If you see a trans woman, like the American  Swimmer Lia Thomas, in an athletic competition  

against women born female, it’s difficult not to  ask whether that’s fair. In this video I want to  

look at what science says about this. How much  of an advantage do men have over women and what,  

if anything, does hormone therapy change  about it? That’s what we’ll talk about today.

The vast majority of humans can fairly easily be  classified as either biologically male or female.  

The two sexes have rather obvious differences in  inner and outer organs and those differences are  

strongly correlated with the expressions  of the twenty-third chromosome pair,  

as you remember from your school days, XY  for men, and XX for women. So far, so clear.

But this classification of the two biological  sexes doesn’t always work. There is a surprisingly  

large variety of what’s known as “intersex”  conditions in which the biological expression may  

be ambiguous or doesn’t align with the chromosome  expression. Some cases are truly amazing.

For example, in 2014 researchers from India  presented the case of a 70 year old man,  

father of four, who underwent surgery. The  doctors discovered he also had a uterus and  

fallopian tubes. His chromosomes however  turned out to be the standard male 46 XY.

Another example is the Spanish hurdler  Martínez-Patiño who has feminine genitals but  

XY chromosomes and internal testes, something that  she was herself unaware of until the age of 25.  

After this was discovered, she was banned  from competing in the 1986 Olympics.

Such cases are called disorders  of sex development, DSD for short.  

They are rare but not as rare as you may  think. According to various estimates  

that you find in the literature they affect  between one in a thousand and one in fifty.

This means that in the United States there  are between about three hundred thousand and  

7 million intersex people, and globally between 8  million and 160 million. Part of the problem with  

making those estimates is that the definition  of an intersex condition is somewhat ambiguous.

Now, most intersex people aren’t transgender  and most transgender people aren’t intersex,  

but the question what to do with intersex people  in competitive sports is a precedent to the newer  

question of what to do with trans athletes.  And Martínez-Patiño illustrates the difficulty.

Her condition is called hyperandrogenic 46XY DSD.  

In the general population, it happens  at a rate of about 1 in 20 000 births.

Women with this condition have elevated  level of the male hormone testosterone.  

In 2014, a study by European researchers  found that in elite female athletes the  

rate of this disorder is about 140 times  higher than in the general population.

Testosterone helps muscles grow, strengthens  bones, and increases levels of hemoglobin in the  

blood, which benefits oxygen transport. Synthetic  forms of testosterone are often used for doping.

In case you think this pretty much settles  the case that high testosterone levels are an  

unfair advantage for women, think again.  Because the reason Martines-Patino  

was assigned female at birth to begin  with is that she has what’s called  

complete androgen insensitivity syndrome.  She has high levels of testosterone, yes,  

but her body doesn’t react to it. After being  banned from the Olympics she appealed, arguing  

that she has no advantage from her elevated  testosterone levels and the ban was indeed lifted.

However, there are other conditions that can  lead to elevated testosterone levels in women.  

To make matters more complicated, there is a  large natural variation in testosterone levels  

among both men and women and since women  with naturally high testosterone levels  

tend to perform well in sports, they  are overrepresented among athletes.

As a consequence, the testosterone  distribution for male and female athletes  

has a big overlap. A paper published  by researchers from Ireland and the UK  

in 2014 showed results of hormonal profiles of  about 7 hundred elite athletes across 15 sports.

They found that 16.5% of men  had low testosterone levels,  

13.7% of women had high levels, and there  was a significant overlap between the them.

So as you see, the business with the  testosterone levels is much more difficult  

than you might think, and that doesn’t  even touch the question how relevant it is.  

The advantages stemming from testosterone  are particularly pronounced in disciplines  

that require upper body strength, and less  pronounced in those that test endurance.

Having said that, let’s then talk about the trans  athletes. Transgender people don’t identify with  

the sex they have been assigned at birth.  Leaving aside the intersex conditions, this  

means a trans man was born biologically female  and a trans woman was born biologically male.  

People-who-aren’t-trans are  commonly referred to as cis.

Some trans people undergo surgery and/or  hormone therapy to adjust their physical  

appearance to their gender. The results of  the transition treatment differ dramatically  

depending on whether it’s started  before or after puberty. During puberty,  

boys grow significantly more than  girls and they develop more muscles  

whereas, to quote Meghan Trainor, women get all  the right junk in all the right places. Physical  

changes during puberty are partly irreversible and  transitioning later will not entirely undo them.

In 1990, a seminar convened by  the World Athletics Federations  

recommended that any person who had undergone  gender reassignment surgery before puberty  

should be accepted to participate under  their new gender. This isn’t particularly  

controversial. The controversial bit is what  to do with those who transition after puberty.

The International Olympic Committee has  been leading the way on passing regulations.  

In 2004 they ruled that transgender athletes  are allowed to compete two years after surgical  

anatomical changes have been completed and  if they’ve undergone hormonal therapy. This  

means in particular that trans women must have  taken hormone therapy for at least two years.

In 2021, the committee issued a framework  that allows international federations to  

develop their own eligibility criteria  for transgender and intersex athletes,  

so there’s no simple rule that  applies to all disciplines.

But does the hormonal treatment make it fair  for trans women to compete with cis women?

In 2019 a team of European researchers from the  Netherlands, Norway and Belgium measured the  

change in grip strength for trans people after a  year of hormonal therapy. They had about 250 trans  

women and trans men each who participated in their  study. So this isn’t a huge sample but decent.

