Is it fair to allow transwomen to compete in female sport?

The rights of male-born transwomen to be included in female spaces clash head on with the sex-based rights of females to exclude males for reasons of privacy, safety and fairness. Nowhere is this more apparent than in the arena of elite sporting competition.

Can it ever be fair for someone born male to compete in female sports? Here we discuss the science behind this controversial and complex question.

Why do males have a competitive advantage in sport?

What is the strength of the scientific evidence regarding Testosterone and athletic performance?

Why are the current trans eligibility rules unfair to female competitors?

How do we ensure both fairness and inclusion for all?


Conclusions in brief:

The sex hormone Testosterone is responsible for the performance advantage enjoyed by males, some of these affects are irreversible but some are not.

Despite this, Olympic rules now allow male-born trans athletes to compete against females if their Testosterone levels have been suppressed for a minimum of 1 year. However, the maximum level of permissible Testosterone is 10 times higher than levels seen in the average female athlete. Genital reassignment surgery is no longer required.

For reasons of fairness, female sports must be reserved for biological females only. We discuss an alternative solution in which MALE-only categories are converted to OPEN categories for all regardless of sex or gender identity.


Why do males have a competitive advantage in sport?

Virtually all elite sports are segregated into male and female competitions. This is to allow females a chance to win and be the best in their field.  It also showcases and celebrates the best of what the female body can achieve. Separate sporting competitions are necessary because females have major disadvantages against males who are, on average, taller, stronger, and faster and have greater endurance due to their larger, stronger muscles and bones as well as having more circulating red blood cells. The ‘performance gap’ between male and female athletes is proven beyond doubt based on decades of sporting records and it ranges from 10-30% depending on the sport. Link HERE and HERE.

It is well established that the difference we see between male and female bodies is driven by exposure to the male sex hormone Testosterone. There are three important points in life when this really matters:

note: nM is a unit of concentration. It stands for nano Molar. 10 nM means there are 10 nanomoles of Testosterone in 1 Litre of blood serum.

In the womb:

In the male embryo, high levels of Testosterone drive the sexually undifferentiated gonads to develop into testes and the formation of male external genitals. In the female embryo, with little or no exposure to Testosterone, the gonads form ovaries and female external genitals form. In most cases the sex of the fetus can be observed before birth by ultrasound scanning.


After birth, circulating testosterone concentrations are essentially the same in both boys and girls, and apart from their genitals, we see no difference in body size, shape or physical performance between the 2 sexes.


At puberty the male testes start to produce high levels of Testosterone, leading to sexual development and enlargement of the genitals. Testosterone levels in the blood also surge from almost nothing to about 5nM-10nM triggering the male body size and shape and massive improvements in physical performance. Females do not experience this surge in Testosterone and pre-pubescent levels remain throughout adulthood (<1nM). Instead, the female body shape changes under the influence of estrogen with the pelvis widening in preparation for childbirth.


High Testosterone (T) persists throughout adult-life for males. Multiple studies show T levels ranging from 7nm – 30 nM in healthy males, building and maintaining muscles, and boosting the number of red blood cells helping more oxygen get from the lungs to the muscles. Females on the other hand remain at childhood levels of T ranging from 0.06nM to 1.67nM gaining none of the strength and stamina advantages that T brings. See figure HERE or link HERE.



What is the strength of the scientific evidence regarding Testosterone and athletic performance?


There is strong evidence that Testosterone correlates with athletic performance.


Studies in Females:

Observational studies in non-athletic women show that muscle mass is increased with mildly elevated T levels due to Poly Cystic Ovary syndrome (PCOS, still lower than 2nM T). Link HERE.

Observation studies in over 1000 elite female athletes show that high natural levels of T correlates with a 1.8 % – 4.5 % increase in performance in some track and field events (although not all). Link HERE.

62 post-menopausal women were given weekly injections of T for 24 weeks achieving an increase in circulating T of 7nM (levels seen in pubertal boys). These women exhibited both an increase in muscle mass (4.4%), an increase in blood hemoglobin (3%) and an increase in muscle strength (12-26%) compared to placebo controls. Link HERE.

