Chapter 20 Selected guidance on approaching questions

Experts and opinion evidence

There is no single correct way to answer a question of this kind. There is scope for different approaches to any ‘critical evaluation’. However, a good answer would probably touch on many of the following issues and draw on the available academic literature.

Inherent problems – comprehension by the tribunal of fact

There is an inherent problem with the admission of expert evidence to which there is no obvious or easy answer. Expert evidence is admitted because necessary, i.e. expertise is called for which the tribunal of fact, whether magistrate or jury, does not have: see R v Turner [1975] QB 834. However, the tribunal of fact, lacking such expertise, is likely to find it very difficult to understand the expert evidence and to evaluate its reliability and, for these reasons, may simply defer to the expert. See The Modern Law of Evidence (‘MLE’) 13th ed at p634. See also, below, under ‘…training for judges…’ and ‘Jury comprehension of expert evidence…and technological aids’.

Inherent problems – the fallibility of experts

There is another inherent problem to which there is no solution: expert evidence is only as good as today’s scientific knowledge. Later research may undermine today’s scientific ‘truths’. However, that, by itself, cannot be a basis for rejecting expert evidence: one cannot speculate about possible future developments. See per Judge LJ in R v Cannings [2004] 1 WLR 2607 at [178], considered at MLE 13th ed at pp651-652.

There is another related inherent problem. In some cases, the scientific knowledge may be limited or incomplete because the science in question is new or at an early stage in its development. The reception of expert evidence in such cases can result in miscarriages of justice, as in R v Cannings itself. In that sort of case, the remedy may be to impose a requirement of additional, independent corroborative evidence.

Accreditation of experts and training for judges and practitioners

The risks associated with expert evidence, including in particular the risks of a miscarriage of justice, could be reduced by a scheme of compulsory accreditation of expert witnesses, with a view to them being appropriately qualified, experienced and reliable. The risks could also be reduced by a scheme of mandatory training for judges and practitioners, with a view to them being better able to understand expert evidence and to assess its reliability. See the House of Commons Science and Technology Committee, Forensic Science on Trial, HC 96-I 2005 and MLE 13th ed at p634.

The reliability of expert evidence

On the key issue of the reliability of expert evidence, English judges very often pay only lip service to the requirement of ‘sufficient reliability’ as a condition of the admissibility of expert evidence (see R v Bonython (1984) 38 SASR 45) . For illustrations of their reluctance to apply such a condition, see R v I [2012] EWCA Crim 1288 (see MLE 13th ed at p646), R v Dallagher [2003] 1 Cr App R 195 (12) and R v Luttrell [2004] 2 Cr App R 520 (41) (see MLE 13th ed at pp648-649) and R v Robb (1991) 93 Cr App R 161 (see MLE 13th ed at p650).

A considerable improvement has been made by the changes to the Criminal Practice Directions, which now incorporate the reliability factors recommended by the Law Commission and into which the courts are now ‘encouraged actively to enquire’. See MLE 13th ed at pp647-649. However, although an improvement, the Practice Direction has attracted justifiable criticism. First, there is no definition of ‘sufficient reliability’ and thus no indication of the weight the expert evidence must have in order to be admissible. See Ward, ‘A new and more rigorous approach to expert evidence in England and Wales?’ (2016) 19(4) E&P 228. Second, the reliability factors raise new complex questions including, eg, whether particular scientific techniques have been generally accepted and whether they may be reliable even if not accepted by a scientific community. See Roberts, ‘Drawing on Expertise: Legal Decision-making and the Reception of Expert Evidence’ [2008] Crim LR 443.

Juror comprehension of expert evidence – research and technological aids

In R v Henderson [2010] 2 Cr App R 185(24) the Court of Appeal gave guidance on the content of the summing-up in cases where evidence to prove guilt consists of expert evidence alone. However, the capacity of jurors to apply the guidance, especially in complex cases, is open to debate. There is a need for more research on jury comprehension of expert evidence and ways in which it can be enhanced, e.g. by greater use of technological aids. See Cohen and Heffernan, ‘Juror Comprehension of Expert Evidence: A reform Agenda’ [2010] Crim LR 195.

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