Jurors should be tested before trials to reduce the effect of prejudices on their understanding of the burden of proof, according to the authors of a study suggesting bias has a ‘significant’ impact on verdicts.

A study, published in the British Psychological Society’s Legal and Criminological Psychology journal, found that pre-trial bias and jurors' understanding of the concept of beyond reasonable doubt have a ‘significant’ impact on the verdict they are likely to deliver in court.

Researchers from Anglia Ruskin University, University of Cambridge and University of Middlesex sampled a group of 118 people aged between 19-63 employed by a pharmaceutical company in Cambridge.

Participants were asked to imagine they were serving on a jury in a ficticious burglary trial. They were given details of the charge, the prosecution and defence cases and the judge's instructions on the presumption of innocence.

Looking at the criminal standard of proof to convict - beyond reasonable doubt - the study found that the average threshold required to find someone guilty was a probability of 95%.

It also found that pre-trial attitudes combined with interpretations of beyond reasonable doubt accounted for 37% of the variability in the verdicts, so are an important predictor of how a juror will decide, regardless of the evidence.

Dr Samantha Lundrigan, senior lecturer in criminology at Anglia Ruskin University, said the study confirms much of what lawyers anecdotally believe - that certain characteristics of jurors can predispose them towards their verdict.

Factors that had the most impact on 'conviction proneness' were how sympathetic jurors are to the prosecution and the police and cynicism towards the defence.

Less important characteristics included views on social justice, racial bias and criminality.

Beyond reasonable doubt, said Lundrigan is a ‘static concept’ that should be applied universally. But, she said the study showed many people did not really understand what it meant.

‘Their interpretation of beyond reasonable doubt is influenced by their attitudes, which in turn influences their verdict,’ she said.

She suggested that initiatives are needed to tackle jurors' lack of understanding of the concept.

Although UK courts do not permit jury selection in the way that US courts do, Lundrigan suggested that pre-trial questionnaires could be used to ensure jurors have an appropriate understanding of the standard of proof and to highlight potential issues with jurors before trials begin.

The judge or clerk could then assist jurors by clarifying difficult concepts and explaining the importance of focusing on the evidence presented before them.

Lundrigan said: ‘The jury system in the UK works well and most of the time the verdict is the right one, but inevitably when dealing with human beings, there will be extra-evidential factors that affect jurors’ decisions.’