The South African’s call for unity stood out amid all the arguments, but it is not only Cas’s verdict that has been criticised

On the opening morning of the sporting trial of the century, Caster Semenya ran unnoticed through Lausanne’s old town to clear her mind. Then, knowing her career was potentially on the line, she stood up at the court of arbitration for sport (Cas) and made the speech of her life. “When I was 18 years old, I won gold at the 2009 world championships in Berlin,” she said. “It should have been a great moment. But sadly it was tarnished.”

How could it not be when a fellow 800m finalist, the Italian Elisa Cusma, had told reporters: “For me, she’s not a woman, she’s a man.” Or when Semenya then had to take medication to reduce her testosterone in order to compete for years afterwards? No wonder she told Cas that she was relieved when the IAAF’s previous hyperandrogenism rules had been overturned in 2015.

Cas tried to provide a clear verdict on Caster Semenya but left a tangled mess | Andy Bull Read more

And now she was resuming hostilities against a familiar enemy, the IAAF, over new rules requiring athletes with differences in sex development (DSDs) to take hormone suppressants to compete internationally at events between 400m and a mile. Semenya, the reigning Olympic 800m champion, insisted these rules discriminate against her. On Wednesday Cas decided she was right. There was discrimination. However there was a brutal sting in the tail. By a 2-1 majority it ruled that the policy was also “necessary, reasonable and proportionate” to ensure fair competition in women’s sport.

The case is widely regarded as being the most complex and contentious one in Cas’s 35-year history. However, apart from a short statement, little has come out about the drama that went on behind closed doors. But the Guardian has spoken to more than half a dozen eyewitnesses to gain an unparalleled insight into a case that touched on so many areas – including science and sociology, gender and genetics, health issues and medical ethics – and also split sport and society right down the middle.

One expert said: 'The judges are impressive people but 35 different scientific opinions is too much'

Given the extraordinarily high stakes it was no surprise that each side came tooled up with many world-leading experts. Semenya even had two legal teams, one from Canada and the other from South Africa, while more than 200,000 words of evidence were filed before the case even began.

In the months beforehand, Semenya’s team had been confident. After all, they had two big trump cards. First, when Cas heard the case of the Indian sprinter Dutee Chand in 2015 it put the onus on the IAAF to show exactly how much high testosterone helped DSD athletes. Second, they knew the evidence the IAAF had subsequently published in the British Journal of Sports Medicine contained a number of errors.

Facebook Twitter Pinterest Caster Semenya arrives with her lawyer, Gregory Nott, at the first day of her Cas hearing in Lausanne. Photograph: Laurent Gilliéron/EPA

However, in the lead-up to the tribunal, the IAAF surprised Semenya’s team by re-framing the debate. Its lawyers went back to the Chand verdict, where the bioethicist Katrina Karkazis had proposed six characteristics to determine sex – including testes and phenotype – and said: ‘OK, let’s look at how this applies to DSD athletes.’ Some of Semenya’s team believe the IAAF was saying that DSDs were biological males. That is disputed by athletics’ governing body. All sides, however, recognised the cleverness of the argument – and, tellingly, that it could have the power to sway Cas.

All along it has insisted DSD athletes have to reduce their testosterone to five nanomoles per litre to compete internationally between the 400m and a mile to ensure fair competition.

How the Caster Semenya controversy has unfolded since 2009 – a timeline Read more

Semenya’s side fought back, insisting that many of the characteristics the IAAF had identified were not clearcut. They also disputed whether high testosterone necessarily always made a big difference. It had long been part of the IAAF case that DSD athletes have testosterone more in the normal adult male range, which is from 7.7 to 29.4 nmol/L, rather than the female range (0.12‑1.79 nmol/L). However, as one of Semenya’s experts told the court, if a DSD woman lacks the hormone DHT she would not get all the benefits of testosterone. “To say that a DSD person is the same as a male athlete is a leap beyond the evidence,” they added.

Semenya’s case was that her genetic advantages were no different from those of other exceptional athletes – such as the Finnish cross-country skier Eero Mäntyranta, who was born with a genetic mutation that increased his haemoglobin level to about 50%, or Usain Bolt with his fast-twitch fibres. However – as with so many other arguments in the case – there was an obvious counterpoint. Sports competitions do not have categories for athletes with low haemoglobin levels or slow-twitch fibres. They do, however, have categories to protect women’s sports – otherwise men would win every Olympic medal and societies would be poorer for it as a result.

Q&A Caster Semenya's discrimination case explained Show Hide The IAAF have introduced new rules requiring female athletes with differences in sexual development (DSDs) to lower their testosterone if they wanted to compete in events between 400m and a mile. The IAAF says the rules are needed to protect female sport, as DSD athletes like Semenya have an advantage. Semenya challenged the rules in the court of arbitration for sport (Cas), claiming they discriminate against her. Her legal team have said “her genetic gift should be celebrated”, and argue that taking hormone suppressants to reduce testosterone is unethical and a health risk. Cas's verdict means Semenya must take medication to reduce her testosterone in order to keep running on the international stage. One expert believes she will run the 800m around seven seconds slower if she reduces her testosterone levels.

Science was one key battleground. Whether the IAAF had discriminated against Semenya another. Semenya’s lawyers made a powerful case that she and other DSD athletes had always been recognised as female. They had been born female, grown up as girls, and became women, competing in female sport along the way. Yet under the IAAF’s new rules a barrier was being hoisted only when they got to the top of their profession. As a result they were faced with a vicious choice: either having hormone-suppressing treatment, robbing them of their abilities, or giving up, robbing them of their dreams. How could that be right?

The IAAF, meanwhile, insisted its rules were proportional and placed far fewer restrictions on athletes with DSD than under the old hyperandrogenism rule, which Cas had kicked out in 2015. After all, the ruling applied only to IAAF Diamond League, Olympics and world championships and four events – the 400m, 800m, 1500m and the mile.

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Towards the end of the week, both sides moved on to the health, ethical, and psychological issues. Again there was wide disagreement. Could having internal testes lead to a greater chance of developing cancer, as some argued? Or could removing the testes effectively plunge someone into menopause overnight? Was it really ethically right to compel athletes to take medication – especially when no one could be sure of the side-effects? One of the more compelling arguments from Semenya’s team, meanwhile, came when they discussed the psychological impact on many DSD people who were told they were males not females.

Not everyone was impressed with the way the case was dealt with at the Cas. As one expert put it to the Guardian: “There are some really important questions of science, some of which have answers, many of them that don’t. And you had adversarial parties – they want the science they want.

“A lot of the back and forth of the science would have best taken care of well in advance of having an arbitration,” they added. “The judges are all impressive people but it is too much to sort out 12 different scientific disciplines with 35 opinions.”

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There were other criticisms of Cas too. Some said the proceedings were not nearly as systematic and rational as an outsider might expect. Others said that topics jumped around too much. “Lawyers from both sides introduced red herrings at various points,” added one source. “The cross-examination of the experts was more than a little unsatisfactory. It was often more about de-legitimising the expert than getting to the heart of the matter.”

However one thing everyone agreed on was just how impressive Semenya was throughout. At the end of the case, she spoke again. And while reiterating some of the themes of her opening address, she also made an appeal for unity, saying that this case need not be a divisive issue. Those wishes, however, now seem more of a pipe dream than ever.