Women runners born with high testosterone levels enjoy a "significant competitive advantage", said a study Tuesday that could reignite debate on the future participation of athletes whose gender was questioned.
The study, jointly sponsored by the sporting agency seeking to ban athletes with hyperandrogenism, comes three weeks before the International Association of Athletics Federations (IAAF) must present expert evidence on "the actual degree" of advantage women could gain.
Hyperandrogenism is a condition that causes high natural levels of the male hormone, testosterone, in women.
Without proof, IAAF regulations excluding women with hyperandrogenism from competition are set to lapse. Track stars such as Caster Semenya and India's Dutee Chand both endured banishment for failing so-called "gender tests".
The new study, published in the British Journal of Sports Medicine, was funded by the IAAF and the World Anti-Doping Agency (WADA).
One of the authors, Stephane Bermon, is an IAAF consultant and a member of its working group on hyperandrogenic athletes.
The other, Pierre-Yves Garnier, is director of the IAAF's health and science department. He returned to work in January after a three-month suspension in a probe linked to Russian athletics doping.
Their research relied on blood data from male and female athletes who competed in the World Championships in 2011 and 2013 - more than 2,100 samples in all.
It found that women with high natural testosterone levels performed better in the 400-metre sprint, 400 m hurdles, and 800 m middle-distance events than women with low levels.
They also outperformed them at pole-vaulting and hammer throw.
Depending on the event, performance improved by between 1.8 and 4.5 percent, the paper said.
This link, concluded the authors, "should be taken into account when the eligibility of women with hyperandrogenism to compete in the female category of competition is discussed."
The study is an observational study that cannot determine conclusively that higher testosterone is what causes the performance boost, merely that an increase in one is associated with an increase in the other.
Testosterone, which can also be injected as a performance-enhancer, increases muscle mass and boosts physical strength.
The issue of hyperandrogenism is controversial because it has pitted principles of fair competition against the rights of women born with a condition they have no control over.
In 2011, the IAAF introduced so-called "hyperandrogenism regulations" after a highly-emotive and public battle with Semenya.
The regulations allowed hyperandrogenic athletes to take medication to lower their testosterone levels to below 10 nanomoles per litre - considered a low level in men.
The natural range for women is about 10 times lower.
Semenya won gold in the 800 m at the 2009 World Championships in Berlin, but was subsequently barred from competing for nearly a year while undergoing gender tests.
Competitors say hyperandrogenic athletes enjoy an unfair physical advantage, but critics say gender testing is arbitrary, discriminatory and psychologically harmful.
In 2015, the Court of Arbitration for Sport (CAS) suspended the IAAF regulations in a challenge brought on behalf of India's Chand, a sprinter.
It said there was not sufficient scientific evidence that natural testosterone boosts performance in hyperandrogenic women, and gave the agency two years to submit expert reports to the contrary.
The deadline of July 27 is fast approaching.
"Our starting position is to defend, protect and promote fair female competition," an IAAF statement quoted Bermon as saying on Tuesday.
"This study is one part of the evidence the IAAF will be submitting to the CAS," he added.
There would be no impact on the World Championships in London in August, as the regulations remain suspended "pending the resolution of the CAS proceeding", the association said.