I appreciate the opportunity to add my voice to the debate on gender variant athletes. Since my area of expertise is on transgender athletes, that is where I shall focus my attention.

As you are probably aware, the general opinion among most athletes is that transgender women (you have used the term XY females) are assumed to be at a competitive advantage with respect to cisgender (you use XX) women. On the other hand, your article claims that “sport […] handicaps XY females from competing against XX females”. I would suggest that the truth lies somewhere in between in the general viewpoint and yours. 

Transgender women have both athletic advantages and disadvantages with respect to cisgender women. Advantages held by transgender women include greater height, size and muscle mass than most women. There are indeed some disadvantages incurred by trans women when their bodies are deprived of previous testosterone levels, although I don’t believe these disadvantages are quite as dire as you or Kristen Worley claim.

In particular, the suggestion that trans women are not able to compete successfully in sport for more than four or five years post-transition does not withstand scrutiny. I could cite several examples of trans women, myself included, who have continued to compete at a relatively high level for many years after transition.

I think, however, that focusing on individual advantages or disadvantages held by trans women when compared to cisgender women is counterproductive. After all, if one compares any two groups of athletes, there will be both advantages and disadvantage held by each group.

The important question is whether or not one can have equitable and meaningful competition between the two groups. Both my published research, and unpublished data that I presented in talks – such as the one you observed [Editor’s note – The Sports Integrity Initiative did not attend Professor Harper’s presentation at the Beyond Fairness event at the University of Brighton in March] – suggest that it is reasonable to have transgender women compete against cisgender women after testosterone suppression.

Perhaps the portion of your article to which I would most like to respond to is your claim that transmen are advantaged over cisgender men by means of their TUE for testosterone, i.e. “sport allows these ‘XX supermen’ to enjoy an exogenous testosterone advantage over XY men”.

Although it is true that taking exogenous T is an advantage, trans men are saddled with many disadvantages when compared to cisgender men. After all, prior to transition, trans men are, athletically speaking, women. Hence, the exogenous T that trans men take does not lead to dominance over cisgender men [Editor’s note: Our article stated that transitioned XX males were beginning to outperform XY men, not that they were dominant over XY men].

For instance, it is true that Chris Mosier has been successful in men’s sport, but he has yet to finish within the top 100 in the world duathlon championships. I don’t think that Mosier’s results support your suggestion that “XX Males (are) beginning to outperform XY Males”. 

Your example of wrestler Mack Beggs is even weaker. Mack wrestled girls in high school wrestling because the state of Texas required him to do so, as a result of his female birth certificate. USA Wrestling requires Mack to wrestle boys in the summer because of his testosterone usage.

Although Mack has been dominant against girls, he has not been extraordinarily successful against boys. Now that Mack has finished high school, he will be wrestling against men, and his chances of future athletic success are cloudy at best.

Personally, I think the most instructive example of a trans man in sports is swimmer Schuyler Bailar. Schuyler was a very big name at the high school level when he swan against girls, and his best time in his specialty – the 100 yard breast stroke – was 1 minute and 3 seconds.

The Harvard women’s swim team offered him a scholarship, anticipating that he would star on their women’s team. Instead, Schuyler wound up on the Harvard men’s team; with testosterone injections, and plenty of hard work over three seasons, he has brought his time in the 100 yard breast stroke down to 57.7 seconds, a phenomenal improvement. He still, however, finds himself in the bottom half of the Harvard men’s team, a result that is admirable, but hardly dominant [Editor’s note: Our article stated that transitioned XX males were beginning to outperform XY men, not that they were dominant over XY men]. 

Overall, the questions raised by transgender participation in elite-level sport are complex. One must look in detail at the advantages and disadvantages held by transgender athletes when they compete against cisgender athletes, and there is still much work to be done before we have the full picture.

With the current state of scientific knowledge, however, the testosterone-based eligibility rules of the IOC, IAAF and NCAA with regard to transgender athletes appear to be the optimal solution. I am hopeful that further experiments will lead to a refinement of the current rules, but it will take time. Although it is extrapolation, I think that the experiences of transgender athletes in sport can shed light on regulations for intersex athletes as well. 

• This article was written as a response to ‘Questions remain over IAAF Differences of Sex Development Regulations’, published by The Sports Integrity Initiative on 19 May 2018. To view that article, click here. Jonathan Taylor of Bird & Bird, who drafted the Differences of Sex Development (DSD) Regulations for the IAAF, also drafted a response to the article, which can be viewed by clicking here.