By RNZ

A suggestion transgender women athletes should compete in their own category to remove any unfair advantage is being challenged.

Otago University researchers say rules that allow transgender women to compete in elite women's sporting divisions need to change because trans women can have an advantage due to higher testosterone and physiological differences.

The peer-reviewed study of scientific literature on differences in the male and female physiology concluded that International Olympic Committee guidelines were putting cis-women at a disadvantage.

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But not all researchers have interpreted the existing studies in the same way, or agree that trans women have unfair advantages.

Human Rights researcher Jack Byrne said studies about testosterone were red herrings because the majority of trans women reduced their testosterone to very low levels.

Even when trans women had higher levels of testosterone, there was not enough clear scientific evidence to prove that affected their sporting ability, he said.

"It's serious because it has real impacts on people's health and wellbeing and research shows us that trans people are less likely to play sport than others in the general population."

Mr Byrne cited a 2014 study published in The Journal of Clinical Endocrinology and Metabolism which "no clear scientific evidence proving that a high level of testosterone is a significant determinant of performance in female sports," and a literature review in Sports Medicine that found no direct and consistent research that transgender female athletes had an advantage in sport.

He said discussion of transgender participation in sport should focus on the positive initiatives being taken.

The University of Otago research was welcomed by the Olympic Weightlifting New Zealand's high performance director Simon Kent.

"The key thing is that the discussion is now being had. More organisations are wanting to understand the topic. And we can get the scientists working together to really fully understand what equates to fairness."

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The New Zealand Olympic committee also backs further research into ensuring a level playing field but in the meantime it would support all New Zealand Team members eligible to compete under the current international rules, it said in a statement.

Those rules allow trans women to compete in the women's division if their testosterone is below 10nmol/L. University of Otago bioethicist Alison Heather, who wrote the review alongside professor Lynley Anderson and Dr Taryn Knox, said those levels were 10 to 20 times higher than a typical female athlete whose average is around 0.87nmol/L a litre. She said even with trans women taking estrogen, it would not lower the testosterone levels to the equivalent of a cis-female.

Associate Professor Heather and her co-researchers said trans women should not be excluded from elite level sport, but instead a separate category could be established for trans and intersex women to compete in. Their research was published in the latest issue of the Journal of Medical Ethics.

- RNZ