Sex, Science, and Sport: Why Biology Isn’t So Simple
Written by and illustrated by the X-plainers, Devyn Deschamps and Jimin Lee
A runner approaches the starting line of a hurdles race. At first, the path ahead looks straightforward: run forward, clear each barrier, reach the finish. But with each hurdle, the race becomes more complicated, requiring constant adjustment and precision.
Similarly, athletes face numerous hurdles just for the opportunity to compete in sports. One major hurdle for athletes is the seemingly simple question of who qualifies to compete in women's sports. In this post, we will take a deep dive into the science of biological sex, a more complex and nuanced topic than it first appears!
For nearly every sport, competition is divided into two categories: women’s and men’s. Sometimes the games diverge entirely, baseball and softball, for example. But in most cases, it is the same game, separated under the assumption that competitors within each category share similar biological capabilities.
Because of this, athletic officials have repeatedly attempted to “verify” who qualifies to compete in the women’s category. These efforts to draw a clean line between “male” and “female” have consistently overlooked how biologically complex sex actually is.
Biological sex emerges from the interaction of multiple developmental components: sex chromosomes, hormones and the cellular responses to hormones, and the anatomy they produce. Each of these components can vary independently, as we will explore below, and their combinations produce a far wider range of human biology than two categories alone can contain.
Biological sex is established in the embryo during a developmental process known as sex determination.
The first part of this process occurs at fertilization, when a sperm carrying an X or Y chromosome meets an egg containing an X chromosome, to generate an embryo that has either XX or XY chromosomes. Typically, males have XY chromosomes and females have XX chromosomes.
Despite these chromosomal differences, XX and XY embryos appear anatomically identical for the first several weeks of development. All embryos begin with bipotential gonads: the original structures that will become either the ovaries, which produce eggs, or the testes, which produce sperm.
Between 6-7 weeks of gestation, the bipotential gonads begin to differentiate into either an ovary or testis. In XY embryos, a gene on the Y chromosome, SRY, triggers a cascade of molecular signals resulting in the development of testes. In XX embryos, which lack the SRY gene, they become ovaries.
These embryonic gonads then secrete hormones, which are sensed by other cells in the embryo, directing them to form the complete internal and external anatomy of the reproductive systems, known as “primary sex characteristics”. At puberty, hormones, such as testosterone and estrogen, act again to generate secondary sex characteristics, such as facial hair or breast development.
But typical development is not the whole story. Variation can occur at every stage, including chromosomal, hormonal, or anatomical, and sex testing in sports is a story of officials stumbling into this biological complexity.
First, all female athletes were subjected to humiliating genital examinations. In the 1960’s when technology to visualized chromosomes became available, officials mandated chromosome tests for female athletes to screen for the presence of a Y chromosome. This test assumed that chromosomes sorted neatly into two patterns; however, human chromosomal variation is more complex than XX and XY.
For example, aneuploidies are genetic conditions where there is variation in the number of chromosomes, such as an additional copy of chromosome 21 in individuals with Down syndrome. A variation in the number of sex chromosomes leads to conditions like Turner’s (X), triple X (XXX) Klinefelter’s (XXY), or Jacobs’ syndrome (XYY). Variation in sex chromosome copy number is very common, occurring in around 1 in 400 live births.
The pathways of sex determination in these individuals are similar to those with two sex chromosomes, where the presence of a Y chromosome (and SRY gene) result in testes and testosterone production and typically male primary and secondary characteristics.
In the 1990s, pushback from physicians and scientists moved sports authorities away from blanket sex testing toward a case-by-case basis whenever officials suspected a male “imposter” in women’s sports. The accused faced invasive testing, including genital exams, chromosome analysis, hormone measurements, internal imaging, and psychological evaluations.
By the 2010s, the focus had narrowed to a single number: testosterone level. Some athletes were required to medically or surgically lower their naturally occurring testosterone to “female levels” in order to compete.
But testosterone, like the sex chromosomes and genes they carry, isn’t unique to one sex - everyone produces it! What differs is the amount, and the body’s ability to respond to it.
The problem is, the science doesn’t really back up the idea that a woman’s own testosterone level decides how she performs. Testosterone taken as a drug (doping) clearly enhances performance. But whether the body’s own natural level makes much difference in women is far less clear.
Most studies measure a group of athletes once and check whether testosterone tracks with better performance, which can only demonstrate an association, rather than causation, especially because training can raise testosterone levels. The groups studied are usually small, and the studies rarely account for the many other things that affect who wins, like coaching, nutrition, equipment, and other biological factors.
To really isolate the effects of testosterone, you would need to follow many athletes over time in their actual events, account for other confounding factors, and make sure that individuals with high levels of testosterone are not doping. Even the researchers who report a signal tend to agree that they are seeing an association, but not proof that testosterone is what is driving any result.
Why does this matter? Because the rules treat “too much” testosterone in a woman as a sign that she falls outside her gender category, and therefore has an unfair edge on the competition. In reality, women vary: some naturally produce more, for example because of a condition like Polycystic Ovary Syndrome (PCOS).
Androgen Insensitivity Syndrome (AIS) complicates things further. Individuals with complete AIS may have high levels of testosterone, but their cells can’t respond to it, so they develop a typically female body.
Another form of sex testing is being instituted at the upcoming Olympics. The International Olympic Committee recently announced that in the 2028 games, under the Policy of the Protection of the Female (Women's), all athletes competing in the female category must be a biological female. This will be determined by a one-time genetic test for the presence of SRY, the male sex-determining gene on the Y chromosome.
The Olympics use SRY gene screening as their main method for determining the sex of female athletes. This test is considered less invasive and more thorough than other sex verification methods. They state that the exceptions to this are athletes with Complete Androgen Insensitivity Syndrome (CAIS) or other rare differences/disorders in sex development.
Whether this is, in actuality, the most comprehensive test is up for debate. Some scientists claim that it is oversimplifying the dynamics of sex, and don’t factor in other characteristics that determine sex.
There are many examples of how this SRY gene testing may not be the best choice for determining if an individual is biologically female.
Mutations in the SRY gene can occur, making this gene not functional, which can lead to the development of typical female characteristics like estrogen production and ovaries in individuals with XY chromosomes.
Additionally, the SRY gene can jump from the Y to the X chromosome. Despite having XX chromosomes, these individuals would develop testes and other male characteristics because the SRY gene would promote testes development and testosterone production.
When it comes to women in sports, each new testing strategy assumes that individual biological markers can definitively determine sex: chromosomes, anatomy, or hormones. Yet, for many individuals, their natural development does not fit neatly into the expected binary pattern of XX–ovaries–estrogen or XY–SRY-testes–testosterone, supporting the idea that sex, instead, is a spectrum.
Perhaps it is time to reconsider the many hurdles female athletes must overcome to participate in sports, and whether they are built on a comprehensive understanding of biological sex and all of the ways it can naturally vary.
Additional resources if you want to learn more:
Podcasts:
Tested: https://www.tested-podcast.com/
Gonads, Radiolab: https://radiolab.org/series/radiolab-presents-gonads/
Scientific American: https://www.scientificamerican.com/podcast/episode/sex-testing-in-the-olympics-and-other-elite-sports-is-based-on-flawed/
News:
This article is a part of the first series in the X-plainers’ initiative, a team of Duke undergraduate students producing scientific communication. This project has been under the supervision of graduate student, Hannah Kubinski & Dr. Adrianna San Roman. To read more about X-plainers, click below.
