Introduction
Expert evidence plays an increasingly significant role in modern criminal trials in England and Wales. From complex DNA analysis to psychiatric assessments, the evidence of specialists can be crucial in helping the jury understand technical matters that fall outside their ordinary knowledge and experience. However, the use of such evidence is not without its dangers. The deference that jurors may afford to an expert, combined with the risk of flawed or unreliable scientific theories being presented as fact, has led to serious miscarriages of justice (Naughton and Tan, 2011). This has prompted extensive debate over whether the rules governing the admissibility of such evidence are sufficiently robust to protect against these dangers. This essay will argue that while the rules on expert evidence have been strengthened in recent years through procedural reforms and judicial guidance, the lack of a formal statutory reliability test means they remain insufficient to consistently guard against the admission of unreliable evidence. The current framework is a noticeable improvement on the previous laissez-faire approach, but it falls short of providing the comprehensive protection that many commentators, including the Law Commission, have argued is necessary.
The Common Law Test for Admissibility
The traditional foundation for the admissibility of expert opinion evidence in criminal proceedings is the common law test laid down in R v Turner [1975] QB 834. The guiding principle from this case is that an expert’s opinion is admissible if it provides the court with information which is likely to be outside the experience and knowledge of a judge or jury. Lord Justice Lawton stated that an expert's opinion is admissible "to furnish the court with scientific information… which is likely to be outside the experience and knowledge of a judge or jury" (Turner at 841). If the issue is one on which the jury can form their own conclusion without assistance, then the expert's opinion is deemed unnecessary and therefore inadmissible.
This core 'helpfulness' test is supplemented by other requirements. The expert must be suitably qualified in their field, and the evidence must be relevant to a matter in issue. Historically, there was also a rule against an expert giving an opinion on the ‘ultimate issue’ – the very question the jury has to decide, such as whether the defendant had the required intent. However, this rule has been significantly relaxed, and the courts now generally permit such evidence provided it does not cross the line of telling the jury what to decide (R v Stockwell (1993) 97 Cr App R 260).
While these rules provide a basic framework, their major weakness has long been identified as the lack of a specific or robust test for the reliability of the expert's opinion or the scientific basis upon which it is founded. Under the common law, as long as the evidence was deemed relevant and presented by a qualified expert, judges were often reluctant to exclude it, preferring to leave questions of reliability and credibility to be tested in cross-examination and ultimately weighed by the jury (Henderson, 2016). This approach proved to be inadequate in a number of high-profile cases, which highlighted the potential for unreliable expert testimony to lead to wrongful convictions.
The Call for Reform and the Reliability Problem
The insufficiency of the common law approach was starkly illustrated by a series of miscarriages of justice in the late 1990s and early 2000s. In cases such as those involving Sally Clark, Angela Cannings, and Donna Anthony, mothers were wrongly convicted of murdering their children based on flawed expert statistical and medical evidence (Law Commission, 2011, p.1). For example, in R v Clark (Sally) (No 2) [2003] EWCA Crim 1020, the conviction was quashed partly due to the misleading statistical evidence given by a paediatrician, which grossly understated the likelihood of two children in the same family dying from Sudden Infant Death Syndrome (SIDS).
These cases exposed the dangers of the court's deferential approach, demonstrating that cross-examination and jury evaluation are not always effective safeguards against 'junk science'. Juries may struggle to assess complex scientific evidence critically and may be overly swayed by an expert’s credentials. This led to a growing consensus that a more rigorous 'gatekeeping' role was needed for judges at the admissibility stage.
In response to these concerns, the Law Commission undertook a comprehensive review, culminating in its 2011 report, Expert Evidence in Criminal Proceedings in England and Wales. The Commission concluded that the common law test was "insufficient to exclude unreliable expert opinion evidence" (Law Commission, 2011, para 1.20). Its central recommendation was the introduction of a new statutory test for admissibility. This proposed test would require the party seeking to adduce the evidence to demonstrate to the court that the opinion was "sufficiently reliable to be admitted". The draft Bill suggested several factors for the judge to consider in this assessment, such as the extent and quality of the data on which the opinion was based, the validity of the methods used, and whether the opinion was properly explained. This approach was heavily influenced by the reliability-focused test established in the United States in Daubert v Merrell Dow Pharmaceuticals Inc 509 US 579 (1993). However, despite widespread support, the government rejected the Law Commission's proposal for a statutory test, citing the cost and potential for delays.
The Modern Procedural and Judicial Framework
In the absence of the statutory reform recommended by the Law Commission, the judiciary and the Criminal Procedure Rule Committee have attempted to address the reliability gap through other means. The main developments have been the strengthening of the Criminal Procedure Rules (CrimPR) and the issuing of key guidance from the Court of Appeal.
