THE recent treble murder trial should give rise to reflection about the role of the expert witness in court and in particular in what sense psychiatrists can be deemed expert witnesses.

The trial in the United States in 1982 of John Hinckley for the attempted assassination of President Ronald Reagan is relevant in this context. After months of examination by psychiatrists, those for the defence claimed Mr Hinckley suffered from schizophrenia; those for the prosecution denied this and stated that Hinckley was not suffering from schizophrenia but from "personality problems". The jury acquitted Hinckley on the ground of insanity and he was committed to a secure hospital in Washington DC. Once he had been committed, it was open to his lawyers to argue that he should be released on the grounds of the expert prosecution testimony throughout the trial. The prosecution lawyers, eager to ensure his continued hospitalisation, now turned to the expert evidence of the defence witnesses in proving he was severely mentally ill. This was expert testimony and the legal system turned on its head.

The issues raised by both these trials are not inconsiderable. Courts call expert witnesses to guide them in matters of which they themselves and the jury have no expert knowledge. It was set out long ago that expert evidence is the opinion of scientific men upon proven facts within their own science. This then poses the question, in the present context, as to whether psychiatrists are people of science and whether the discipline in which they are trained and which they practise can be correctly regarded as a science. It also asks whether the courts, because of an adversarial judicial system, induce, encourage or oblige them to give opinion on matters outside their science and on which they have no greater skill to speak than the average person.

Psychiatry has been striving hard over many years to establish its scientific credentials, not least in deciding whether a person suffers from mental illness and the nature of that illness. To this end, psychiatrists are expected to familiarise themselves with and be competent in the diagnosis and classification of mental illnesses in accordance with increasingly strict but yet not completely scientific criteria. The limitations imposed upon them in this exercise are several, but one may be mentioned as of primary importance and that is whether the information given to them by a person is truthful (the defendant in this recent case freely admitted some of the information he supplied to examining psychiatrists was untruthful).

Thus, can a psychiatrist arrive at an opinion as to an individual's subjective state supplemented by his observation of the person's behaviour - and of the reporting of his speech and behaviour by third persons and attempt to arrive at a diagnosis. In most psychiatric conditions this is all the psychiatrist has to work on; there are no objective tests such as those which, in physical illness, can reveal that a person is suffering from diabetes or raised blood pressure.

IT IS common knowledge that psychiatrists differ in the weight they put on their perception of an individual's description of his subjective experience and of his behaviour and, not surprisingly, there are often diagnostic differences between psychiatrists dealing with the same case. Even under the strictest of research conditions, where highly trained clinicians use the same standardised diagnostic techniques, variations between psychiatrists in diagnosis occur.

Taking this into account, it is questionable whether psychiatry has established the criteria necessary for it to be declared a science, having regard, inter alia, to its lack of consistency and predictability.

This does not mean psychiatrists may not be regarded as expert witnesses - but tee courts must recognise the limitations of their expertise. Thus, it is perfectly appropriate for psychiatrists to describe to the court the symptoms exhibited by an accused person and to bring them together in a diagnosis. What is not acceptable is that they should be asked to give an opinion on matters on which they have no expertise.

The current position is that to establish the defence of insanity it must be shown the defendant, while performing the criminal act, "was labouring under such a defect of reason, from disease of the mind, as not to know the nature and quality of the act he was doing, or if he did know it, that he did not know what he was doing was wrong". It is for the expert witness to provide the court - with the clinical evidence that there was or was not "a defect of reason from disease of the mind". That should be the end of it as far as the psychiatrist is concerned.

But psychiatrists (either spontaneously or as a result of examination and cross examination) now enter an area for which they have no qualifications to make judgment - that is, as to whether the defendant knew what he was doing was wrong and whether he could resist doing it. It is strictly legitimate for a psychiatric expert to say the accused was hearing the Devil's voice telling him to kill the deceased and to express an opinion as to whether he believed what the accused tells him in this regard is true. It is unfair for the psychiatric expert to be asked whether the accused could resist the command. But the psychiatric expert witness is often regarded by the court not just as competent in his own expertise of describing the mental state of an accused person and giving a diagnosis to it, but of being skilled to decide upon matters of morality on which he has no more expertise than the common citizen.

THE rules which currently decide in law that one is either insane or sane are out of step with contemporary psychiatric thought, which has abandoned the sane/insane dichotomy and recognises intermediate states. Unfortunately, expert witnesses are no more likely to agree on these states than they are on the polarities. But the matter is, in a judicial sense, important because on it hinges whether an individual will spend a substantial apart of his life in prison or in a secure hospital setting and it is inappropriate that such decisions, irreversible as they mostly are, are arrived at in such a fashion.

In short, the present conduct of criminal prosecutions in murder charges, to the extent they call on expert evidence, is unsatisfactory: it is so on two principal grounds. The first is that the adversarial system polarises expert opinion and diminishes its value to the court. Secondly, it invites experts in psychiatry to out strip their competence and become experts on morality.

The current system does little for an accused person and does nothing for the discipline of psychiatry which comes to be seen as confused, muddling, unscientific and venal. There surely must be a role for the Law Reform Commission in these muddied waters.