It is unusual for chess to make front page news. World champion Magnus Carlsen went into the recent Sinquefield Cup with a 53-game unbeaten streak, but unfortunately it was not his chess skill that has been the focus. Instead, it is his withdrawal from the tournament after an upset win by 19-year-old Hans Niemann – widely interpreted as an accusation that Niemann secured his victory by cheating.

Whether Niemann in fact cheated may never be known, and the tournament organisers have said they have no evidence of unfairness. However, Niemann’s ensuing statements have provided an object lesson in the importance of maintaining credibility – and how easily it can be lost.

The first blow for Niemann was in his post-game interviews, where his explanations of his moves were regarded by many chess masters as surprisingly unsophisticated – and prone to errors – for a player who had just defeated the world champion. That is not itself surprising – he was doubtless exhausted after his game – but the reaction from spectators provides a clear lesson for expert witnesses: be prepared to explain every part of your workings, because anything you cannot explain can provoke suspicion.

The second blow came when claims surfaced that Niemann had previously cheated in online games, which was confirmed by his admission to have previously cheated when he was 12 and 16 years old. This is what the courts would refer to as “similar fact” evidence, or evidence of propensity. The argument is simple and alluring: he did something similar before, so he probably did it this time. This type of evidence is carefully controlled by the courts because it can be highly prejudicial while often having limited relevance. The Niemann case is a striking example: does the fact that someone admitted to cheating online as a child really help us to determine whether they cheated in person as an adult? Probably not. And yet that is exactly the assumption that was widely made.

The third and (so far) final blow was a 72-page report published last week by chess.com, which alleges that Niemann cheated in more than 100 online games, including as recently as 2020, contradicting his statements. The report is a perfect demonstration of the importance of credibility. There is no way to verify the claims now made by chess.com – neither their data, nor their methodology are public. Where people believe the report, it is purely because it is seen as independent and scientific, and therefore credible. When they disbelieve Niemann it is because his credibility has been damaged.

Lawyers should pay attention to this episode. The lesson is not about what actually happened. Only one person knows that. Instead, it is about what people were prepared to believe. Witnesses called to give evidence must be alive to the fact that their evidence is only as strong as their credibility. Consistency of the witness’ evidence with what is agreed or clearly shown by other evidence, along with the internal consistency of their evidence, is imperative, especially in cases where supporting documentary evidence is scant and/or there is an absence of other corroborating witnesses.