7.5 Opinion Evidence & Expert Witnesses
Key Takeaways
- The general rule is that opinion evidence is inadmissible: witnesses give evidence of facts they perceived, and the tribunal of fact draws the inferences.
- A non-expert may state an opinion as a compendious way of describing perceived facts, such as speed, identity, handwriting, or that a person appeared drunk.
- Expert evidence is admissible only where the subject matter is outside the ordinary experience and knowledge of the tribunal of fact, following Turner.
- Competence to give expert evidence rests on skill and experience rather than formal qualification, as Silverlock establishes.
- An expert's overriding duty is to the court and overrides any obligation to the party instructing or paying them, and the weight of the evidence is always for the jury, which is not bound to accept even uncontradicted expert opinion.
7.5 Opinion Evidence & Expert Witnesses
The exclusion of opinion evidence protects the function of the tribunal of fact. Witnesses supply the facts they perceived; the jury (or bench) draws the inferences. The rule has two exceptions — one narrow and practical, the other structurally important.
1. The general rule
A witness may not give evidence of their opinion, belief or inference about a matter the tribunal has to decide. So a bystander cannot say "he was obviously trying to kill him"; they say what they saw and heard, and the jury decides intent.
2. The non-expert exception
A witness of fact may state an opinion where it is a compendious way of conveying perceived facts that could not sensibly be broken down. Established examples:
- The speed of a vehicle;
- The identity of a person or of handwriting the witness knows;
- Whether a person appeared drunk, angry, or unwell;
- The age of a person seen;
- The general condition of an object.
The limits are that the witness must have perceived the underlying facts, and the opinion must be one on which an ordinary person is competent to comment. Where the opinion depends on specialist knowledge, only an expert may give it.
3. Expert opinion evidence
Expert evidence is a genuine exception: an expert may give an opinion about matters the jury cannot assess without help. Four questions must be answered.
(a) Is the subject matter appropriate?
Turner [1975] is the governing authority: expert evidence is admissible only where the subject matter is outside the ordinary experience and knowledge of the jury. "Jurors do not need psychiatrists to tell them how ordinary folk who are not suffering from any mental illness are likely to react to the stresses and strains of life."
Admissible subjects therefore include: pathology, toxicology, DNA and other forensic science, fingerprints, ballistics, accident reconstruction, foreign law, handwriting comparison, the effects of mental illness or personality disorder, and the reliability of confessions by a suspect with a very low IQ or abnormal suggestibility. Inadmissible subjects include the credibility of a normal witness, and the likely behaviour of a person without any relevant disorder.
(b) Is the witness competent?
Silverlock [1894]: expertise may be acquired by study or experience, and formal qualification is not required. A solicitor who had studied handwriting as a hobby for ten years was competent to give handwriting evidence. The question is always whether the witness has acquired sufficient knowledge and skill in the relevant field, and it is decided by the judge.
(c) Is the evidence sufficiently reliable?
The Criminal Practice Directions and CrimPR Part 19 require the court to consider the reliability of an expert's opinion, including the extent and quality of the data, the validity of the methods used, whether the opinion relies on an inference or assumption that has not been properly explained, whether it is based on a hypothesis that has not been subjected to sufficient scrutiny, and whether it falls outside a recognised body of knowledge.
(d) Has the procedure been followed?
CrimPR Part 19 governs expert evidence:
- The expert's overriding duty is to the court, and that duty overrides any obligation to the person from whom the expert receives instructions or by whom they are paid (r.19.2);
- The report must comply with r.19.4: the expert's qualifications and experience, the literature and information relied on, the facts stated as within the expert's knowledge, the substance of instructions, a summary of the range of opinion where there is one and the reasons for the expert's own view, a statement that the expert has complied with the duty to the court, and a declaration of truth;
- The expert must disclose anything that might detract from their credibility or impartiality;
- Where two experts are instructed, the court may direct a discussion and a statement of the points of agreement and disagreement, which is admissible only with the parties' agreement.
(e) The ultimate issue
The old rule that an expert could not express an opinion on the ultimate issue has been much eroded and is now more a matter of form than substance: an expert may in practice express an opinion on a matter the jury has to decide, provided the judge makes clear that the decision remains the jury's.
4. Weight, function and the jury's freedom
Expert evidence is evidence like any other. Its function is to furnish the tribunal with the scientific or specialist criteria against which to test the facts.
Three propositions follow, all of them examinable:
- The jury is not bound by expert evidence, even where it is unchallenged. In Stockwell the Court of Appeal confirmed that the jury must be directed that the expert's opinion is not binding and that they must reach their own conclusion — though a jury that rejects uncontradicted expert evidence must have some rational basis in the other evidence for doing so.
- Where the whole case depends on expert evidence that the defence cannot realistically challenge, and the jury would be bound to accept it, the position is different: there is no room for the jury to reject it capriciously.
- The facts on which the opinion is based must be proved. An expert may rely on the general body of learning in their field (a preserved common law hearsay exception under s.118 CJA 2003) but not on unproved facts specific to the case. If the assumed facts are not established, the opinion falls with them.
Exam traps in this section
- Turner excludes psychiatric evidence about how a normal person reacts to stress; it does not exclude evidence about a recognised disorder.
- Silverlock: experience can suffice — a lack of formal qualification is not fatal.
- The expert's duty is to the court, and it overrides the duty to the instructing party — this is r.19.2 and it is regularly tested.
- The jury may reject even uncontradicted expert evidence, on a rational basis.
A defendant charged with murder wishes to call a psychologist to say that a person of the defendant's ordinary personality, having been provoked in the way alleged, would be likely to lose self-control. There is no diagnosed mental disorder. Is that evidence admissible?
An expert instructed by the defence forms a view that is, on balance, unhelpful to the defendant, and defence solicitors ask her to omit that part of her analysis from the report. What is the expert's position?
At the close of a trial the only evidence on a technical issue is that of a single prosecution expert, which the defence did not challenge with expert evidence of its own. How should the judge direct the jury?