The Laurel Hubbard debate: The science behind transgender athletes

It’s one of the hottest topics in sport - the debate on whether transgender females should compete in cisgender female sport.

James de Lacey

By James de Lacey
Last updated: April 20th, 2024
15 min read

James de Lacey

By James de Lacey
Last updated: April 20th, 2024
15 min read


  1. Introduction
  2. Should Transgender Athletes Compete In Female Sports?
  3. Argument 1 – Sport Is Unfair
  4. Argument 2 – There Are No Differences Between Genders
  5. Argument 3 – The Power of Testosterone: Does reducing it really make it fair?
  6. Argument 4 – Advantages Don’t Mean Medals
  7. Are There Any Real Solutions?


It’s one of the hottest topics in sport – the debate on whether transgender females should compete in cisgender female sport. With the recent qualification of Olympic weightlifter Laurel Hubbard for the Tokyo 2021 games, this argument has been reignited and doesn’t seem to be getting any less controversial.

Rarely grounded in science, the debate is often fuelled by a mixture of emotion and political correctness, with added concerns of inclusivity and fairness. Arguments based on emotion or personal views tend to lead to incorrect facts and misinterpreted science. That’s why this topic so complicated.

Now, I am all for having the right to do what you want with your body, and have no problem with anyone who wants to transition from male to female or vice versa. The problem comes when physicality and competition are involved. But before we get into it, I want to preface this piece by clarifying that I will solely be referring to male-to-female transgender athletes. (Author’s viewpoints are their own).

Laurel Hubbard of New Zealand competes in the women

Should Transgender Athletes Compete in Female Sports?

Find any news article shared on social media about a transgender athlete competing in a cisgender female sport, and it’ll be littered with comments expressing opposing views. Some believe transgender female athletes should be able to compete in cisgender female sports, while others believe they should have their own category, or not compete at all. Here are the common reasons why many believe they should be allowed to compete in female categories:

  • Sport is unfair by nature. We allow bigger, stronger cisgender females to already compete, so why can’t transgender females also compete?
  • Gender is a social construct, so there is no difference between males and females. So, anyone should be able to freely compete in any division.
  • Transgender females need to undergo hormone therapy for at least one year to reduce their testosterone levels to be under those identified by the IOC or IAAF. This should render any advantages they may have null or even a disadvantage. Alternatively, some believe that testosterone isn’t the reason for performance differences in the first place.
  • Certain transgender females aren’t even going to win, so why does it matter?

These are the four most common arguments in favour of transgender females competing in cisgender female sports. I’m going to break down each point and illustrate why they go against conventional science. To quote my favourite science educator Neil deGrasse Tyson: “The good thing about science is that it’s true, whether or not you believe in it.”

Argument 1 – Sport Is Unfair

One of the top reasons many believe transgender females should be allowed to compete in cisgender female sports is fairness. The IOC guidelines state their stance for transgender athletes to compete against cisgender females is to “guarantee fair competition” (1).

Sport is already inherently unfair. Athletes with access to better coaching, facilities, and resources have an advantage over those without. Also, some athletes are naturally gifted with talent or physical attributes that others will never possess, even with years of training. This means shorter athletes may lose out to taller athletes in basketball, and smaller athletes may lose out to larger athletes in rugby.

These examples of unfairness in sport can be considered “tolerable unfairness” (2). Tolerable unfairness is grounded in the idea that athletes begin from roughly the same starting point but ultimately end up at different levels due to uncontrollable factors (2). There are some instances where tolerable unfairness is controlled to ensure it is more tolerable and doesn’t overshadow more important aspects of performance, such as skill.

While the “skill thesis” has issues, the initial premise is sound. It suggests that if sport was to be decided based purely on skill, any genetic advantages would need to be mitigated through handicap systems or luck scales. After all, that is the purpose of competitive sport – to identify the most skilful (3). This is the very reason that combat sports have weight classes – if a heavyweight matched up with a lightweight in any combat sport, the heavyweight is likely to win purely because of their size advantage – it would be difficult to determine who was the most skilful.

If a transgender female has a performance advantage over a cisgender female athlete, there’s a chance that she also might win a contest without being the most skilful (2). So, any advantage they may have should ideally be considered as tolerable unfairness, or have some controlling factor introduced to lessen its effect. This isn’t necessarily possible though, because the performance advantages that transgender females may have are not only because of their size but because of what caused them to grow in the first place – more on this later.

