A new study has identified two types of juror, with one group whose minds work emotionally rather than rationally, judging defendants by looks rather than evidence.
Both sets of jurors convicted attractive defendants at the same rate, the study found. But the emotional group were far more likely to find unattractive defendants guilty, believing they looked more capable of committing the crime.
And when asked to pass sentence in the trial, the emotional group additionally gave unattractive defendants harsher punishments.
The researchers, from New York's Cornell University, believe there will now be pressure from lawyers for the introduction of psychological tests in the jury selection system to stop those jurors who are motivated more by emotion than reason taking part in trials at all.
Their study, 'When Emotionality Trumps Reason', to be published in the academic journal Behavioral Sciences & the Law, sheds new light on how jurors reach their decisions. Researchers say they detected an "unattractive harshness effect" at work in jury rooms.
The study involved using psychological tests to split a group of mock jurors into those who make decisions emotionally and those who do so rationally. They were then asked to study a criminal trial transcript and look at pictures of alleged defendants who varied in physical attractiveness.
Justin Gunnell, a New York lawyer who led the research, explained: "There has been a lot of work done on the concept of whether attractive defendants do better in court."
"We suspected that potential jurors who were more prone to base their decisions on emotion or intuition would be more likely to consider legally irrelevant factors such as defendant attractiveness when rendering verdicts. The results supported our suspicions."
Gunnell, who worked with psychology professor Stephen Ceci on the study, now believes scientists will come up with a quick test to enable US attorneys – who, unlike their Scottish counterparts, are allowed to challenge jurors without giving a reason – to weed out those who rely on gut feeling.
Scottish lawyers rejected the idea of psychological testing of potential jurors, saying they believed juries usually balanced themselves out. But Peter Lockhart, who is dean of the Faculty of Solicitors in Ayr, said the theory about convictions being based on defendant attractiveness was probably correct.
He said: "We are not supposed to speak to jurors but I do remember one coming up to me months after a trial and telling me, unprompted, that my client had 'just looked like a bad 'un,' " he said.
"In Scotland we really know very little of what goes on in the jury room because there has not been very much research done. However, I would say that most jurors take their responsibilities very seriously."
Other Scottish lawyers also fear the "unattractive harshness" effect may play a part in some cases. For example, men accused of rape may sometimes fare better if they are good-looking – with jurors who don't understand the nature of the crime assuming the defendant "would not need to rape".
John Scott, an Edinburgh-based defence agent, stressed he would never point out an accused's looks in such cases but would encourage a defendant to try to look their best.
He said: "We always try to reduce the impact a client's appearance might have, because we know the jury will be watching them closely, especially if they don't take the stand. And it is true an attractive, articulate liar is likely to do better in court, as a defendant or as a witness.
"But I can't think of a fairer way of doing things than using a completely random jury of the kind we have in Scotland."
Aamer Anwar, who has been involved in some of the country's highest-profile trials in recent years, agreed. "It should be a jury of your peers," he said. "We have 15 men and women who could be working-class or middle-class, white or black, rich or poor."
But the Cornell researchers think the scale of the offence also has an effect, with jurors who work on emotional lines becoming more rational if the case merits it.
Gunnell said: "I think it kicks in when there is more ambiguity about a case or when it is dealing with less serious offences, such as theft," he said. "Murders tend to focus the mind."