The current legal rules involving procedure and evidence are not consistently designed based on robust evidence about how the juries make decisions, but the system could function better, according to a new book.

Dr Rebecca Helm, from the University of Exeter, outlines how juries are likely to struggle to make effective legal decisions in predictable case types, including cases involving sexual offences in which testimony from a defendant and complainant is often central evidence.

In such cases juries are particularly susceptible to reaching decisions that are based on uninformed intuition and biased by legally irrelevant information. However, well-designed legal infrastructure can improve the ability of the jury to make decisions in these cases.

How Juries Work: And How They Could Work Better, published by Oxford University Press, highlights how existing changes to legal procedure have failed to address many of the underlying causes of wrongful convictions in trials by jury in England and Wales. Recently there has also been growing dissatisfaction with failures to effectively prosecute defendants, and the potential role of juror bias in these failures.

Dr Helm said: “The procedure surrounding the modern jury is, in many ways, still grounded in common-sense and in tradition from as far back as the 1200s. However, today we have the knowledge and the data to allow us to design procedure to help the jury convict the guilty and acquit the innocent, and to do so in a way that does not exacerbate inequality in the criminal justice system.

“Jurors, as human decision-makers, utilise their own intuitions to interpret case evidence and to reach a verdict. Although these intuitions are likely to be highly effective in allowing us to function in society, they also have predictable weaknesses particularly in the legal context.

“For example, intuitions are likely to be misleading where they arise in the absence of sufficiently representative experience or where they arise as the result of mental shortcuts which we are all regularly influenced by.”

“Jurors are particularly likely to make sub-optimal decisions when they are considering cases involving a part of society that they are not familiar with (e.g., people from a different socio-economic group), and where evidence is ambiguous or difficult to understand.”