While gender is a social construct where all can be viewed as equal, sex is biological. So is there really no difference between males and females?

Argument 2 – There Are No Differences Between Genders

Let’s start at the top – the elite of the elite. Below is a graph showing the clear performance gap between males and females at the elite sporting level (4). With female performance set to 100%, we can see that endurance-based sports have a performance gap of around 10-13 % in best performances, whereas strength and power-based activities have differences up to 50 %. It seems there is a trend where the more upper body that is involved in the activity, the greater the performance gap. Previous research suggests males are more often than not bigger, faster, and stronger than their female counterparts (5). This has been found in several sports including handball (6) and weightlifting (4) where performance differences range between 31-3 7% across all weight classes. Females who are 60 % heavier than males still don’t make up this performance gap difference.

Figure 1. The male performance advantage over females across various selected sporting disciplines (4).

These are the elite performers. But does this performance gap exist in amateur athletes? In young adults, VO2max in males was 56 % higher than females (7), with two separate studies finding males possess 89 % one-rep maximum(1RM) strength, 57 % greater muscle size, 109 % greater isometric strength (8), and 162 % greater power than females (9).

We can go even further with performance differences between males and females pre-puberty before any hormonal changes.

Nine-year-old males are 10 % faster over short sprints (10), 16 % faster over 1-mile, jump 10 % further, do 33 % more push-ups, and have a 14 % stronger grip than 9-year-old females. Performance differences are even seen in children as young as six-years-old where males can perform 16% more shuttle runs and jump 10% further (11).

With all of these performance differences from young children ranging to elite athletes, what would happen if male and female divisions were abolished and anyone could compete against anyone? Well, we would likely see the end of females competing in some high-level sports. For example, the world record 100 m sprint time for females is 10.49 seconds. The 100 m sprint record in the U18 men’s category is 10.06 seconds – set by a 16-year-old boy.

So, it seems there are clear differences between males and females when it comes to physical performance and physical attributes. But what is it that is causing this large performance gap?

Argument 3 – The Power of Testosterone: Does reducing it really make it fair?

There is one hormone that influences these dramatic performance differences between males and females, and that is testosterone. Testosterone is a sex hormone considered highly beneficial in all sports for enhancing many physical attributes. As the IAAF stated in their 2019 eligibility regulations, “To the best of our knowledge, there is no other genetic or biological trait encountered in female athletics that confers such a huge performance advantage [than testosterone].”
The average male testosterone level sits between 7.7-29.4 nmol/L. The average female ranges from 0-1.7 nmol/L (12). Raising testosterone outside of a normal female range is so potent that even at levels just below the lowest male range of 7.3 nmol/L, we see 4.4 % increases in lean muscle mass, 12-14 % increases in lower body strength and power, and a 26 % increase in upper body power (12). This testosterone level is 156 % greater than the female average.

In fact, when males hit puberty, their testosterone levels increase 20x whereas female’s testosterone levels remain steady, resulting in males having a 15x higher testosterone than females (12).

These data suggest that testosterone through puberty is primarily responsible for these large performance gaps between males and females. Even at the early to middle stages of puberty, males exhibit testosterone levels of approximately 6.9 nmol/L which is far greater than the average female (12).

As this movement of inclusivity in sport grows, there are more and more people trying to discredit testosterone as a powerful performance-enhancing hormone.

However, scientific research suggests otherwise. The simplest way of discerning the effects of testosterone on performance is to study the effect of exogenous testosterone (i.e. supplementing extra testosterone) in males and females.

Let’s start with muscle mass and strength.

Figure 2. the strong dose-response relationships of muscle mass shown as (A) “lean” or “fat-free” mass or volume of (D) thigh and (E) quadriceps muscle and (C) of leg muscle strength with increasing testosterone dose (12).

Figure 2 represents increasing doses of testosterone in males and its effect on muscle mass, leg press strength, thigh muscle volume, and quadriceps muscle volume (12). We can see a clear dose/ response relationship between testosterone and body composition, strength, and muscle mass, meaning the more testosterone is used, the greater the gains that are made.

