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Seven the ideal number for juries

The traditional 12-member jury is too large to reach true consensus, according to a study by Scottish psychologists.

The study of 150 people in mock decision-making situations found that seven is the magic number when it comes to reaching a unanimous verdict. In larger groups, discussion tends to be dominated by forceful individuals who heavily influence the outcome.

Professor Simon Garrod of the University of Glasgow told the recent meeting of the British Association for the Advancement of Science of his team's results.

"Large groups with 10 or more participants, such as juries, are not a good arrangement for gaining true consensus," Professor Garrod said.

"In a group of 10, two people won't open their mouths ... and three people could really drive a decision through."

"Smaller groups of up to seven people are much more successful in gaining consensus. Dominant speakers might say more in a small group, but they will have less influence than in a large group, because of the way in which the issues are discussed."

The researchers studied 20 discussion groups, half with 10 participants and half with five. Subjects read an article about student plagiarism and the psychologists gauged their opinions with a questionnaire. They were then asked to discuss the issue in groups of five or 10 before giving their opinions again.

In the bigger groups, discussion often deteriorated into a series of monologues, "with each speaker in turn broadcasting to the rest of the group", Professor Garrod said. They were dominated by small cliques who could easily dominate, and interruptions were infrequent. A balanced consideration of the issues became more difficult.

In the smaller groups, all five participants contributed to a consensual decision, with people often interrupting each other but mostly getting their point across.

To establish the ideal size, the team later ran the same process through groups of six, seven, eight and nine people.

"The sixes and sevens were identical in response to the fives, but there was a clear jump from eight and up," said Professor Garrod. "Above seven, people appeared to run out of capacity to consider all of the points of view."

Professor Garrod said the results might explain why so many court cases end up with a majority verdict, rather than a unanimous one.

"The size of the jury has a terrific impact on the decision-making process," he said.

He said the findings have important implications for many decision-making groups, from a volunteer organisation to the boardroom, or even government cabinet.