The rules which allow trans women to compete in elite women's sporting divisions need to change because trans women have a physiological advantage, University of Otago researchers say.
An externally peer reviewed study of the scientific literature on the differences in the male and female physiology has concluded that International Olympic Committee guidelines are putting cis-women at a disadvantage.
The rules allow trans women to compete in the women's division if their testosterone is below 10nmol/L.
Otago physiologist Alison Heather - who wrote the review, alongside professor Lynley Anderson and Dr Taryn Knox - said those testosterone levels were 10 to 20 times higher than a typical female athlete whose average is around 0.87nmol/L a litre.
"Science demonstrates that high adult levels of testosterone, as well as permanent testosterone effects on male physiology during in utero and early development, provides a performance advantage in sport and that much of this male physiology is not mitigated by the transition."
She said even with trans women taking estrogen, it would not lower the testosterone levels to the equivalent of a cis-female.
Prof 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.
"We need more than two divisions to account for gender diversity because we can't allow every woman that identifies as a woman that has come from former high levels of testosterone to compete in the female division."
An alternative was that a handicap be applied which would take testosterone levels and other physical and social factors into account, but professor Heather said further research was needed on this.
The New Zealand Olympic committee said in a statement that the issue of transgender athletes in elite sport was extremely complicated, "and a balance needed to be struck between protecting an individual's human rights and ensuring the field of play remains fair."
They were supportive of further research into the matter but in the meantime would support all New Zealand Team members who were eligible to compete under current international rules.