But what about females? Do they show the same response?

Figure 3. Dose-response effects on lean (muscle) mass and three measures of muscle strength as a result of increasing doses of weekly testosterone enanthate injections in women (12). 

As demonstrated in Figure 3, the same dose-response relationship is evident in females as in males; increases in muscle mass, leg press strength, chest press power, and lower body power.

So, what exactly does more testosterone do to the human body? Well, testosterone has a strong effect on bone density where males exhibit 10% greater bone surface area compared to females (2). Greater bone surface area allows more muscle to be supported which is one reason why males are generally bigger and stronger than females on average.

Additionally, males on average are 7-8 % taller than females with longer, denser, and stronger bones that allow greater leverage in sports that involve jumping, throwing, and explosive activities (12). Further, females suffer more lower body stress fractures compared to males due to the difference in bone density.

While many believe that testosterone only improves muscle mass and strength, it also has a large positive impact on aerobic capabilities. Males have a diaphragm that sits lower than females allowing for greater lung capacity. During puberty, increases in testosterone increase the quantity of alveoli (small air sacs) in the lungs allowing more oxygen to travel from the lungs into the bloodstream.

Males also have a larger heart, increasing stroke volume. Females will pump 33% less blood per heartbeat compared to males.

Finally, males display greater haemoglobin concentrations improving the ability to carry oxygen to the working muscles during exercise. All of these attributes are influenced by testosterone (2).

With all of these positive impacts related to testosterone, what is the limit for a transgender female to compete? The current IOC guidelines for transgender female athletes are as follows:

  1. They have to declare their gender for sporting purposes to be female for at least four years.
  2. Their testosterone must be under 10 nmol/L for at least 12 months before competing.

The IAAF guidelines have since lowered the required testosterone level to 5 nmol/L which they state is to “ensure a level playing field for athletes” (19). If you remember, the average female testosterone is 0-1.7 nmol/L. And levels approaching the lower range for males is enough to significantly enhance performance and body composition in females.

But what about the belief that transgender females are at a disadvantage because they have to suppress their hormones? There is a strong body of research investigating male-to-female transitions and the effect of hormone therapy on various performance markers. Unfortunately, it seems that reducing testosterone in transgender females doesn’t reverse all of the adaptations cisgender males undergo during growth.

After 24 months of hormone therapy, transgender females retain their bone density and, in some cases, may even be preserved over 12 and a half years (13, 4). After 12 months of hormone therapy, no increases in bone fracture rates were found to support retaining bone density (14).

The largest reduction in muscle mass seen in male-to-female transitions is 12% after 3 years of hormone treatment (21). When looking at a 1-year timeline, as per the IOC guidelines, we see an approximate 3-5% loss in muscle mass (15).

The fact that cisgender males on average have 40% greater muscle mass than cisgender females, suggests that transgender females have large advantages in this department over their cisgender female counterparts (4).

After eight years of hormone therapy, muscle mass is only reduced by 17% placing them in the 90th percentile for women and grip strength is reduced by 25% placing them 25% higher than cisgender female values (4). Further, when testosterone is reduced to within the normal cisgender female range after 12 months of hormone therapy, grip strength is only reduced by approximately 4% (18).

Let’s take the hot topic and use Hubbard as an example. Her previous records before transitioning in 1998 were a 135 kg snatch and 170 kg clean & jerk, for a total of 300 kg.

21 years later in 2019, she hit a 131 kg snatch, and 154 kg clean & jerk in competition for a total of 285 kg. That is a 5% decline in performance. When there is a 30 % strength difference between males and females in Olympic weightlifting, that doesn’t bring her much closer.

While no research to date has measured the effects of male-to-female transitioning with hormone therapy on endurance performance, the research suggests that transgender females retain most of the lean muscle mass, strength, and bone density even up to and past eight years in some instances.

Even with all these advantages, some transgender females still don’t have a chance of winning elite competition.

Argument 4 – Advantages Don’t Mean Medals

This is often a viewpoint parroted on social media, and there is some truth behind it. Not every transgender female competing at the elite level is going to surpass all cisgender female athletes. There are a lot of factors that make up a truly elite athlete, from skill level to genetic background.

But there are many reasons why, regardless of whether they win or not, allowing transgender females to compete with cisgender females is a slippery slope.

