A New Model for Group Decision-Making

By Anna Tarasova ’19

Figure 1: The integrity of the judicial system depends on an unbiased jury.

Making a decision in a group requires integrating individual and group beliefs. It has been previously demonstrated that one is more likely to assign greater credibility to the beliefs of a larger group than to those of a smaller group. Prior researchers have also theorized that the adoption of group beliefs by individuals is driven by a desire to mitigate potential conflict. However, Dr. Seongmin Park and his colleagues at the Centre National de la Recherche Scientifique propose that the driving factor is instead a lack of confidence in individual beliefs.

The team conducted a series of experiments in which 25 participants were asked to individually decide how many years to incarcerate a criminal in a murder case and rate their confidence in this judgment. Then, they were told the average number of years assigned by a jury of either 5 people or 20 people who had high confidence in their judgment, and offered the opportunity to reconsider their initial judgment. Overall, individuals tended to adjust their initial judgment to be closer to that of the group, especially if their confidence in the first judgment was low.

Three different statistical models were used to evaluate the data computationally. The model that best fit the collected data was the Bayesian model, which predicts that individuals make their initial judgment based on private estimates and that the credibility assigned to social information is based on group size. Furthermore, the Bayesian model states that the brain assesses the extent to which group beliefs need to be integrated. The two brain regions examined in this experiment were the dorsal anterior cingulate cortex (dACC) and the frontopolar cortex (FPC). The dACC is responsible for belief updates in an individual and the FPC evaluates changes in credibility assigned to social information. Increasing functional connectivity between these two areas in an individual correlated with the degree to which group size influenced their confidence in group belief.

Dr. Park’s findings suggest a new neural model for the basis of group decision-making as well as reevaluating the reasons behind the integration of group and individual beliefs. Studies such as this one can help find the optimal jury size and improve other decision-making groups.



  1. S. Park et al., Integration of individual and social information for decision-making in groups of different sizes. PLOS Biology 15, 1-28 (2017). doi: 10.1371/journal.pbio.2001958.
  2. Image retrieved from: https://upload.wikimedia.org/wikipedia/commons/d/de/New_York_Court_of_Appeals_hearing_oral_arguments.jpg

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