Many sports are tightening transgender policies. Can inclusion co-exist with fairness, physical safety and integrity?

The inclusion of trans women athletes—women who were assigned male at birth—is one of the most controversial issues in world sports.

International and national governing bodies, sports teams and athletes themselves 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 "framework on fairness, inclusion and non-discrimination on the basis of gender identity and sex variations." 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 have tightened their rules, 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?

New research 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 seven to eight times more testosterone than adult females.

Boys generally have better motor skills, strength and muscle development from an early age. 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 than females.

The gap in sports performance and physical abilities favoring males widens markedly with maturity.

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, U.S. Volleyball specifies 12 months prior to competition.

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

However, the advantages gained from male puberty cannot be completely erased, 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.