Part 19 of the CrimPR now imposes clear duties on expert witnesses and the parties who instruct them. CrimPR 19.2 states that an expert has an overriding duty to give opinion evidence that is "objective and unbiased", and this duty overrides any obligation to the person instructing or paying them. Furthermore, CrimPR 19.4 sets out in detail what an expert's report must contain. This includes a statement of their qualifications, the substance of all material instructions, a summary of the facts and instructions given to them, and information on which their conclusions are based. Crucially, the expert must now also provide a statement that they understand their duty to the court and have complied with it. These rules are designed to enhance the transparency and integrity of expert evidence.
The Court of Appeal has also taken a more robust stance. In R v Dlugosz [2013] EWCA Crim 2, the court, while acknowledging the lack of a statutory reliability test, effectively endorsed the factors proposed by the Law Commission as guidance for trial judges when assessing admissibility. The court stated that a judge should consider the reliability of the evidence, drawing upon the Law Commission's proposed criteria when necessary. This judicial guidance, combined with older cases like R v Bonython (1984) 38 SASR 45, which required the field of expertise to be sufficiently organised or recognised as reliable, creates a more structured framework for judges. This approach essentially encourages a more active gatekeeping role for the trial judge, moving away from the old position of leaving reliability almost entirely to the jury.
An Evaluation of Sufficiency
The key question is whether this combination of procedural rules and judicial guidance is a sufficient substitute for the statutory test proposed by the Law Commission. On one hand, the current framework is a clear improvement. The requirements of CrimPR Part 19 promote better practice among experts and provide the opposing party and the judge with more information to scrutinise the basis of an opinion. The guidance from the Court of Appeal in cases like Dlugosz empowers judges to consider reliability as part of the admissibility decision, using the Law Commission's well-reasoned factors as a checklist. This means that a judge is now better equipped to exclude evidence that is patently unreliable before it ever reaches the jury.
However, on the other hand, this 'soft law' approach has clear limitations which suggest it is not wholly sufficient. Firstly, because the reliability test is not statutory, its application can be inconsistent. The guidance from the Court of Appeal is helpful, but it does not have the same binding force as a legislative provision (Ormerod, 2011). A judge may still feel that, in the absence of a clear statutory duty to exclude, the traditional approach of admitting borderline evidence and leaving its weight to the jury is the safer option. Secondly, there remains a concern that without a statutory test, there will be insufficient wholesale change in culture. The reforms depend on judges feeling confident enough in their own ability to assess scientific reliability, a task for which many may not feel adequately trained. The formal structure of a statutory Daubert-style hearing would arguably force a more rigorous and focused inquiry into reliability than is likely to happen under the current, more discretionary regime. The continued admission of certain types of evidence, such as some forms of forensic gait analysis, suggests that the gatekeeping function is still not as robust as it could be (Edmond and Cunliffe, 2016).
Conclusion
In conclusion, the rules governing the admissibility of expert evidence in criminal trials have undergone significant evolution and are now more sufficient than they were two decades ago. The old common law test from Turner, with its narrow focus on 'helpfulness', has been supplemented by a more robust procedural framework under the Criminal Procedure Rules and important judicial guidance that encourages a focus on reliability. This has undoubtedly strengthened the court's ability to act as a gatekeeper against flawed expert testimony.
However, the sufficiency of the rules remains questionable. The government’s decision not to implement the Law Commission's proposed statutory reliability test means that the current system is a patchwork of case law and procedural rules rather than a clear, unified code. This leaves open the risk of inconsistency in application and a continued reluctance on the part of some judges to exclude evidence, preferring to see reliability as an issue of weight for the jury. While the situation has improved, the rules are not yet fully sufficient to ensure that only reliable expert evidence is placed before a jury. The system has moved in the right direction, but the absence of a firm statutory footing remains a critical weakness.
References
Edmond, G. and Cunliffe, E. (2016) 'Cinderella Story? The Social Production of a Forensic "Science"', Journal of Criminal Law and Criminology, 106(2), pp. 219–275.
Henderson, T. (2016) 'The Admissibility of Expert Evidence: A Contested Doctrinal and Normative Landscape', in: Roberts, P. and Stockdale, M. (eds.) Forensic Science and Expert-Witness Testimony. Edward Elgar Publishing.
Law Commission (2011) Expert Evidence in Criminal Proceedings in England and Wales. Law Com No 325. London: The Stationery Office.
Naughton, M. and Tan, G. (2011) 'The need for an 'innocence commission' in England and Wales to review alleged miscarriages of justice', International Journal of Evidence & Proof, 15(4), pp. 317-336.
Ormerod, D. (2011) 'Sounding the retreat? The future of expert evidence', Criminal Law Review, (9), pp. 707-709.
Daubert v Merrell Dow Pharmaceuticals Inc 509 US 579 (1993).
R v Bonython (1984) 38 SASR 45.
R v Clark (Sally) (No 2) [2003] EWCA Crim 1020.
R v Dlugosz [2013] EWCA Crim 2.
R v Stockwell (1993) 97 Cr App R 260.
R v Turner [1975] QB 834.
