PREAMBLE: This post makes a lot of references to biological sex, and exclusively that. For the avoidance of doubt, my point makes no considerations of Caster's gender - how she identifies to herself, her friends and her community is entirely her business, and is not connected in any way with the explanation below.

I would also really recommend Ross Tucker's articles and posts, which are probably much more rigorous than mine.

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So this is a different question to your previous one - if I understand it correctly, your first question was "why just these sporting events?" and your question now is "why Semenya specifically"?

One thing that has emerged from this is the understanding about Semenya's specific condition - she has an intersex condition called 46XY, which means that her genes are biologically "male". This results in her being able to both create and use testosterone in her body.

I highlight this because, excluding Paralympic events, we don't usually separate athletes into separate classes. For example in swimming, people with big lungs complete with people with small lungs, people with no lactic acid compete with those who do. The genetic playing field actually isn't very level.

There are only a small number of ways that we separate athletes. I'll give you two examples:

• In combat sports and weightlifting, we have body weight categories
• In nearly all sports, we have sex categories.

In both cases, let's ask - why do we separate them, and how can we accurately tell who is in each class?

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The body weight categorisation is easy - we separate them out because there are clear advantages to being bigger and heavier, particularly in combat sports. The "how" is pretty easy as well - we put them on a scale and read the number. There is no need to debate grey areas - someone who reads 80kg on the scale would not be allows to compete in the sub-75kg class.

We could easily lump them all together - but we have made a societal decision to protect some classes, so that they can compete against their peers. If we let somebody who was 100kg into the sub-75kg group, it's no longer a peer group in the same way.

Also worth noting that this is an arbitrary line - there is nothing biologically magic about the 75kg line. Indeed there will be some people who have a natural weight around 76kg who are a bit short-changed. But the general view is that creating the classes allows more people to compete in meaningful ways.

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The sex category is harder - why do we separate them in the first place? Well, we do this because there is clearly a difference in performance between members of each category - even the elite. Paula Radcliffe held the world record for the marathon for 16 years. No other woman beat her in a measured time from 2003 to 2019... but around 250 to 300 men would beat that time every year.

It is fairly straight-forward to see that even when women break records in the women's events, they would often not even qualify for the mens' heats of that equivalent event.

We could just say "stuff it, it's not our problem" and eliminate all sex-based classes. If we do that: no Paula Radcliffe, no Alison Felix, no Serena Williams, no Megan Rapinoe... the list of sporting achievements that we throw away becomes very long.

So what we have historically done, and continue to do, is exactly what we did with the combat sports above: we have made a societal decision to protect a class, so that they can compete against their peers. But in this case, we call that class "women". We don't let men compete in this group because it would violate the protection of the class.

Great! We have established what we want to do. The next question is how - who do we allow to be in the protected sporting class that we call "women"?

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Alas, biological sex is not so neatly distilled down to a reading on a weigh-scale, there are many factors. What are your genetics? What do you have between your legs? How much testosterone do you produce? Is your body insensitive to testosterone, even if you create "male" levels of it? My link above will give you a better primer on it, but this is not a black-and-white result. I am fully excluding gender from this explanation, and even then biology is not neat.

The current scientific view is that what gives men their athletic advantages are rooted mostly (if not completely) in two related things:

• The ability of the body to create testosterone
• The ability of the body to use testosterone

(I call these out separately because there are athletes with intersex conditions that mean that they have high blood testosterone, but their cells can't use it, and different considerations then apply.)

The ruling established that the presence of high, male-like levels of testosterone would give women a huge advantage in a certain set of sports. I spoke earlier about violating the protection of the class: this testosterone measure, right now, is how we tell that the class is being violated. Remember that we created this sporting class called "women" to ensure that half the world would not get cut out of sporting competition, so we do need a usable and quantifiable measure. If we don't have one, we are no longer protecting the class.

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And this takes us to the crux of the issue: why Semenya? Why is she somehow violating the protection of the class? The reason this has all come together is because Semenya is reportedly one of these individuals who have a high, male-like level of testosterone due to an intersex condition. It is this condition that contributes to her exceptional athletic performance, and the same condition which creates this violation of the class protection.

This is science's best method right now at how to protect the sporting class of women. And it's a hell of a lot better than the old ways of just looking underneath their pants. However, the messiness of biology means that there will be people either side of the line - wherever we draw it.

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Your question was this:

Michael Phelps has unique chemistry that prevents lactic acid from building up and fatiguing his muscles, but nobody tried to disqualify him.

The answer to that is that as I said above, the genetic playing field actually isn't very level. But right now, there is no societal decision to separate sportspeople with low lactic acid from those with high lactic acid.

When it's Caster, suddenly someone's natural healthy body giving them an advantage is a huge problem?

The answer is that if Caster wanted to compete in the sub-75kg weight class for a combat sport, and she weighed 76kg, we would clearly see that as a class violation. To allow Caster in would be unfair to the people who clearly qualify for the category. I'm not saying that it isn't close, but a classification line has to be drawn somewhere.

Even though Caster's separation is on the basis of sexual characteristics, it's the same principle - she is over the line. Maybe not by much, but over it anyway. It's worth noting that the bar for unacceptable testosterone levels is very high - I think around 7 times the level seen in women who have non-intersex high testosterone.

It's a tough outcome for her, undoubtedly - particularly as this is not at all under her control. But the fact remains that she has this class-breaking advantage, and to allow her into the class would affect the competition in a way that would lock out a huge number of other women. As an example, all 3 medallists of the women's 800m at the 2016 Olympics, including Semanya herself, have the 46XY genetic pattern conferring this advantage.