Is there an unfair advantage? Should transgender women be banned from competing in female categories? Should there be a separate category established? Or should sport be more inclusive?

The debate centres around the balance of inclusion, sporting fairness and safety in women’s sport – essentially, whether trans women can compete in female categories without their biological sex giving them an unfair advantage or presenting a threat of injury to other competitors.

As part of The Sports Desk podcast’s look at the debate, BBC Sport spoke to two scientists who offer views from opposing sides.

• Ross Tucker is a sports scientist who says the physiological differences established during puberty can create “significant performance advantages (between men and women)”.

• Joanna Harper is a sports scientist and is transgender herself. She studies the effects of transition on female transgender athletes.

Here, Tucker and Harper answer the key questions being debated from a science perspective. You will also hear from Dr Seema Patel, a lawyer who argues other factors – such as human rights – must be considered.

Do transgender women hold an unfair advantage over female athletes?

Harper: Advantages are not necessarily unfair, and let me use two examples, one where the advantages aren’t unfair and one where they are.

Left-handed athletes have advantages over right-handed athletes in many sports. It is perhaps most marked in fencing where 40% of elite fencers are left-handed versus 10% of the population is left-handed.

But right-handed fencers and left-handed fencers can engage in meaningful competition despite the advantages that left-handed fencers have.

However, you never put a big boxer in the ring with a little boxer, no matter how good the little boxer is. No matter how hard the little boxer works, trains, how competitive they are, they can’t beat a big boxer. The size difference means there’s no such thing as meaningful competition between big boxers and little boxers.

So the question isn’t ‘do trans women have advantages?’ – but instead, ‘can trans women and women compete against one another in meaningful competition?’ Truthfully, the answer isn’t definitive yet.

Trans women can have disadvantages because their larger frames are now being powered by reduced muscle mass and reduced aerobic capacity, but that’s not as obvious as the advantages of simply being bigger.

Yes, it’s true that competition can often come down to a very small margin, but there are, in any competition, many factors that come into overall performance and just saying that ‘oh, somebody has an advantage’ in one factor doesn’t necessarily determine the outcome.

Tucker: When boys reach the age of 13-14, things start to change physically and we see increased muscle mass, bone density; [it] changes the shape of the skeleton, changes the heart and the lung, haemoglobin levels, and all of those things are significant contributors to performance.

Lowering the testosterone has some effect on those systems, but it’s not complete, and so for the most part, whatever the biological differences are that were created by testosterone persist even in the presence of testosterone reduction – or, if I put that differently, even after testosterone levels are lowered.

It leaves behind a significant portion of what gives males sporting performance advantages over females.

Should transgender women be banned from women’s sport?

Tucker: The point of the women’s category is to exclude male advantage, which comes as a result of testosterone.

Until it can be shown that that advantage doesn’t persist or exist in trans women, then I would say that there’s no basis to allow trans women in.

The point of all that is that if there were no evidence at all, I would say that an exclusion policy would be the prudent start point.

However, we do have evidence – we have 13 studies that show significant retained advantage. We have a number of other studies of males with lower testosterone levels with prostate cancer, we know what happens with training, and so I think collectively the picture is quite strong to suggest that advantages are retained.

Finally, I think that what that leads to is the prediction that over time you will see athletes like Lia Thomas and Emily Bridges, so they are in effect the manifestation of what we know will happen physiologically.

So I would be quite confident at this point that a policy that regulates women’s sport by excluding male advantage, which includes trans women, is the evidence-based one.

It’s not impossible that in time evidence will emerge to challenge that and then we can reconsider that, but I think [the IOC] got it backwards in the beginning by allowing it in until proven otherwise. It should have been excluded until it could be shown that the advantages can be removed.

Harper: The science is in its infancy and we are not going to have definitive answers for probably 20 years.

There are some, including the IOC, that have said until we know [more] we shouldn’t restrict trans athletes.

What I would say is that until we know for sure, sport’s governing bodies should do the best they can with the data that exists, with the knowledge that we have today, with the understanding that any policy they create now should be subject to change one we get more data.

So for instance, World Athletics has said that once transgender women reduce testosterone for 12 months, they should be allowed in. That’s not a perfect policy – nobody is saying it is – but World Athletics has said this is the best we can do with the available science.

That I think is a more reasonable approach than either saying there shouldn’t be any restrictions on trans women or we shouldn’t let trans women in until we know for certain.

Should there be a separate category for transgender athletes?

Harper: In recreational sports, we should be creative; we can look at different ways of dividing. Do we need a male and female category in every case? Could we separate in other ways? Perhaps there may be cases where there is a third category that might be effective.

But the problem is if you strictly require all trans athletes to go into a trans category, then you have three categories – one with 49.5% of humanity, the other with 49.5% of humanity, and one with 1% of humanity.

