The Science Behind the New Olympic Eligibility Rule
The International Olympic Committee (IOC) has recently announced a new policy that will determine whether athletes are eligible for women’s events by mandating a once-in-a-time screening for the male-determining gene, SRY. This decision has sparked widespread debate and raised critical questions about how “female” is defined, whether trans women have an athletic advantage, and whether a truly level playing field in sport is even possible.
Understanding the SRY Gene
In humans and other mammals, the SRY gene plays a crucial role in determining sex during early embryonic development. It initiates the formation of testes and the production of androgens—male hormones such as testosterone—which drive male development. The SRY gene is located on the Y chromosome, which males possess along with an X chromosome, while females have two X chromosomes.
Over the years, sex testing methods have evolved from simple anatomical inspections to more advanced techniques, including microscopic examination to detect the presence of X or Y chromosomes. However, these traditional methods were slow and often led to misdiagnoses, particularly among athletes with sex chromosome variations.
To address this, a test was developed to directly detect the SRY gene. This is the same test that the IOC will now use to determine eligibility for women’s events. But the science behind sex determination is far more complex than it appears.
The Complexity of Sex Determination
The SRY gene activates a network of dozens of genes that promote testis development or block ovary formation. Variations in any of these genes can lead to individuals who may appear female but have the SRY gene, or those who appear male but lack it. For example, some women may have an inactive form of the SRY gene that doesn’t trigger testis development, while others may have a typical SRY gene but cannot utilize male hormones due to other genetic factors.
This means the SRY test could misdiagnose athletes with variant sex genes and chromosomes. Some women might be incorrectly labeled as biological males and banned from competing, while men without the SRY gene could be allowed to compete in women’s events.
Evidence of Male Advantage in Sport
Physiological studies over the decades have shown that men, on average, have larger hearts, more efficient lung function, and greater muscle mass than women. While there is overlap between the sexes, on average, men tend to be bigger and stronger.
Recent research suggests that these differences go beyond just physical traits. A 2017 study found that nearly one-third of our 20,000 genes act differently in men and women, not just in reproductive tissues but also in the heart, lungs, and brain. These sex differences are also observed in monkeys and are apparent before birth.
In three types of muscle cells, 2,100 genes work differently in men and women. This indicates that sex differences are much more profound than previously understood. Traditionally, these differences were attributed to the effects of androgens, but experiments with genetically manipulated mice suggest that some physiological traits, such as fat and energy metabolism, are linked to the number of X chromosomes rather than the SRY gene or hormones.
Transgender Athletes and Athletic Advantage
The evidence regarding transgender athletes’ advantages in sport is less clear. Transitioning from male to female involves hormone replacement therapy, which suppresses androgens and introduces estrogen. This leads to changes such as breast development, increased body fat, and reduced muscle mass. Trans women may also take puberty blockers before going through male puberty, which prevents irreversible physical changes.
The question of whether trans women have a physical advantage over cis women depends on what irreversible sex differences occurred before and during puberty, as well as any non-hormonal differences that might affect performance. Studies show mixed results, with some indicating that trans women may have longer limbs, stronger hand grip, and greater muscle mass. However, after two years of hormone therapy, their cardiac and respiratory function tends to resemble that of cis women.
There is no data on gene activity in trans women athletes, so many questions remain unanswered. Do the 2,100 genes in their muscle cells revert to a female pattern? Do other genes on the Y chromosome protect their heart and kidney function? Does the absence of a second X chromosome improve their fat and energy metabolism?
The Challenge of a Level Playing Field
The IOC’s new rule raises important questions about fairness in sport. While trans women who underwent male puberty may have some advantages in organ size and function, these differences may be subtle. In elite sports, where winning can come down to fractions of a second, even small advantages can matter.
However, human variability complicates this issue further. There is already significant variation among cisgender athletes in attributes like androgen levels, leading to calls for regulating hormone levels. This raises the question: do we ban exceptionally tall women from basketball?
Participation in sport is vital for health and social connections, especially for trans women. Exploring alternatives, such as open competitions or categories based on something other than sex, remains essential.
Ultimately, the idea of a truly level playing field in sport may be unattainable. Elite athletes are likely to have extreme physical and physiological attributes that set them apart from the general population. Whether this is fair or not is a broader ethical question that goes beyond the scope of this discussion.
