Expert witnesses aren't what they seem - and I should know

By Dr Theodore Dalrymple
(Filed: 02/02/2003)

When medical experts open their mouths the truth is supposed to emerge, especially in court. However, experts may disagree among themselves; moreover, like every other type of human being, they sometimes have feet of clay. In the case of Sally Clark, the solicitor wrongly convicted of the murder of her two young children, the evidence of three medical experts helped to convict her. One of these experts was changeable in his opinions, another concealed or forgot to mention facts that did not support his conclusions, and a third relied upon erroneous statistical reasoning. And yet they were all eminent men, in one case world famous in his field: a Home Office pathologist, Dr Alan Williams; a professor of forensic pathology, Michael Green; and a professor of paediatrics, Sir Roy Meadow.

How does such a lamentable thing happen? What are the perils and temptations of being an expert witness? Occasionally called to court myself in that capacity, I have observed the perils and felt the temptations.

The expert in our adversarial system is employed by one side or the other, by the prosecution or the defence in criminal cases, by the plaintiff or the defendant in civil cases. It is a natural human desire to want to please, and this desire exerts a subtle psychological pressure on the expert to come up with something pleasing to the side that employs him.

Moreover, it is possible to make quite a lot of money from giving expert evidence. Daily fees of #1,000 plus expenses are not unheard of, and written reports frequently cost as much again. In complicated cases, experts can earn #10,000 or more. Clearly, an incentive exists to become known as the expert of last resort: the one who will always be able to come up with something.

Thus medical experts in criminal cases may become known as being generally favourable to the prosecution or to the defence - more often the defence, because it pays more generously than the prosecution, and because outlandish theories are of more use to the defence, since it has only to sew doubt in minds, not produce certainty in them.

Once an expert has produced his written report, the lawyers may ask him to alter it to produce an impression more favourable to the case they are presenting. It is far from unknown for lawyers to ask medical experts to change the whole meaning of their reports, either by addition or subtraction, and if the expert has a weak personality he may agree to change much more than he should. He then commits himself to something that he knows is wrong: he is trapped.

Once the expert has taken a position, he feels compelled by a sense of pride to defend it, irrespective of its flaws and shortcomings. He thus resorts to sophistry to back up his position: like the lawyer, he becomes more interested in a good argument that serves his purpose than in the truth. I have seen experts of world fame defend the indefensible in the witness box, merely because their amour propre will not allow them acknowledge the validity of a point against their own opinion. If the cross-examining lawyer is any good, such experts end up looking like fools; but if he is no good (and barrister, too, may have feet of clay), their flawed opinions go by default.

Experts, being only human, sometimes let themselves be influenced by extraneous considerations. When I am sent the papers in a criminal case by the defence lawyers (or for that matter by the prosecution), my first reaction on reading them is sometimes "This bastard should go to prison for ever". But I am not being asked about what I consider a man's just deserts: I am being asked a technical question.

Sometimes objectivity compels one to cook a decent man's goose, or help a scoundrel get away with it. It is not always easy, though, to cleanse one's mind of prejudices. Recently, I saw an allegedly expert report that was little more than a lengthy diatribe against the character of the person about whom it was written.

Some experts are crusaders: they are so convinced of a general truth that they have difficulty in descending to the particular case before them. Something like this might have happened in the Sally Clark case to Professor Sir Roy Meadow, the man who discovered Munchausen Syndrome by Proxy, the pattern of behaviour in which a person in charge of a child (usually the mother) deliberately produces symptoms and signs of illness in that child. Prof Meadow is one of the world's greatest experts on the subject of child abuse, of which he has seen so much that unexplained deaths, such as those of Sally Clark's babies, must inevitably raise suspicions in his mind.

It was this, no doubt, that led him to use the patently false argument in this case that if the chances of a cot death occurring in a middle class family were one in 8,543, then the chances of two cot deaths in a middle class family were one in 8,543 squared, that is to say the now notorious one in 73 million.

This would have been true if cot deaths were purely random events: but they are not, and therefore the figure is nonsense. Prof Meadow's visceral (and laudable) desire to protect children from the terrible abuse he had so often seen probably got the better of him and his reputation, and his confident presentation of spectacular statistics must have dazzled the jury.

It is also a mistake to underestimate the influence of incompetence in human affairs. Experts have to have knowledge and experience; they do not have to have common sense. I have seen expert opinions expressed in reports that were staggering in their absurdity or self-contradiction. Moreover, in general, experts who are world famous are sometimes the worst offenders. They can be deceived by their own eminence. They can come to believe that what they say must be true because it is they who have said it; they therefore do not do the detailed and time-consuming work that is necessary to produce a really valid opinion.

I don't want to suggest, however, that the medical experts who appear in British courts are mostly knaves or fools. In most cases, at least in my experience, medical experts do their job conscientiously, and we have not yet reached the level of absurdity displayed, for example, in the Louise Woodward trial in America (Miss Woodward being the British nanny accused of the murder of the baby in her care), in which the defence medical experts blatantly contradicted the authoritative textbooks - which they themselves had written. However, at $900 an hour, most people are prepared to say anything, and risk a little embarrassment in cross-examination before moving on to the next, equally lucrative case. This has not yet happened in Britain, and I hope it never does.