So is the UK going to be able to field a transgender football team? And if so, will any other country field a transgender football team? Will the UK transgender football team have anybody to play?

In team sports especially, it’s virtually impossible to consider a transgender category – it’s not going to work in elite sport.

There are situations where it might be beneficial to be flexible and look outside the male/female dichotomy.

When we separate into categories, we don’t necessarily eliminate advantages, but we reduce them to the point where anyone who is in the category can enjoy meaningful competition with anyone else in the category.

That is true when we subdivide into male and female. If we want to see women winning Olympic gold medals or earning professional sports contracts then we can’t be having men in the category.

Can we have trans women who have gone through male puberty in the category? That, admittedly, is not yet a settled question.

Tucker: It might be that in the future – that’s where we head to.

It would in some respects be quite a positive step, but I don’t think that the world is really ready for that, and I don’t just mean the sports world.

The one obvious problem is there would be so few athletes that I’m not sure they would be able to sustain a sporting competition or even a category that is viable.

The other problem is that there is still a lot of stigma attached to being trans and I’m not sure that trying to force or create a platform through sport would help overcome that. If anything, there might be certain barriers that are created.

There are some countries in the world where it would be deemed illegal, so I’m not sure that society is necessarily ready for that and that it would be fair.

That said, I think it might be a solution at some point in the future, but I just don’t think that we are necessarily there yet.

Would transgender women ‘take over’ women’s sport?

Harper: Trans women are never going to take over women’s sport. First of all, trans people make up roughly 1% of the population.

The best example of a population study to look at comes from America. If you look at NCAA sports, there are more than 200,000 women competing every year in NCAA sports. Trans women make up 0.5-1% of the population so we should be seeing 1,000-2,000 trans women every year.

The NCAA 11 years ago allowed trans women to compete, based on hormone therapy. We should be seeing 1,000-2,000. We see a handful every year.

So 11 years after these hormone-therapy-based rules went into effect, trans women are not taking over NCAA sports. They are still hugely under-represented.

Tucker: The problem is one of concept – not scale and number – and if you ask women about that, they’ll say ‘well, how many would you accept?’ Would you accept five? Would you accept 10? Does it need to be 50?

We’ve had a handful [of transgender women athletes] in the last while. There are a number of others in the United States who might not be receiving global attention, but they’re certainly winning titles in the US.

And again, they’re taking places from women within the women’s sports category, so playing the numbers game to me seems really dangerous because by 2028, at the Olympic Games beyond Paris, we could be seeing half a dozen, maybe a dozen. Who knows?

It seems to be a problem that’s only growing.

What do you think of the IOC’s policy?

New guidance from the International Olympic Committee in November states there should be no assumption that a transgender athlete automatically has an unfair advantage in female sporting events. It invites individual sports to find the right approach.

Tucker: It’s quite clear that the IOC from the very beginning was intent on finding inclusion at the expense of the integrity of women’s sport.

Where the IOC are now is that they’ve compromised it even further. No presumption of advantage – it’s an extraordinary statement at a time when they have more knowledge than they would have had even seven, eight years ago.

So, despite the fact that we now know more, all the evidence, as I mentioned previously, points towards retained advantage, never mind existence of advantage in the first place, and yet they’ve gone in the direction of saying that we no longer need to measure testosterone, so there is a fundamental failure of scientific integrity on the part of the IOC.

Most Olympic sports don’t have the capacity to make this decision, let alone the evidence, so I think the IOC has failed in its leadership by not giving them a stronger framework with which to work.

They’ve gone with a scientifically bereft policy guideline and now sports will have to sort it out as this problem continues to grow.

Harper: The new IOC framework doesn’t provide enough substance, and certainly saying there shouldn’t be any restrictions until we have data, I don’t agree with that.

The IOC has prioritised inclusion, and I think inclusion is valuable, but again I would prefer the example of World Athletics, which is taking a more proactive stance, where they have said this is what we think we should do, we know it won’t work for all sports, but they are showing more leadership. In that sense it is possible to criticise the IOC.

Trying to create transgender policy is extremely challenging – you can criticise anybody for any policy that they have.

I certainly wish the IOC had done something other than what they did do, but it’s also true that both trans women and cis women have been put in a very difficult spot.

Sports governing bodies have also been put in a very difficult spot, and we should have some sympathy too.

What about the impact on transgender athletes?

Harper: I have met Emily Bridges – she’s 21 years old. She’s a world-class athlete. She should be free to do her sport and go to uni and live a normal life.

Now, she has the weight of the world on her shoulders, people are debating her all across the world, she’s been called some horrible things, there are some people who think she’s a heroine, and she just wants to ride her bike, go to uni, have friends.

She’s been devastated by this, and it’s heartbreaking to see what has happened to Emily.

