Law of evidence

As a general proposition, the rule in relation to criminal trials is that opinion evidence is inadmissible; a witness may only testify with regard to facts which he actually perceived and not of inferences drawn from those perceptions. In a criminal trial, the drawing of inferences is for the jury or other tribunal of fact and it is up to them to reach an opinion in any given matter, not the witness. Traditionally, the reason for this is that an opinion on its own is seen to have little, if any, probative weight and as such, if admitted, would usurp and undermine the very function of the trier of fact.

In civil trials, the rule has been largely abrogated, however this is probably due to the fact that the vast majority of civil cases are not heard in front of a jury; it is felt that the trained judge is more able to determine between fact and mere opinion; or at least as to how much weight should be attached to such. It is arguable that rigid adherence to the rule in criminal trials can lead to the exclusion of potentially useful material, which could at times be detrimental to both the prosecution and the defence.

However, if the recent proposals of the current Home Secretary gain favour and further restrictions on the defendant's right to elect trial by jury are adopted, it could be that more Stipendiary magistrates will be employed and this will lead to a course more in line with civil procedure. Moreover, there is an underlying policy behind the rule that is to ensure that a convincingly presented lay opinion does not unduly persuade or seduce the trier of fact into an easy acceptance of it; albeit that they do have the option of disregarding opinion evidence, when considered in light of other evidence that is presented.

There are two major exceptions to the general rule of excluding opinion evidence. Firstly, with regard to non experts. A statement of opinion on any matter not calling for expertise, if made by a witness as a way of conveying relevant facts personally perceived by him, is admissible as evidence of what he perceived,1 for example, the identification or estimated speed of a car. The non-expert is not permitted however, to testify as to the ultimate issue or as to those issues which are within the competence of the trier of fact.

The second exception is in cases where certain matters call for special skill or knowledge which the judge or jury do not possess and with which they will need assistance. On such matters, the opinion evidence of so called 'expert' witnesses may be admissible. 2 '[… the opinion of scientific men upon proven facts may be given by],' "men of science within their own science". 3 The common law has always accepted expert evidence; for example, in 1782,4 the opinion of an engineer was admitted on the issue of whether an embankment had caused the silting up of a harbour.

The opinions of experts however, must be restricted to their field of expertise. If the issue does not require specialised knowledge and is within the competence of the trier of fact, the witnesses opinion will be inadmissible. This has, in the past, produced some rather peculiar results, particularly in relation to psychiatric and psychological evidence. 5 Examples of where the judge or jury may currently require assistance include scientific, medical or forensic evidence.

It is apparent that this type of evidence is often regarded as admissible because there is a real chance that a jury would not be able to draw the appropriate inferences and form proper opinions from the facts which require expert opinion. 6 The weight to be given to such evidence however, remains a matter for the jury. It ought to be borne in mind that situations do change and in some fields, expert testimony may become unnecessary as it is felt that perhaps judicial knowledge may be a more appropriate approach or that through the passage of time, the matter may have come within the competence of ordinary people.

i. e. the jurors. As new fields of knowledge emerge, the courts must be satisfied that such evidence should indeed be given. In their time, both fingerprinting and DNA testing have been looked on with suspicion; and not without good reason when a brief study is made of some of their 'scientific' predecessors. Throughout history there has been a ceaseless search for a suitable means of identifying criminals which has led to some of the most ingenious and it has to be said, preposterous propositions.

Some early criminologists and researchers, like Cesare Lombroso, believed that there was such a person as 'Criminal Man', whose physiological features would reveal his predisposition to crime; "Humped backs, which are rare in Murderers, are more frequent among Ravishers, Forgers and Incendiaries; … Thieves often have a grey iris and in stature … are inferior to Highwaymen and Murderers. Scanty beards are rare". 7

In the eighteenth and nineteenth Centuries, equally exotic claims were made for Physiognomy,8 and Phrenologists were convinced that criminal tendencies could be read in the bumps of the head. 9 Modern forensic science has made huge advances over the last two decades, with the most significant techniques probably being fingerprinting, DNA profiling, facial mapping, and with so many recent miscarriages of justice, Electrostatic Detection Apparatus, (ESDA). These techniques have highlighted the fact that far less emphasis can be placed on the reliability of confession evidence and witness identification.

See for example the witness confusion that arose over compiling an E-Fit of the suspect in the recent killing of television presenter, Jill Dando. It is clear that in the future, convictions will be made to rely more and more on indisputable forensic proof. In English law, the particular specialist field need not be generally accepted before expert testimony is admissible but it must be sufficiently established to be reliable. The telephone is now in such common use that it has become an everyday tool of the criminal just as it has for the rest of us.

The New York State Police say that as an aid to identification the 'voiceprint' may prove to be as valuable as the fingerprint and are setting up what they call a 'Talking Rogues Gallery,' where voice samples are routinely taken of felons convicted of appropriate crimes; such as ransoms, bomb threats and obscene calls. 10 In R v. Robb,11 techniques of voice identification were in issue. The evidence consisted of recordings of telephone ransom demands compared with control recordings of the accused's voice. The court accepted that voice identification or 'voiceprinting,' was a field that called for expert testimony.

The witness concluded that there was no difference between the two examples; however, the 'expert's' technique simply involved listening carefully to the two tapes! This was neither scientific or a respected approach within the field of phonetics, but the court took a pragmatic approach and treated any doubts about the accuracy of the technique as a question of weight for the jury. "Studies show that … the accuracy of [Health Professionals] judgements does not necessarily surpass that of lay persons, thus raising substantial doubt that [they] meet the legal standards for expertise".

12 In contrast to the examples above, psychological profiling and the polygraph have yet to be accepted as valid evidence. With regard to psychiatric evidence, there is much controversy surrounding the very existence of certain syndromes and disorders such as battered woman's syndrome,13 rape trauma syndrome, and so on; and while it is commonly accepted that labelling and treatment of such conditions may have a beneficial therapeutic value, it is argued that such scientifically unproved concepts may be unsuitable for courtroom presentation.

 With regard to the polygraph; as an instrument of scientific accuracy, it is flawed in almost all respects – not least in that it cannot determine whether a person is telling a lie – only whether there has been a change in their metabolism. It seems that this particular device is destined to remain outside of the English courtroom and in the world of popular detective fiction.

Before an expert witness can give opinion evidence relating to any particular issue, he or she must first establish their credentials and ultimately it is a matter of fact for the judge to decide whether the witness is indeed an expert. This may depend on whether or not the witness had undergone formal study giving the requisite expertise in that area or whether his experience and or training is such as to make him an expert.