17 female-born trans people showed a 19.2% increase in muscle mass and 15% increase in red blood cells after 12-month of T treatment, with T levels reaching 31 nM (similar to levels seen in adult males). Link HERE.

23 female-born trans people administered adult male T doses produced striking increases in total
body muscle size and limb muscle size (by 6.5% to 16.6%) and grip strength (by 18%) compared with age-matched untreated control women. Link HERE.

Increases in muscle size and performance observed in East German females doped with T in 1930s. Link HERE, HERE


Studies in Males:

400 healthy men first had their T levels reduced for 16 weeks (to female levels of 0.7 nM), some were then given doses of T to raise their levels to 7 nM (as seen in pubertal males). An increase of 2.3% total muscle mass and 3% thigh muscle mass was observed, along with a  5.5% increase in leg press strength. Link HERE.

17 male-born transsexual people were monitored before and after genital reassignment surgery (reducing T levels from an average of 21 nM to 1 nM). 1 year after surgery total muscle area was reduced by 10% and red blood cells down by 14%. Link HERE.

An unpublished, retrospective analysis of the competition results of 6 elite male-born trans athletes showed a reduction in athletic performance. Link HERE.

But there is currently no evidence that lowering Testosterone to normal female levels can fully remove male-competitive advantage.


The only available studies looking at athletic performance in male-born transgender athletes have serious limitations. Link HERE.

Athletic performance data was acquired from just six athletes (one transgender sprinter, one rower, one cyclist, and three distance runners) both before and after transition. However, past performance is based on historical competition records and current performance is in some cases up to 18 years after transition. Results had to be adjusted to compensate for the natural deterioration of performance due to ageing, seriously limiting the accuracy of this currently unpublished study.

For example, one long distance runner used to run a half-marathon in time of 1h 16s at age 28. Their time increased to 1h 35mins at age 33 (5 years after transition). Performance has decreased as expected but this will be a combination of both age and the impact of T reduction.

See figure HERE to see the results obtained for all six trans athletes.


Why are the current trans eligibility rules unfair to female competitors?

In 2015 the International Olympic Committee (IOC) relaxed the eligibility criteria for male-born transgender people to compete in female sport. The requirement for genital reassignment surgery was removed and eligibility instead determined by reducing Testosterone to 10 nM for at least 12 months. More recently, the International Athletic Association Federation (IAAF) reduced their limit to 5nM T for some track and field events.


The maximum level of  Testosterone allowed for male-born transgender athletes is set too high.

It is currently set at 6-12 times higher than the average level observed in females (0.8 nM).  Link HERE.

The current 10 nM limit is actually closer to the pubertal level of T observed in boys (5-10nM). To put this into context, the high school 100m sprint record set by a teenage boy beats that of the fastest ever female. Link HERE and HERE.

7nM T has been shown to cause a 12-26% increase in muscle strength when administered to women. Link HERE.

In a recent study of over 700 male elite athletes 25% had T levels below 12.5 nM. A considerable proportion of males would be expected to naturally fall below the 5-10nM limit. Link HERE.


Removal of Testosterone today does not remove the advantages of Testosterone in the past.


Life-long advantage of larger bones and body size:

Although it is clear that reducing T to female levels does reduce current muscle size and red blood cell levels in males, it does not reverse the impact that T had during puberty. Puberty and early adulthood was when the size and shape of the male body was formed. It is well known that males are larger, have longer limbs, a larger rib cage with larger organs like heart and lungs, bigger hands, their legs are more vertical due to a narrow pelvis, the list goes on. The male body is optimised for physical performance. In contrast, in females there is an evolutionary trade-off between physical performance and the ability to bear children.


Different opportunities in childhood:

During childhood, T levels are low for both boys and girls. However, sex stereotypes, lack of female role models and social pressure means that sporting opportunities for girls are significantly lower than for boys. Girls also have to manage the onset of periods and very significant and visible body changes like breast growth. The overall impact is that girls are much less likely to participate in sports than boys, a trend that carries through into adulthood. This well established fact is discussed HERE, HERE, HERE, HERE, HERE, HERE.


