The science on transgender women in sport, from the person whose scientific work informed World Rugby's controversial new proposed policy
The man who helped write guidelines on transgender players in women’s rugby says he hates that his work has created a platform for people to attack transgender people as cheats.
“That’s not the case,” says sports scientist Ross Tucker. "I wish that society would be accepting.”
The report was leaked a couple of weeks ago by The Guardian, which suggested World Rugby was considering changing its policy around male-to-female transgender players, potentially banning them from playing women's rugby.
The policy change was based on the scientific report co-written by Tucker, which said, even after testosterone treatment, trans athletes retained a significant performance advantage over biological women - manifesting in "at least a 20-30 percent greater risk of injury when a female player is tackled by someone who has gone through male puberty".
The proposed ban was predictably polarising: NZ Rugby immediately came out and said while it would seek feedback on the proposal, it wasn't keen on a ban.
Wellington rugby player Alice Soper told RNZ's Morning Report she would have no problem lining up against women who underwent male puberty, and described the proposal as "TERF-y".
On today's episode of The Detail, Emile Donovan speaks to Tucker about how he came to these conclusions, the physiological differences between men and women, and the extraordinary situation sports administrators find themselves in.
This is not a discussion about whether trans women should be able to play sport. That isn't a question; sport is a human right.
This is about an intersection of competing values: of inclusion, safety, and fairness, in a situation where there isn't a clear way to balance all three.
A fundamental question in this discussion is why sport is split along the lines of biological sex.
"[We split sport] within a sometimes arbitrary range, because if we didn't do that, the smallest boxer would never win,” says Tucker. “The athlete with the most severe cerebral palsy would never have the opportunity to win that medal ... and, similarly, those who are female and therefore physiologically different in their biology would have zero chance of winning those medals if they had to compete in an open category against males.
"The scientific, biological differences between men and women are so large, they would render women irrelevant in elite-level sport. Therefore, we protect a category of people who do not have the advantage in order for their sport to have the same meaning.
"Now, we can have a Usain Bolt, who won the 100m gold medal, and we can give the same medal - of equal value - to Shelly-Anne Fraser-Price, because they've both expressed the attributes that we recognise as making them the world's best sprinter."
How big is that physical discrepancy? Pretty big, says Ross Tucker.
When it comes to speed, men on average enjoy about a 10 percent advantage over women.
When it comes to strength and power, it's even larger: between 20 and 30 percent.
In other areas the difference is even greater.
That disparity is almost invisible in pre-pubescent children - in fact, girls tend to outperform boys around the age of 10 - but that all changes as soon as puberty hits.
"Males have testes, and testes produce testosterone - along with a few other hormones - which collectively are known as 'androgens', which means 'male-making'.
"Androgens are responsible for generating 'male-ness' ... and the bits of that that are relevant to sport are: increased muscle mass; increased skeleton growth; changes to the shape of the skeleton; the cardiovascular system changes ... this creates, in males who've undergone puberty under the influence of these androgens, greater strength, greater power, greater endurance capacity, and the ability to produce forces at higher speeds than in women."
Of course, that's not universally true: there are plenty of small, weak men, and plenty of large strong women.
But, Tucker says, if you look at the extremities of athletic performance, they will all be men.
"There are 10,000 men who are faster than the fastest women in the world over 100 metres. That group of men includes 15 and 14-year-old boys.
"The argument for sport is this: we don't want to compare a very fast woman to a mediocre man. So the comparison between, say, Paula Radcliffe, or Katie Ledecky in the swimming pool, is not against someone like you or me, it's against someone who is competing at the same relative level."
Currently, sport is separated along binary lines: we have men's sport and women's sport.
But we don't live in a binary world. An increasing number of people are discovering their biological sex does not match their gender identity, and have the means and ability to take steps to remedy that: transgender people.
And remember, playing sport is a human right. Guaranteeing the inclusion of trans people is fundamental.