It is really hard to see on a personal level somebody you know go through this level of suffering.

Tucker: What happened with the Bridges situation is a perfect illustration of the mess that sports have got themselves into and the reason that that exists is because, simply put, they haven’t listened to their own athletes.

I’m uncomfortable at times with the way the conversation goes when it starts to talk about individuals, and you’ll hear people saying that they’re deliberately cheating; they’re only identifying as women in order to win women’s sports or to get into changing rooms and so forth.

I find those kinds of – if you can even call them contributions to the debate – very uncomfortable and sometimes unpleasant. I wish that we could have this debate impersonally and without ever having to refer to individuals, because I think it is unfair on them and they’re abiding by the rules. The rules are the problem.

Now I don’t know that that’s enough to exempt individuals from the consequences of their own decisions and so forth, but I do think that we could have this conversation without needing to have exhibit A, exhibit B, exhibit C. It’s quite invasive and sometimes unpleasant.

I would really encourage people to debate the policy as opposed to the person.

What happens next?

Tucker: The reason this is so controversial is that there isn’t an ideal scenario where the ideal is defined as keeping everyone equally happy.

The reality is that you cannot restore fairness by lowering testosterone, so therefore you either have to have fairness by exclusion of trans women or you must accept a degree of unfairness through the inclusion of trans women.

Sports will have to make that difficult decision.

Unfortunately, sports leaders will have to say: do they want inclusion of trans women or do they want to protect the women’s category and therefore by necessity have to exclude trans women?

I can’t see a compromise solution. It has to be a choice and I think some sports have leaned that way.

Harper: The world has, over the last few hundred years, moved towards being more inclusive of minorities, whether that’s people of colour, LGBT people… In many ways we’ve come to understand that human beings share more than we differ.

So trying to be accommodating of the differences that humans have is a valuable process and one that I hope will continue.

One of the things is that many people don’t actually know any trans people, so this idea that trans women are men who think they’re women, that isn’t true. The gender identity that trans people have is such an essential part of our being that there is no way to separate that out.

Trans people are who we say we are. I am a woman who was born with different physiology than other women, and so my place, I believe, should be with other women.

That is something that an inclusive society would recognise. I do admit that when it comes to sports, things are a little more complicated.

What I would suggest is that it is impossible to maximise inclusion, maximise fairness, maximise safety in sport, all three of those, without some impact on the other.

If we maximise inclusion, it does come at some cost to fairness and safety. But I think that we can come up with solutions which, while they may not maximise any one of these three parameters, comes pretty close to maximising all three, and that none of these three important parameters – inclusion, safety and fairness – are overly impacted.

Should factors other than science be considered?

The Equality Act 2010 legally protects people from discrimination that is based on gender identity. However, there are exemptions when it comes to sport.

Section 195 of the Act, which deals with sport, says it is lawful to restrict the participation of transgender people from sporting competitions where physical strength, stamina or physique are important factors in deciding who wins, but the restriction can only be done to ensure the competition is fair or the other competitors are safe.

Dr Seema Patel is a senior lecturer in law at Nottingham Law School. She has a PhD in discrimination in sport, looking at the regulatory balance between inclusion and exclusion in competitive sport. She has almost 20 years’ expertise on the specific topic of transgender athletes and the law.

Patel: The focus thus far in this debate has been on the science, with less attention from my perspective on the legal, regulatory and human rights factors that really should be balanced within this debate as well.

When sports bodies are determining eligibility rules for gender diverse athletes such as trans athletes or athletes with natural sex variations, there cannot be just one consideration. It cannot just come from science and medicine.

There has to be a discussion from sociologists, from lawyers, from academics in the field of law and regulation, from the athletes themselves, to determine what’s best and how to move forward.

The current guidance from the IOC is groundbreaking. It’s the first time an international governing body has sought to ensure that everyone can participate in sport irrespective of their gender identity or sex variations over the history of eligibility rules for gender diverse athletes.

We have never had guidance that focuses on principles of inclusion – no presumption of advantage, dignity and respect for athletes.

The focus on human rights is necessary to ensure a balance; it shouldn’t be one or the other. They should be considered alongside each other to ensure the best possible outcome for athletes.

So I do think it’s welcome guidance, but the test will be whether governing bodies actively engage and adopt those objectives set out in the guidance, which currently doesn’t seem to be the case.

The science should not be the turning factor on this. It has to be also a matter of law, regulation, sociology, athletes and human rights ultimately, because these athletes are individuals.

There’s been some coverage of the exemption clause that exists within the Equality Act and the Gender Recognition Act, which permits a sex segregation on the basis of physiology and biology.

But it’s worth noting that that exemption is highly problematic. So to base an argument on that for the exclusion of trans athletes, I think is highly controversial and something that the government would need to look at in terms of any amendments to future legislation in this area.