7. Opinion evidence

There are two reasons for excluding opinion. It is usually irrelevant, because the court wants the facts. And it usurps the function of the judge or the jury. Hearsay is an exclusionary rule of its own, mentioned where it bites. It is not given a chapter of its own.

An expert may give an opinion on a matter outside the court's own competence. The fields are science, medicine, architecture, engineering, handwriting, ballistics, mental condition and foreign law. Folkes v Chadd is the old illustration. The harbour at Wells was silting up. Lord Mansfield allowed an engineer to give his opinion on the cause. The cause was a question of science, not a fact the witnesses could simply describe.

The expert does not decide the case. In R v Chard a prison doctor was not allowed to say that the accused had not formed the intention for murder, where there was no issue of insanity or diminished responsibility. Intention was the jury's question. In R v Jeffries a detective's opinion of what drug lists meant was excluded. R v Anderson is the obscenity case: the statutory test is not handed over to an expert. R v Silverlock shows that the expertise can be acquired by study rather than by a profession. A solicitor who had made a hobby of handwriting was allowed to give an opinion on it. R v Oakley allowed an experienced traffic officer to give an opinion within that experience. The jury is not bound by the opinion.

Psychiatric evidence is required for insanity, non-insane automatism and diminished responsibility. It is not required, and often not allowed, for states of mind a jury already understands. R v Turner is the leading case. Turner killed his girlfriend and relied on provocation. A psychiatrist could say he was not mentally ill, and could describe a personality that was likely to be provoked. That evidence was excluded. Jurors do not need a psychiatrist to tell them how ordinary people react to being told of infidelity. R v Weightman followed Turner. R v Lowery, in the Privy Council, is the caution. Two men were charged with a sadistic murder and each blamed the other. Psychological evidence of their personalities was admitted. The case has not been followed and is not a general rule. R v Masih refused evidence that the accused was of low intelligence within the normal range. R v Reynolds refused evidence about an inability to distinguish reality from fantasy where there was no mental illness. The Canadian case R v Lupien the other way: psychiatric evidence of a man's reaction to a homosexual advance should have been admitted. R v Smith (Stanley), on automatism during sleep, is cited for the point that expert evidence is necessary where the condition is outside ordinary experience.

A non-expert may give an opinion where it is only a compendious way of stating facts he perceived. The list is handwriting, identity, age, voice, what a person is doing on a recording, bodily condition, illness, emotion, value, speed and distance. R v Davies draws the line. A bystander could say the car was going very fast. He could not say the driver was drunk and therefore unfit to drive. A witness may also describe his own condition: how much he had drunk, or that he felt well until he ate the food.

Communications between an expert and the solicitor who instructs him, for the purpose of the litigation, fall under legal professional privilege. The opinion, once it is put in evidence, does not.