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The inclusion of trans women athletes—women who were assigned male at birth—is in world sports.

International and national governing bodies, sports teams and are grappling with how to balance trans inclusion with safety and fairness for cisgender women.

The term "cisgender" (pronounced "sis-gender") refers to people whose gender identity and expression matches the biological sex with which they were born.

In 2021, the International Olympic Committee produced a "." It includes 10 principles to guide sporting bodies to create or revise transgender inclusion policies relevant to their sporting needs or types of competition.

Recently, though, international sports organizations for rugby, cricket, cycling, athletics, netball and swimming , making trans women ineligible to compete in elite women's competitions.

But why are these new rules in place and why do they only apply to trans women, but not to trans men?

by me and my colleagues might help explain.

The key is testosterone

Testosterone is a hormone that helps develop male reproductive organs and increases muscle and bone mass.

On average, adult males have about more testosterone than adult females.

Boys generally have . They consistently outperform girls in ball skills, running and jumping.

After puberty, males typically have larger bones, more muscle mass with greater strength and power, stronger ligaments and tendons and larger hearts and lungs .

The gap in sports performance and physical abilities favoring males .

When trans women transition, they take hormone blockers to lower their testosterone levels and appear more feminine.

To compete with cisgender women in many sports, trans women athletes have to keep their testosterone levels below 10 nanomoles per liter (nmol/L) for a certain period—for example, specifies 12 months prior to competition.

In some sports, the limit is now even lower at 2.5nmol/L, for example with .

However, the advantages gained from male puberty , giving trans women a physiological edge over cisgender women.

Trans men in male sports do not have this performance advantage. Even with hormonal therapy to increase testosterone, they can rarely compete with the physical prowess of cisgender males, so they do not pose a threat to male sporting wins or records in many events.

Consequently, very few trans men have participated in elite male sports.

Can inclusion be fair and physically safe for women?

Science suggests that for events that rely on speed, strength or power, it is not fair to cisgender women to include trans women competitors.

in current world records for many athletic and aquatic events is 4–16% in favor of males.

For example, in the 100m butterfly, champion US swimmer Missy Franklin is nine seconds slower than her male counterpart Ryan Lochte, even though they are matched for height and arm span.

, the male world record is 30% higher than the female record for the same body weight. In , about 10,000 males have a personal best time faster than the current female Olympic champion, Elaine Thompson.

Therefore, if a trans woman competes in female events, she might set records that cisgender women can never match.

This means there may be fewer opportunities for cisgender women athletes to be selected in a team, to represent their country internationally, or to emulate feats of female role models.

Our has identified 53 trans women athletes winning podium places in 18 different sports in national or international championships worldwide. This figure is likely an underestimation due to privacy rights.

Â鶹ÒùÔºical safety may also be an issue

In sports where size, speed, power and force matter, smaller and weaker bodies are at a higher risk of injury. This is especially true in collision sports like rugby, or combat sports like boxing and martial arts.

In 2020, World Rugby was the first governing body worldwide to .

Following suit, , , and the determined that only trans women who haven't gone through male puberty may compete, to ensure fairness and safety.

In April this year, announced that trans women couldn't compete in international competitions for the same reasons.

However, some Australian codes, such as , permit trans participation at the elite level.

Other sports, such as the International Basketball Federation and soccer's governing body, FIFA, are yet to publish their guidelines.

What about integrity?

Recently, Sebastian Coe, former Olympian and president of World Athletics, defended World Athletics's science-based ban on trans women in elite female athletics "(cisgender females) have to believe they are capable of going from the playground to podium with some protection." He added, "If it's a judgment between inclusion and fairness, we will always fall down on the side of fairness."

Those who side with Coe argue the integrity of many women's events will be diminished if the outcome can be influenced by the biological advantage of a trans woman.

What's the solution?

For sports where physical advantages such as physique, strength or power don't matter (such as equestrianism), the athlete's biology isn't as important. Therefore, trans men and women can safely and fairly compete against cisgender competitors.

But for most sports, female athletes should be able to compete on a level playing field.

One idea is to create an "open" category for all comers: trans- and cisgender men and women. , the and have all introduced this model.

This could be a way to include everyone while still protecting the integrity of women's sports.

Maybe this is the answer.

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