While it has long been recognised that testosterone (T) level differences is the principal cause of the athletic advantage that men hold over women, there has been little published data indicating that T differences among women can result in significant advantage for those with higher T.

This lack of scientific data was one of the main reasons the CAS panel issued a temporary ruling in favour of Dutee Chand’s team in the 2015 trial against the IAAF.

And that is why the new study published by Stéphane Bermon and Pierre-Yves Garnier in the BJSM is so important. The authors showed that there was a statistically significant difference in the performance of higher-T female athletes versus lower-T athletes at the 2011 and 2013 IAAF world championships in five selected events.

The bar for statistically different performance is actually quite high, as the five minute advantage by the higher-T women in the marathon was not considered significant. Additionally, the difference in the T levels between the two groups of women in the Bermon/Garnier study was not nearly as large as the difference between most women and those with hyperandrogenism. And it is this massive testosterone-based advantage held by many intersex women that was the foundation for the now-suspended IAAF rules.

The hyperandrogenism regulations initiated by the IAAF in 2011 were a vast improvement upon the chromosome-based rules that were used by the organisation for two decades in the twentieth century. The previous rules erroneously assumed that anyone with a Y chromosome held a substantial advantage over athletes with two X chromosomes.

By contrast, the 2011 regulations zeroed in on “functional testosterone” – ie the testosterone taken up and used by the body, as the source of the advantage. The 2011 regulations also permitted high-T women to reduce their testosterone and become eligible for women’s athletics.

The most significant limitation of the new study is that the T levels measured at the championship meets are only a snapshot in time. It is undoubtedly true that a sizeable fraction of the athletes who qualified for Daegu and Moscow were using anabolic agents during their training phase but ceased using the drugs before the competitions. The authors recognised this fact when they discussed the T levels for male throwers. I don’t, however, think that this limitation affects the extremely high value of the study.

At this point it would only be fair to note that I was a witness for the IAAF at the Chand trial, and am not entirely impartial. On the other hand, there is a clear separation between the IAAF and those of us who provide advice to them on an ad hoc basis.

I have little doubt the Bermon/Garnier study is a cornerstone of whatever response the IAAF is planning to make to the CAS panel before the late-July deadline. One can only hope that the IAAF submission will be effective in persuading the CAS panel to reinstate the hyperandrogenism rules in short order.

While there is no perfect solution to the vexing question of how to decide who gets to compete in women’s sport, I believe the most optimal method we have available to us in 2017 is to use testosterone levels to divide athletes into male and female categories.

Joanna Harper is a scientist who has published research on gender variant athletes. She has also written extensively for general audiences on matters of gender and sports.