The jury in the iconic film 12 Angry Men would have reached a very different verdict if not for the Henry Fonda character, "juror 8". His lone dissenting voice prompts a fierce debate in which prejudices are exposed, and a previously reticent young juror finds the confidence to share a knowledge of knives gleaned from his tough upbringing to convince the others that the accused could not have inflicted the fatal wound.
But in real life, not every jury has a juror 8. Research in the US has found that it is common for a third of jury members to contribute little to discussion, while a significant minority remain completely silent. It's difficult to conduct similar research in the UK, where there are strict laws preventing jurors talking about what has gone on in the jury room, but it seems likely the results would be similar. A jury of 12 is supposed to represent a cross-section of society and bring a wide range of experience to the deliberation process, but that's not going to happen if some members can't get word in.
Psychologists at the University of Portsmouth have been looking at how to improve the quality of jury debate and have come up with a simple solution: rearrange the chairs so they begin their deliberation in groups of four. "Four is the magic number, because it's the maximum number of people you can interact with effectively at any one time," says the lead researcher, Dr Bridget Waller.
In simulations, the researchers found that the arrangement increased the involvement levels of all jury members and there was less of a tendency for some people to dominate. They also noted that the group discussion evolved naturally into a whole-jury consensus, and jurors felt they contributed more to the verdict.
Why is the number four so important? "Observations of social settings suggest that four is a cut-off point in spontaneously forming groups," says Waller. "If you look at a restaurant table of eight people, there will be two conversations going on."
In group conversations, there's a lot to keep track of, she explains. For example, you need to remember who has said what, read each person's body language and keep track of conversational processes such as turn-taking. It's very difficult to manage this with more than four people.
Waller also points to other research that suggests there are evolutionary as well as practical reasons for our tendency to gather in groups of four. The evolutionary anthropologist Robin Dunbar has put forward the idea that language has evolved partly as a form of social "grooming", which functions in a similar way to the physical grooming practised by other primates to maintain bonds.
There is a relationship between the amount of time a species spends grooming each other and the size of their social group – chimpanzees live in groups of around 50, so spend a lot of time grooming. While animal grooming takes place on an individual basis, language allows us to "groom" more than one person at a time. Because the average person is connected socially to around 150 others, we need to be three times as efficient at grooming as chimps. Group talk is a way of achieving this, although it gets difficult once you get beyond the group of four.
In the Portsmouth research, 120 participants watched a video of a mock trial and were then divided into "juries", some in the traditional way, with all 12 members seated around one table, and others arranged into groups of four. The only instruction given was not to move the chairs. "We thought they might be confused as to whether they were in a group of four or 12," says Waller. "What surprised us was how quickly people intuitively understood what they were supposed to do. They began discussions very quickly in their group of four, but also had a very fluid exchange of ideas between the sub-groups and the main configuration. In the groups of 12 we didn't have that – it was a lot quieter and there was a lot more hesitation at the start. What came up in our feedback at the end of the simulations was that personality types seemed to be exposed more in the group of 12 than in our sub-divided groups, and some individuals dominated the discussion." Leaders still emerged in the group setting, she adds, but their influence was diluted.
"What you tend to get in large groups is the "groupthink" phenomenon, which means you can easily get swayed towards the majority consensus. You're less likely to have that in smaller groups, so if you do have an alternative position, you're more likely to vocalise that."
Some members of the team are also looking at other ways of improving the jury experience. In a related study, Dr Lorraine Hope developed a specially structured notebook with trial-relevant headings that jury members can use to take notes. Her research found that those using it reported significantly more accurate information in a post-trial recall task.
As yet it's unclear what effect, if any, jury groupings will have on verdicts. Almost all the juries in the simulation found the defendant not guilty. The team now plans to conduct further studies using more divisive cases, and to look in detail at how non-verbal behaviour helps participants to understand the motivation and intention of others. "What might be happening in groups bigger than four is that you get the linguistic sentiment – the actual words they're saying – but not the emotional value or emphasis they're putting on it or how confident they feel about it. Looking at someone's face and gestures and listening to the sound of their voice helps you interpret what they're saying. I think what we might find is that in our small groups of four there's a much better emotionally communicative process than in the large groups."