Grace McKenzie started playing rugby in 2018. While at a technology conference in San Francisco, she was approached by a few ruggers who recommended that she try out for a recreational team. As a transgender athlete, McKenzie jumped at the chance to participate in a sport whose international governing body promoted the motto "Rugby for All.”

"Before rugby found me, I was at a low,” McKenzie, 26, said, explaining that she often encountered people who disrespected her gender identity. "In rugby, I found people who accepted me for who I am.”

In a sport that had embraced athletes of different sizes, shapes, abilities and gender identities, McKenzie and other transgender ruggers felt blindsided in recent months when word spread that World Rugby planned to exclude transgender women from women’s teams at top international events, even though none yet are known to have played at the sport's highest levels.

"It may have a minimal impact right now, but it certainly sets a cement ceiling,” said Joanna Harper, a researcher in England who is transgender and has long studied transgender athletes.

On Oct. 9, World Rugby became the first international sports governing body to institute a ban on transgender women competing in global competitions like the Olympics and the women’s Rugby World Cup. Each country can determine whether to continue to permit transgender women to participate in domestic rugby competitions.

After nine months of review and deliberation, World Rugby said that in a collision sport where at least one injury typically occurs per match, "safety and fairness cannot presently be assured for women competing against trans women in contact rugby.”

Rugby officials said they worried that transgender women could cause serious injuries when tackling cisgender women. (Cisgender people are those whose gender identity matches their assigned sex at birth.)

At puberty, male athletes generally gain physiological advantages for many sports, including a larger skeletal structure and greater muscle mass and strength. As a result, men and women compete in mostly separate divisions.

There is little or no scientific research regarding the performance of elite transgender athletes, experts say. But some studies have indicated that residual strength and muscle mass advantages largely remain when people assigned as males at birth undergo testosterone suppression for a year.

World Rugby’s ban was announced months after a prominent Swedish study of 11 transgender women showed that after a year of testosterone inhibition, the women maintained the muscle strength in their thighs and lost only 5% of the muscle mass.

"If sports don’t take care of the safety issue, sports will die,” said Ross Tucker, an exercise physiologist from South Africa who advised World Rugby on the ban. "Mothers won’t put their children in these collision-type sports because of the danger.”

But ruggers and activists who oppose the ban say it is a solution in search of a problem.

A number of leading rugby-playing nations oppose the ban. And athletes who are in favor have been reluctant to come forward in fear of backlash. That viewpoint, in a time of heightened political polarization, generally goes against the inclusivity espoused by international sports, including at the Olympics.

"It’s about policing female bodies,” said Verity Smith, 39, of Britain, a transgender man who competed on women’s teams for 26 years before transitioning and was a silent observer at the World Rugby deliberations. "These governing bodies automatically assume that all female-bodied athletes are not as strong as male-bodied ones, when that simply isn’t the case.”

No scientific studies have been conducted specifically on transgender women in rugby, Harper said.

And there are no examples of transgender women causing serious injuries to cisgender women, said Anne Lieberman, the director of policy and programs for Athlete Ally, an advocacy group for women and LGBTQ participants in sports.

The rugby issue plays into a wide, complex question throughout sports about how to balance inclusivity, safety and fairness when considering athletes who transition from male to female. The sports world has tended toward inclusivity, though the issue remains contentious.

A federal judge in Idaho recently halted a ban on transgender women competing in all sports in the state. And in New Hampshire, Franklin Pierce University changed its policy of allowing transgender athletes to compete to help settle a case with the U.S. Department of Education. That challenge surfaced when CeCe Telfer won an NCAA track championship, taking first place in the Division II women’s 400-meter hurdle race last year.

The Tokyo Olympics, set for next July and August, and the 2021 Rugby World Cup in New Zealand next September would be the first major international competitions that would block transgender ruggers at the door if they happen as scheduled given the coronavirus pandemic.

However, no transgender women would have been expected to compete in the Rugby Sevens at the Tokyo Olympics or in the 2021 women’s Rugby World Cup, regardless of the ban.

USA Powerlifting, which does not oversee an Olympic sport, bars transgender women from competition. But Olympic-related sports other than rugby do not completely block transgender women from competing on teams that match their gender identity. Track and field requires transgender women to suppress and maintain their testosterone levels below a certain limit to remain eligible to compete in certain running events.

Four women’s rugby powers, the United States, England, Canada and Australia, said they would ignore the ban when holding domestic competitions. New Zealand is expected to follow suit.

"We want to make sure we strike the balance between safety and fairness so won’t be rushing into any decision,” said Ken Quarrie, New Zealand Rugby’s chief scientist.

Transgender women ruggers said none were invited to discussions by World Rugby officials before the ban was implemented.

"They didn’t listen to feedback from rugby-playing nations,” Lieberman said. "Looking at major rugby countries, this ban is largely unacceptable.”

Isabella Macbeth Cain, 33, of South Carolina, who started playing the sport after she transitioned, said World Rugby did not show interest in hearing her voice or similar voices.

"My team wanted me there, they were pulling me into the fray of it,” she said of the decision. "The World Rugby ban is so the opposite of everything I’ve come to know about rugby.”

Harper, the researcher, recommended, as a compromise between a ban and no restrictions, that teams be allowed one transgender player per international competition. Because there are few transgender women in rugby - for instance, there are four who are self-identified in Britain, where the sport is popular - setting a limit would not exclude athletes at the moment, she said.

Transgender men are permitted to play on men’s teams as long as they sign a waiver and undergo a physical proving it is they are safe for them to play. They are not allowed to compete on women’s teams after starting testosterone enhancement treatments. Transgender women can play on elite men’s teams, however there are no known examples of transgender women wanting to do so.

Transgender women can still play mixed-gender, noncontact rugby, World Rugby announced, although guidelines for elite mixed-gender rugby are still being formulated. Transgender women who transitioned before puberty can also play women’s rugby after confirming medical treatment that would have prevented biological changes that occur primarily via testosterone during puberty.

Even with the recent ban, the prohibition of transgender women ruggers is not considered permanent. More research will be conducted, World Rugby said, and the guidelines will be formally reviewed every three years.

"Sports want to be a reflection of society, and society wants to be inclusive,” said Tucker, the South African scientist. "It’s a difficult, complex issue — the most complex issue in collision sports.”

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