“”I'm an expert witness. Why? Because I say I am.
|—Satan, The Garden of Allah; Don Henley|
Sometimes, they do good work… sometimes, they don't. In general an "expert" witness may be brought in as some kind of spurious argument from authority and people will accept what they say regardless of whether they might be wrong. This is irrelevant of whose "side" they are actually arguing for. For example, in the murder trial of Sally Clarke, who was accused of killing both her children, an "expert" witness was brought in to state that the odds of two cot deaths are 1 in 73 million. this seemed to imply that the odds of her being guilty of murder were 72,999,999 out of 73 million and people bought it hook, line and sinker because it was from an expert. It seemed beyond question. The trouble is, even such experts are prone to cognitive biases and since the general public aren't doctors or scientists trained to keep an eye on such things, they didn't realise this. In the Sally Clarke case, the 1 in 73 million figure was irrelevant in the case, as the two children were already dead. Only the relative odds of double murder vs double cot death were relevant, and the expert witness (and the judge, jury and defence team) completely missed this blatant misuse of statistics.