They found that grip strength decreased in  trans women by minus 1 point 8 kilogram but  

increased in trans men by 6 point  1 kilogram. In trans men, but not  

in trans women, the change in grip strength  was associated with change in lean body mass.  

So it seems that hormonal therapy does  more for trans men than for trans women.

Another team of researchers from Sweden  followed 11 untrained trans women and  

12 untrained trans men before and up to one  year after gender-affirming hormonal therapy.

They found that in trans women thigh muscle  volume decreased by 5 percent and quadriceps  

cross-sectional area decreased by 4 percent,  but muscle density remained unchanged and  

they roughly maintained their strength  levels. In trans men, on the other hand,  

thigh muscle volume increased by 15 percent;  quadriceps cross-sectional area also increased  

by 15 percent, muscle density increased by 6  percent, and they saw increased strength levels.  

Again it seems that hormonal therapy does  more for trans men than for trans women.

It’d be rather tedious to list all the papers,  so let me just say that this finding has been  

reproduced numerous times. A meta analysis of  from March last year surveyed two dozen studies  

and concluded that, even after 36 months of  hormonal therapy, the values for strength,  

lean body mass and muscle area in trans  women remained above those of cis women.

These numbers aren’t directly applicable to  athletes because in the general population  

trans men have an incentive to build muscles while  trans women have an incentive trying to lose it.  

But those studies pretty much agree that hormone  therapy makes a faster difference for trans men  

than for trans women, and after 3 years  the difference hasn’t entirely disappeared.

There is basically no data on what this  hormone treatment does in the long run.  

A 2021 paper from Brazil suggests that after  about 15 years differences between trans and  

cis women have basically disappeared. But this  was a very small study with only 8 participants.  

And in any case, if you ask athletes to wait  15 years, they’ll be too old for the Olympics.

So let us come back to the question then whether  it’s “fair” for trans women to compete with  

cis women. It seems clear from the data that  trans women keep an advantage over cis women,  

even after several years of hormonal therapy.  I guess that means it isn’t fair in the sense  

that no amount of training that cis women  can do is going to make up for male puberty.

But then, athletic competition has never  been fair in that sense. To begin with,  

let’s not forget that for athletic performance the  most important factor isn’t your sex, it’s your  

age. And some people are born with an advantage  at certain types of sports, being male is only one  

of them. Usain Bolt has long legs. Michael Phelps  has big feet. And American Basketballers are tall.

Really tall. Here’s the American under16 Women’s  Basketball team with the team from El Salvador.  

The Americans won 114-19. Is that fair?

And those are just the visible differences.  There are also factors like bone density,  

cardiac output, or lung volume that  are partly genetically determined. I  

never had a chance to become an  Olympic swimmer. Is that fair?

No. Athletes are biological extremes.  “Fairness” has never been the point of  

these competitions. They’re really more like  freak shows. Kind of like physics conferences.

There’s another aspect to consider, which  is that these competitions should also  

entertain. I guess this is why the researcher  Joanna Harper, who is a trans athlete herself,  

has suggested we talk about “meaningful  competition” instead of “fair competition”.  

We have historically segregated men and for  women in sporting events because otherwise  

competition becomes too predictable, too  boring. In some disciplines we have further  

categories for the same reason,  like in weight lifting and boxing.

Now we’re asking if not we need  additional categories for trans athletes.  

Alright, we could do this. But if you  follow this logic to its conclusion  

then really the only person you  can compete with is yourself.

Or you will have to try and measure every single  parameter that contributes to athletic performance  

in a given discipline and then try to adjust for  it. The result may be that the person who comes  

in last in a race is the winner, after you  adjust for heart valve issues, testosterone  

levels, age, slightly misaligned  legs, under average lung volume,  

and a number of other different conditions.  And that would be “fair” in the sense that now  

everyone had chance to win provided they trained  hard enough. But would people still watch it?

The question of entertainment brings up another  issue. Most of the sport disciplines that are  

currently widely broadcast favor biological  characteristics typically associated with  

men, with the possible exception of long distance  swimming. But generally sex differences decrease  

the more emphasis a discipline puts  on endurance rather than strength.

A 2019 study among casual athletes found  that men still have an edge over women  

in marathns but somewhere between 100  and 200 miles, women begin to win out.  

Though this currently doesn’t reflect in the world  records where men are still leading, it seems that  

men have less of an advance in endurance  disciplines. Which brings up the question:  

Why don’t we see more of such sporting events? I  don’t know for sure, but personally I find it hard  

to think of something more boring than watching  someone run 200 miles. So maybe the solution is  

that we’ll just all just do esports in the end. But let’s come back to the trans athletes.  

Researchers from the University of California  estimated in 2017, that the percentage of  

transgender people in the United States is about  0 point four percent (0.39%). A similar estimate  

for Brazil put the number there at about 0 point  seven percent(0.69%). If these number are roughly  

correct, transgender people are currently  underrepresented in elite level sports. That  

isn’t fair either. This is why I think sporting  associations are doing the right thing with  

putting forward regulations based on the best  available scientific evidence, and as long as  

athletes comply with them, they shouldn’t have  to shoulder accusations of unfair competition.

That said, professional sports associations will  soon have a much bigger problem. Like that or not,  

genetic engineering has become reality. And as  long as athletes can make a lot of money from  

having a genetic advantage, someone’s going  to breed children who’ll bring in that money.  

This is why I suspect a century from  now professional athletics will not  

exist anymore. It creates too many  incentives for unethical behavior.

I hope this brief summary has helped you  make sense of a somewhat confusing situation.  

Let us know what you think in the comments.

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