Future advantage due to muscle memory:

There is considerable scientific evidence now that muscles built under the influence of Testosterone will be easier to build in the future even in the absence of Testosterone. Links HERE, HERE. ‘Muscle memory’ means that if an elite athlete takes performance-enhancing drugs, they may continue to be at an advantage over their competitors even if they stop taking the drugs. This is why elite athletes can have their titles removed retrospectively if they are later found to have taken performance enhancing drugs at any point in the past (not just at the time the title was acquired).

Therefore it is likely that male-born transgender athletes who once trained under the influence of natural testosterone will forever be at an advantage over female-born competitors never exposed to such high T levels. Read an excellent short review on this topic HERE.



Even low levels of male-competitive advantage will dramatically change sporting outcomes.

Suppression of Testosterone by male-born transgender athletes undoubtedly reduces their athletic performance. However, the key question is ‘by how much?’ It is highly likely that some competitive advantage will remain due to being born and developing as male. This is also borne out by the observation that many male-born trans-athletes are rising to the top of female competition. Link HERE, HERE, HERE, HERE, HERE.


Seeing male-trans athletes rising to the top of female sport is inevitable because the difference between winning and losing at elite level is so small. Analysis of the performance differences over the last 3 Olympics confirm exactly how small that difference is. Link HERE. The average difference between winning Gold vs Silver is only 0.8%, for Bronze and no medal is 0.4% and making the final or not is 0.3%.

Male-bodied people start out with a 10-30% performance advantage over females. Even if just a 1% residual advantage remains post-transition this is still more than enough to radically skew the medals table and world records.


How do we ensure both fairness and inclusion for all?

Balancing the rights and needs of all competitors.

Rachel McKinnon, the male-born transgender cyclist and now a world champion in female track cycling,  quotes the International Olympic Committee (IOC) Charter, which says: “The practice of sport is a human right.” McKinnon supports the view that since ‘transwomen are women’, and in some countries can be legally recognised as members of the female sex, then they have an absolute right to compete as females in female sporting competition. They say any performance advantage they may have through having being born in a male body is simply a natural female advantage, no different than being tall for example.



The 2018 Olympic Charter does indeed state that “The practice of sport is a human right”. However, the right to sport is not the right for any given individual to play in any sport in any category at any level, but instead is contingent on the central importance of both non-discrimination explicitly referencing sex, and of fair play, fair competition, and a level playing field.

Anti-discrimination legislation is well established in law in many countries and in the UK for example is covered by the Equality Act.  This permits the lawful exclusion of people with the protected characteristic of gender reassignment from single sex sport for the purposes of (a) fair competition, or (b) the safety of competitors.  This exemption applies regardless of whether the transgender competitor has legally changed their sex (has a Gender Recognition Certificate).

Human rights never exist in isolation and must always be considered alongside the rights of others. In this instance there is a clear conflict between the sex-based rights of females to fair competition and transgender-based rights of trans people to compete as their preferred gender identify.

This conflict of rights cannot be ignored.


An alternative solution.

Female sport is a protected category to ensure meaningful competition due to the SEX differences between males and females. For this reason eligibility rules must be based on someone’s SEX, with the inclusion of XX females and exclusion of XY males.

Special rules will of course be necessary for female DSD/intersex athletes with genetic disorders that may confer a performance advantage. However, any rules put in place to limit a natural competitive advantage in females should not be extended to include XY male athletes (including males who identify as women) who are not intersex.

Transgender athletes must be allowed to participate and compete in sports. However, opening up the female protected category of sports to a subset of the male sex based on gender identity is not the only option.

Instead the MALE category could be converted to an OPEN category. This category could be open to any athlete, regardless of sex, gender identity, hormone levels and allows full inclusion. Although some unfairness still inevitably remains in this solution, it is significantly less than the unfairness under the current rules and the impact on a very large and already disadvantaged group within sports; females.


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