short person is unlikely to become an elite basketballer but may self-select and excel in gymnastics. In contrast, the all-purpose benefit held by transwomen means that it would be difficult for ciswomen to self-select into a sport in which low testosterone levels or female physiology would be advantageous. Transmen could face similar issues when self-selecting into competition against cismen. Finally, the advantage that cismen have over ciswomen is considered unfair as evidenced by the gender separation in sport. Hence, consistency demands that competition, as it stands between elite cis- and elite trans-athletes, is also considered unfair.
In most aspects of life, gender identity should be sufficient for determining inclusion in the category of male or female. However, the inclusion of trans-athletes into elite sport is not so straightforward, as inclusion conflicts with fairness. While treating elite trans-athletes differently from their cis-counterparts may be justifiable, we argue for replacing the gender binary in elite sport with something that recognises the difference between people (and so is fair), while creating space for all athletes (thus satisfying inclusion). The gender binary could be replaced with a multifaceted algorithm, similar to that used by the Paralympics . The Paralympics use an algorithmic classification system based on function or the ‘degree of activity limitation’ relative to a particular sport, rather than athlete's medical condition . The philosophy behind this functional approach is ‘to ensure the success of an athlete is determined by skill, fitness, power, endurance, tactical ability and mental focus’  rather than physical ability, i.e. naturally occurring advantages. Just as the Paralympic algorithm is sport-specific and considers function rather than diagnosis, our proposed algorithm would be tailored to individual sports and account for a range of physiological and social factors that affect on athletic function. Physiological factors would include, but not be limited to, present testosterone levels and differences retained from prior male or female physiology. Social parameters would include, but not be limited to, gender identity and the socioeconomic background of the athlete's country of residence. Such factors will help to ensure that the most skillful athlete — not the most fortunate lucky athlete — will win. Our algorithm helps address unfair advantages, and so is fair. As it is applied to all athletes, it is inclusive.
Some will argue that an algorithm that can meet the needs of all athletes is unrealistic. They might point out that there are problems with the Paralympic algorithm, such as the lack of valid, sport-specific measures of impairment  and the risk that athletes may potentially exacerbate their impairment to gain a competitive advantage . We agree that realising such an algorithm will be complex, require robust scientific research specifically concerning elite trans-athletes, and will generate challenging normative dilemmas. However, this is a poor reason to discount the notion of an algorithm. We could retain the gender divisions currently used in sport; however, we have an opportunity to develop a progressive response that adapts to evolving scientific evidence, as well as addressing social issues such as gender identity: ‘Inclusion is not bringing people into what already exists; it is making a new space, a better space for everyone’  (p. i). The suggested algorithm provides the possibility of creating a new and better space for all instead of attempting to shoehorn athletes into our current flawed binary structure.
The principles of inclusion and fairness are important in elite sport.
Elite transwomen athletes have an advantage over their cis-counterparts due to relatively high testosterone levels and prior male physiology. Elite cismen athletes may also have an advantage over elite transmen athletes.
Hormone therapy (decreasing testosterone levels for transwomen and increasing testosterone levels for transmen) will not entirely eliminate the advantage.
The advantage afforded by testosterone and male physiology is unfair because (a) it is not attainable by ciswomen or transmen and (b) testosterone provides an all-purpose benefit.
An algorithm that accounts for social and physiological parameters would satisfy the principles of inclusion and fairness.
The Authors declare that there are no competing interests associated with the manuscript.
Appendix. Glossary of terms
refers to a person's physical characteristics, including their reproductive system, hormones, chromosomes and external genitalia.
refers to one's sense of self as male or female (or something else such as gender-neutral or gender-fluid).
- Cisgendered people
(i.e. ciswomen and cismen) are those for whom whose sex and gender align.
- Transgendered people
(i.e. transwomen and transmen) are those for whom the sex they were assigned at birth does not align with their gender identity.
were assigned the male sex at birth but identify as female.
were assigned the female sex at birth but identify as male.