Firstly, there’s the matter of fairness. A cisgender female could potentially lose her spot for selection because of the advantages a transgender female has. The cisgender female athlete is not allowed to supplement with exogenous testosterone to match the transgender athlete. Further, cisgender females haven’t gone through the surge in testosterone that occurs during puberty, leaving them even further behind.

But it’s not just the elite level that suffers. It’s the amateur level, too. For example, Gabrielle Ludwig, the 6’6”, 230 lb transgender female played college basketball (21). And Hannah Mouncey, the Australian Rules football player playing in the VFLW who previously represented Australia in men’s handball at 6’2” (22). Most recently, France has allowed transgender females to play women’s rugby in France (23). As it becomes more acceptable for transgender females to compete at the elite level, it will continue to trickle down to the amateur game.

While transgender females may not medal at the elite level, at the amateur level, the average male who transitions can potentially compete at a much higher level than they could as a cisgender male. This creates a small butterfly effect – their advantage may allow them to reach the representative level or win local events, which means a cisgender female athlete at that level doesn’t get scouted or selected. This can mean a lack of scholarship or sponsorship, potentially leading to the athlete leaving the sport altogether.

Are There Any Real Solutions?

Many believe if males transition before puberty, the effects of testosterone will be negated as they wouldn’t have been exposed to high levels of testosterone. However, as previously discussed, children as young as six show a large performance gap favouring males. Consideration also needs to be given to the teens undergoing female-to-male transition. Mack Beggs, the American high school wrestler won the Texas State girls’ wrestling championship with a combined 89-0 record throughout both seasons whilst using steroid therapy treatment whilst transitioning from female to male. Texan public high schools require students to compete under the sex assigned at birth, though Beggs stated, “[Texas policymakers] should change the laws and then watch me wrestle the boys. Because I’m a guy. It just makes more sense.” (24).

It seems that suppressing testosterone to the cisgender female range still isn’t enough to mitigate the performance advantages transgender females have. UK Athletics has called for an open category allowing transgender athletes to compete with men, protecting the female division. However, this has been contested by the UK government highlighting that the Equality Act 2010 does not allow the exclusion of transgender athletes on grounds of fairness (25).

However, at this point, there aren’t enough transgender females to warrant this (perhaps a case of ‘build it and they will come’?). The evidence isn’t yet comprehensive enough to provide a clear answer, and until the research is conducted, there is unlikely to be an immediate consensus.

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  2. Knox, T., Anderson, L. C., & Heather, A. (2019). Transwomen in elite sport: scientific and ethical considerations. Journal of medical ethics45(6), 395-403. [Link]
  3. Bianchi, A. (2017). Transgender women in sport. Journal of the Philosophy of Sport44(2), 229-242. [Link]
  4. Hilton, E. N., & Lundberg, T. R. (2020). Transgender Women in the Female Category of Sport: Perspectives on Testosterone Suppression and Performance Advantage. Sports Medicine, 1-16. [Link]
  5. Ryman Augustsson, S., Bersås, E., Magnusson Thomas, E., Sahlberg, M., Augustsson, J., & Svantesson, U. (2009). Gender differences and reliability of selected physical performance tests in young women and men. Advances in Physiotherapy11(2), 64-70. [Link]
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  13. Fighera, T. M., Ziegelmann, P. K., Rasia da Silva, T., & Spritzer, P. M. (2019). Bone mass effects of cross-sex hormone therapy in transgender people: updated systematic review and meta-analysis. Journal of the Endocrine Society3(5), 943-964. [Link]
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James de Lacey

James de Lacey

James was the Head Strength & Conditioning Coach for the Romanian Rugby Union. He has previously worked in America’s professional rugby competition Major League Rugby with Austin Elite and the NZ Women’s National Rugby League Team. He is a published author and has completed a MSc in Sport & Exercise Science from AUT, Auckland, NZ.

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James de Lacey

James de Lacey

James was the Head Strength & Conditioning Coach for the Romanian Rugby Union. He has previously worked in America’s professional rugby competition Major League Rugby with Austin Elite and the NZ Women’s National Rugby League Team. He is a published author and has completed a MSc in Sport & Exercise Science from AUT, Auckland, NZ.

More content by James
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