Less than a third of trial jurors fully understand a judge's legal directions, a ground-breaking study has suggested.
Jury members also looked on the internet for information about their case, despite being told not to by the judge, the research showed.
Other findings show that all-white juries do not discriminate against black defendants and juries convict more often than they acquit in rape cases, contrary to public perception.
The two-year study, led by Professor Cheryl Thomas of University College London, analysed 68,000 verdicts across Crown courts in England and Wales, and also staged simulated trials.
In relation to judge's directions - where a judge gives crucial guidance to jurors about what they have heard - the research team asked jurors at Winchester Crown Court to recall two key questions that the judge gave in a case where a defendant was charged with violence.
Only 31% of jurors accurately identified both questions, it was found. A further 48% correctly identified one of the two questions, and a fifth did not correctly identify either question.
Researchers found a written summary of the judge's directions on the law for jurors improved their comprehension of the law. Prof Thomas has recommended a pilot test of written juror guideline cards for a sample of courts to work out how they can be best introduced.
In 2008, the Lord Chief Justice suggested that courts might need to present more information visually to reflect greater use of technology like the internet. The report, called Are Juries Fair?, also tested verdicts from all-white juries concerning white and black and Asian defendants.
It was found that verdicts at both courts showed no tendency for all-white juries to convict a black or Asian defendant more than a white person. White defendants accused of racially motivated crimes were also not more likely to be acquitted by all-White juries than racially mixed panels.
Justice Secretary Jack Straw said: "The study's findings on the fairness of jury decisions, including for people from black and minority ethnic backgrounds, will help to maintain public confidence in juries and the jury system. But we cannot allow complacency about the justice system. We will carefully consider the recommendations for helping jurors do their job to the best of their ability."
