A cell layer that has developed to protect your body from the outside doesn’t work like a cell layer that has developed to protect your body from the inside.
The cells lining my vagina are not the same cells, and they don’t have the same function, as the ones wrapping your penis.
There’s a name for what happens when you subject dry-adapted “outside skin” to wet-adapted “inside conditions”.
My vagina - “inside skin” - hothouses a healthy microbiome that promotes health and healing, and imparts immune function onto small humans that happen to come out of it.
Yours? Less so.
My vagina is a muscular organ, adapted to my healthy female function of receipt of peen, expelling menstrual products and pushing out small humans.
Let’s move the discussion from available techniques for sex screening and to matters of process.
Ross @Scienceofsport has described the need for detailed technical documents that inform sports federations in robust implementation of a sex screening policy. I’ll link to his video next.
But here, I’m going to take a wander through running an assay, highlighting standards and procedures.
First, this is Ross’ video of the overall process, highlighting the need for coherent implementation practices. He - correctly - evokes the reams of technical documents used by WADA in their anti-doping programmes.
Even the simplest of lab assays can have pages of instructions associated with it.
So, the assay for sex screening will be detection of the SRY gene. This is the ‘make male’ gene that is the master switch for testes-not-ovaries.
The assays out there are very sensitive and specific. That means they can detect SRY when it’s present, and they don’t give a signal when it’s absent. They aren’t 100% on either metric, but near as dammit.
In 2025, Jon Pike and I argued that exclusion of athletes with androgenising XY DSDs from female athletics is justified, because these athletes are male, not female.
@runthinkwrite This followed a 2024 paper where we, along with Ross Tucker, Tommy Lundberg, Cathy Devine and many others, argued for a return to sex screening to secure eligibility for female sport.
@runthinkwrite @Scienceofsport @TLexercise @cathydevine56 This followed another 2024 paper where we critiqued the (now former) IOC policy on inclusion of trans-identifying males in female sports.
Alvares 2025, n=7, fat mass is higher in females as both absolute and relative values. This is logged as "favours cisgender", which is kinda odd because high fat mass isn't usually considered favourable for sports, but whatever.
TIMS: 16.2 kg (24%). F: 19.5 kg (26%).
But Ceolin 2024 is also logged as "favours cisgender" when their values are:
There are little-to-no controls for physical fitness in the individual studies.
Yet they conclude: “transgender women do not exhibit significant differences in upper-body strength, lower-body strength or maximal oxygen consumption relative to cisgender women after 1–3 years of GAHT.”
You haven’t controlled for fitness!!!
Their "performance" data. Can you see one study that really sticks out as an outlier?
The claim that won't die: trans-identifying males are "underpowered" and therefore "disadvantaged" in sport.
"One can imagine a large car with a small engine competing against a small car with a small engine, and that summarizes the playing field." Joanna Harper, Huff Post, 2016.
"You have a bigger body, and you have a smaller engine to move that vehicle around." Yannis Pitsiladis, BBC, 2019.
"giving trans women the disadvantage of having to power larger skeletal frames with reduced strength and aerobic capacity." Jamie Agapoff, 2025.
What happens when a trans-identifying male suppresses testosterone?
They lose a bit of muscle mass.
Their haemoglobin drops to female-typical levels.
The claim that won't die rests on the idea that trans-identifying males retain their skeletal frame and most of their muscle mass, but become unable to move it around a sports fields, rendering them "disadvantaged".
The words "underpowered" and therefore "disadvantaged" are carefully chosen, and typically leave the reader to infer that this means "underpowered" and therefore "disadvantaged" compared to females.