Expert Witnesses in Criminal Justice: Problems and Appeals Reform

The criminal justice system relies heavily on expert witnesses, particularly in medical cases where technical knowledge is crucial for understanding evidence. However, this reliance has revealed significant systemic issues that contribute to miscarriages of justice.

Expert Witness Problems Leading to Miscarriages of Justice

Expert witnesses in medical cases often face challenges that can undermine justice. Medical professionals may present opinions with more certainty than the evidence supports, especially in complex cases like sudden infant death, non-accidental injuries, or disputed cause of death. Courts and juries, lacking technical expertise, may give undue weight to these opinions without recognising their limitations or potential biases.

Several high-profile miscarriages of justice have stemmed from flawed expert testimony:

Wrongful convictions based on disputed "shaken baby syndrome" diagnoses
Cases where parents were falsely accused of harming their children due to rare medical conditions being misinterpreted as abuse
Death investigations where natural causes were mistakenly attributed to criminal acts
Convictions based on scientific evidence that was later discredited or significantly refined

These miscarriages often share common features: overconfidence in expert opinions, failure to consider alternative explanations, and courts deferring too readily to established medical authorities.

Expert Witness Employment and Bias Concerns

The employment structure of expert witnesses raises significant concerns about impartiality. In the UK system:

Instructed and paid by one side: Experts are typically instructed and paid by either the prosecution or defence, creating an inherent risk of allegiance bias. This financial relationship may, even unconsciously, influence their conclusions to favour their instructing party.
Career expert witnesses: Some professionals develop careers as frequent expert witnesses, particularly for either prosecution or defence. This specialisation can create conscious or unconscious bias, as their professional reputation and future work may depend on delivering helpful opinions to their typical clients.
Institutional pressures: Experts from within institutions (such as NHS consultants in the Letby case) may face institutional pressure to reach conclusions that align with their organisation's interests or previously established positions.
Confirmation bias: Experts may be provided with selective case information that frames their analysis in a particular direction, leading to confirmation bias in their conclusions.
Absence of true independence: Unlike some other jurisdictions, the UK lacks a robust system of court-appointed, independent experts with no connection to either side. While judges can appoint joint experts, this remains relatively uncommon in criminal cases.

The question of whether experts can be truly unbiased is profound. Even well-intentioned experts are vulnerable to cognitive biases, professional allegiances, and the subconscious desire to please those who have instructed them. The adversarial system magnifies these issues by encouraging each side to find experts who support their narrative rather than identifying objective truth.

The Lucy Letby Case: A Concerning Example

The Lucy Letby case highlights several problematic aspects of expert testimony in criminal proceedings:

Statistical evidence interpretation: Prosecution experts presented statistical arguments about the improbability of multiple deaths occurring during Letby's shifts. However, concerns have been raised about how these statistics were calculated and presented, including potential prosecutor's fallacy issues where coincidence is mistaken for causation.
Absence of definitive medical evidence: In several of the deaths, there was no definitive medical evidence of murder. Some medical experts have questioned whether natural causes could explain some of the incidents, particularly given the vulnerable nature of premature infants. They have also pointed out that substandard care may have been a contributing factor to deaths and collapses.
Disagreement among experts: There was significant disagreement among medical experts about the causes of death, with some defence experts offering alternative explanations but were not called to present this to the Jury.
Lead expert changing cause of death theory: A key prosecution expert witness changed their opinion on the cause of death for at least one infant. Initially, the expert testified that death resulted from air being pushed down a nasogastric tube, but post-conviction changed this opinion to air embolism caused by a syringe being used to push air into the venous system. Such a fundamental change in the mechanism of death raises serious questions about the reliability and certainty of expert testimony presented to the jury.
Contextual bias: Experts may have been influenced by knowledge of other allegations against Letby, potentially biasing their interpretation of ambiguous medical evidence.
Jury comprehension challenges: The highly technical nature of the medical evidence presented challenges for jury comprehension, particularly when evaluating competing expert opinions about rare medical conditions and statistical probabilities.

Appeal System Deficiencies

The appeal system's criteria for new evidence are particularly problematic. Currently, fresh evidence must typically satisfy strict requirements:

It must not have been available at the original trial
It must be credible and relevant
It must have a real possibility of affecting the verdict

These criteria create a high threshold that many valid claims cannot meet, especially when the issue involves evolving medical understanding or reinterpretation of existing evidence rather than entirely new facts.

When scientific understanding advances, defendants face significant hurdles in getting cases reconsidered. The system often treats expert disagreement as a matter already settled at trial rather than as potential evidence of a miscarriage of justice.

CCRC Limitations

The Criminal Cases Review Commission (CCRC) faces structural limitations in its role. Instead of conducting truly independent reviews, the CCRC often feels constrained to predict how the Court of Appeal would view a case. This approach creates a circular problem where the CCRC essentially second-guesses the appeal court rather than providing a genuinely fresh assessment of potential miscarriages of justice.

This conservative approach means many genuine miscarriages remain uncorrected, particularly in cases involving disputed expert evidence. The CCRC's referral rate remains low, with many compelling cases never reaching the Court of Appeal.

Reform Proposals

Reform is needed to create a more balanced system that:

Recognises the fallibility of expert opinions, particularly in evolving medical fields
Allows for more flexible consideration of new expert interpretations of existing evidence
Empowers the CCRC to apply independent judgement rather than merely anticipating court decisions
Creates clearer standards for evaluating competing expert testimony
Establishes independent scientific review panels for cases involving disputed medical evidence
Improves training for judges and lawyers in evaluating scientific evidence
Considers a move toward court-appointed, truly independent experts in complex scientific cases
Implements stronger disclosure requirements for experts regarding their history of work for prosecution or defence

Justice requires a system that can acknowledge and correct errors, especially when lives and liberty are at stake based on contested expert opinions. Addressing these issues would significantly reduce the risk of miscarriages of justice continuing uncorrected.