"A decent, tolerant society understands that there are people whose gender identity does not match their biological sex ... and a tolerant society accepts this is the case and wants those people to have every right to identify.
"The trouble for sport is it creates a situation, effectively, of colliding rights. Because there's a question now over whether that person's self-identification imposes on another person's rights for fair or safe competition."
The solution thus far has been testosterone treatment: Olympic guidelines require trans women athletes declare their gender and do not change that assertion for four years, as well as reducing testosterone levels to a certain point for at least one year prior to competition. The theory is that this reduction in testosterone would take away any inherent biological advantage.
But Ross Tucker says there's a problem with that course of action: it doesn't work.
"Only about one-fifth of the initial advantage is taken away.
"Where lean muscle mass is 40-50 percent difference, studies show [testosterone treatment] takes away about 5-8 percent.
"Where strength is 40-60 percent difference, the studies show between 5-10 percent reductions in strength when you lower testosterone. So the biological conclusion is that the lowering of testosterone does impair performance by between 0-10 percent.
"But the initial differences are between 40 and 60 percent. And therefore it does not achieve even a quarter of what would be required to guarantee fairness and safety."
And this can lead to safety concerns.
The risk of injury isn't very high in running or weightlifting, but that's not the case in rugby, Tucker says.
"Rugby's stated priority for the last five-10 years has been player welfare. So, when we look at a question like this we have to also factor in safety elements.
"And to be very clear, rugby wants, desperately, to be inclusive. It positions itself as an inclusive sport.
"But when the inclusiveness starts to create potential issues for fairness, and safety, then the question shifts slightly: now you're asking whether we can achieve a balance between these things.
"If the answer is 'no', you have to start to think about making some difficult decisions. There are trade-offs now. And the World Rugby position is that the prioritisation is player welfare. And it was felt - based on the published evidence at this point in time - that this creates an increased safety risk."
The thing is, size disparities already exist in rugby: just over the weekend we saw all 171cm and 83 kilograms of Aaron Smith line up against the 198cm, 120-kilogram lock Patrick Tuipolotu.
How is this any different?
"There is a disparity within a group of males and a group of females between the smallest and the largest.
"Those disparities span around 30-40 kilograms. The lightest men are about 40 kilograms lighter than the heaviest men, and the same is true of females.
"The issue for this question is what happens if you allow crossover. For instance, we know the typical male rugby player is 40 percent heavier than the typical female player. That's about the same difference from the heaviest female to the lightest female."
In other words, the heaviest female players are about the same weight as an average male player.
"The problem is the heaviest male is about 70 percent heavier than the typical female. The range increases by a large degree when you allow crossover.
"That's just mass, it doesn't account for strength. That male player also has a 30-40 percent difference in strength. They're 15-20 percent faster than the female player. And mass and strength and speed are the three most important risk factors for injury.
"If a person who is biologically male with typical attributes for strength and mass and speed were to compete against typical or even extreme women players, they would introduce a set of variables that is simply not seen in the women's game. And that would create the theoretical injury risk in that group."
So, what are the solutions? There aren't any easy fixes, Tucker says.
Some sports might consider open competition, like many social leagues do, with players signing up voluntarily.
Administrators could also create a third category for trans athletes, though Ross Tucker says this idea is shaky due to insufficient numbers and the stigma attached to being trans still present in some countries around the world.
"But I understand there is another group involved here, and it's the biological women who - let's be honest - have fought for 50 or 60 years to get women's sport elevated to the point where they can play it with some degree of recognition.
"We know that women have had to fight for a place in society for many decades. And I am therefore as sympathetic to them when they say our space, our opportunities, our fairness and safety is being encroached by this issue.
"If the solution to this can be ring-fenced and applied only to sport, and if sport can facilitate other ways for inclusion and to recognise [trans people] as equal in society, then I hope we can do that."
Want more from The Detail? Find past episodes here.