A compelling new paper published in the Northern Ireland Legal Quarterly uses the Lucy Letby case as a lens to expose something criminal lawyers everywhere should take note, says Jae Gerhard, forensic DNA expert and Principal Forensic Scientist at Independent Forensic Services.
The central argument is stark. The problem with contested expert evidence in criminal trials is not simply one of bad experts or bad procedures. It is structural.
The criminal process lacks the epistemic infrastructure necessary to properly evaluate complex scientific evidence in a principled and consistent way. The authors argue that England and Wales relies on a 19th century adversarial logic, cross examination and judicial discretion to safeguard against unreliable expert evidence. In an era of increasingly complex forensic and medical science, this is no longer adequate. Australia is in the same position.
| “You Can’t Handle the Truth” is a famous line from the film A Few Good Men (dir. Rob Reiner, 1992), where Colonel Jessup, played by Jack Nicholson, delivers this line during a tense courtroom scene. The movie revolves around a lawyer, Daniel Kaffee, defending two Marines accused of murder, exploring themes of honour and duty. |
High-profile criminal cases increasingly depend on complex scientific and medical evidence, yet the legal framework governing its admissibility in England and Wales remains strikingly underdeveloped.
The article by Amel Alghrani of the University of Liverpool and Saraha Bennett of Liverpool John Moores University, examines that tension through the case of Lucy Letby, the neonatal nurse convicted of the murder of seven infants and the attempted murder of seven others at the Countess of Chester Hospital between 2015 and 2016, where the prosecution case relied heavily on expert interpretation of clinical events.
It argues that the current admissibility regime, rooted in common law principles and supplemented by procedural rules, remains permissive and lacks sufficiently robust mechanisms to rigorously evaluate the reliability of expert testimony.
Situating these concerns within the Law Commission’s 2011 proposals and comparative experience under the United States Daubert standard, the article contends that neither procedural reform nor formal gatekeeping tests have resolved the underlying epistemic tensions between law and science.
The problem is structural: the criminal process lacks the epistemic infrastructure necessary to evaluate contested scientific evidence in a principled and consistent manner. In response, the article advances a set of integrated reforms, including more structured judicial reasoning, a proportionate pre-trial reliability review mechanism, clearer expectations around accreditation and continuing expertise and enhanced judicial engagement with scientific reasoning. Ultimately, it argues for a shift towards “epistemic literacy” as the foundation for a more transparent and defensible approach to expert evidence.
Re The Letby Case (summary)
“You acted in a way that was completely contrary to the normal human instincts of nurturing and caring for babies and in gross breach of the trust that all citizens place in those who work in the medical and caring professions.” (Sentencing, par 3)
In sentencing Letby to whole-life orders, Mr Justice Goss’s sentencing remarks above capture the gravity of the crimes committed, which he went on to condemn as ‘a cruel, calculated and cynical campaign of child murder involving the smallest and most vulnerable of children’. However, recent post-conviction developments have sharply divided public opinion as to the reliability of the convictions, with high-profile figures such as Member of Parliament Sir David Davis describing it as ‘one of the major injustices of modern times’.
In the absence of eyewitness testimony, the case was a circumstantial one. The prosecution relied heavily on expert medical evidence and circumstantial material said to cumulatively establish Letby’s guilt.
Letby was convicted of seven counts of murder, seven counts of attempted murder, found not guilty on two counts of attempted murder and the jury was discharged on 18 August 2023 after failing to reach verdicts on six additional counts. On 21 August 2023, the applicant was sentenced (in her absence) to life imprisonment, with a whole life order on each count on which she was convicted, Goss J stating, ‘you will spend the rest of your life in prison’. Letby was subsequently struck off the nursing register.
BUT …
In February 2025, Letby’s legal team convened an ‘astonishing’ press conference, presenting medical evidence from an international panel comprising 14 ‘experienced and distinguished paediatricians and neonatologists’. The panel, chaired by Dr Shoo Lee, Professor Emeritus, University of Toronto, President, Canadian Neonatal Foundation) undertook a detailed examination of the 17 cases forming the basis of Letby’s prosecution and produced what was described as an ‘impartial, evidence-based report’. Dr Lee stated that they had concluded that ‘in all cases, death or injury were due to natural causes or simply poor medical care’ and that ‘the medical evidence does not support murder in any of these babies’.
Rather, he identified multiple systemic deficiencies at the Countess of Chester Hospital, remarking that the standard of care was so deficient that ‘if this was a hospital in Canada, it would be shut down’.
These deficiencies included staff caring for infants ‘probably beyond their expected ability or designated level of care’; unsafe delays in both diagnosis and treatment, as well as in the transfer of high-risk neonates to specialist tertiary centres; inadequate resuscitation and intubation skills among clinical staff; insufficient supervision of junior doctors during critical procedures such as intubation; and a ‘lack of teamwork and trust between the health professionals’.
Dr Lee opined that many of the witness statements from Letby’s trial, provided to the panel alongside medical records by her legal team, highlighted ‘serious resourcing and infrastructure deficiencies’ within the hospital. These included ‘inadequate numbers of appropriately trained personnel in the unit, lack of training for assigned nursing roles, inadequate staffing and workload overload and poor plumbing and drainage resulting in the need for intensive cleaning in the unit’. Dr Lee’s report has been submitted in support of Letby’s application to the CCRC.
While the outcome of that process remains uncertain, the application illustrates that questions surrounding the evidential basis of the convictions remain live.
The Letby case must be situated within a broader and enduring epistemic tension between law and science, which continues to shape the treatment of expert evidence in criminal trials. This tension arises from the fundamentally different ways in which each discipline constructs and validates knowledge. Scientific reasoning proceeds through hypothesis, testing and revision and is characterised by uncertainty and provisional conclusions. By contrast, the criminal law is institutionally committed to finality and binary outcomes: guilt or innocence, conviction or acquittal.
When scientific knowledge is introduced into the courtroom through expert testimony, these differing epistemic commitments collide, requiring courts to translate probabilistic and contested interpretations into determinate legal judgments. It is this epistemic mismatch that underpins the law’s approach to admissibility. The requirement that expert evidence demonstrates ‘sufficient reliability’ is intended to mediate this tension, ensuring that only evidence capable of safely informing the fact-finding process is admitted.
However, the operation of this standard remains uneven and often deferential, reflecting a continued reliance on judicial discretion and adversarial testing to resolve questions that are, at their core, epistemic rather than purely procedural. These epistemic tensions are translated into legal form through the rules governing admissibility, which seek to regulate expert evidence through a combination of common law principles and procedural safeguards, albeit with uneven success.
The paper ends with this damning paragraph: “A justice system that cannot adequately explain why it trusts expert evidence risks substituting authority for reason. Only by addressing these deeper epistemic challenges can the law maintain both its integrity as a truth-seeking institution and its moral authority to determine guilt and innocence.”
Again, Australia is in the same position.