“Social brain” changes structure with evolving concept of fairness

An important aspect of social interactions and cooperation is “fairness,” which is determined by perceptions of others’ intentions and preferences for “egalitarianism” or equal outcomes. Others’ intentions have a considerable influence on our ultimate decisions, because we tend to reciprocate intention rather than outcome. Interestingly, this relative importance is determined by age. Younger children usually prefer egalitarian outcomes, whereas older children and adolescents consider reciprocity and intention. In a study published in August 2017 in Scientific Reports, led by first author Asst Prof Sunhae Sul from Pusan National University, scientists showed that these behavioral changes are strongly linked to structural changes in the brain.

The “social brain” represents a set of regions that are involved in understanding and inferring others’ mental states. These include the medial prefrontal cortex (MPFC), posterior cingulate cortex, temporo-parietal junction, and posterior superior temporal sulcus. But, how do these regions develop or change in order to support affiliation? This is exactly what this group of scientists set out to explore.

Consider, for example, the ultimatum game, where a “proposer” chooses what proportion of an endowment they’d like to offer and the “responder” decides whether or not to accept it. The responder’s decision is usually based on whether they considered the offer to be fair. However, because mature humans are sensitive not only to their own gains and losses but also to those of other people, the same “unfair” offer might be accepted if the proposer demonstrates good intentions.

But, when does this type of reasoning develop and how is it related to the brain structure?

In this study, the scientists investigated the link between changes in the perception of fairness and development of cortical matter in participants aged between 9 and 23 years. The participants responded to multiple rounds of ultimatum game proposals, which involved the exchange of money. The proposer had to choose between two different parts of $10, and the responder had to decide whether or not to accept the given part. Using computational modeling, the scientists recorded the degree to which the participants considered the division of endowment “fair.” They analyzed this along with measurements of cortical thickness from high-resolution MRI scans. Younger children (between 3-5 years old) preferred a more equal distribution of money, and older players, i.e., adolescents and young adults, were more prone to consider the other participants’ intentions.

The scientists found that this development of intention-based reciprocity was significantly correlated with the decrease in the cortical thickness of the social brain in late adolescence, specifically the MPFC. They called this “cortical thinning.” Another region of interest was the posterior temporal lobe, which had not been previously associated with the “social brain.” As Prof Sul puts it, “it is interesting to speculate how changes in this region correlate with developmental changes in more sophisticated preferences for intention-based reciprocity.” This study proves that structural changes in the MPFC and posterior temporal lobe are linked to a change in the perception of fairness and the ability to infer others’ intentions. Prof Sul concludes, “The findings of our study might enable large-scale human cooperation at a societal level and provide a granular understanding of the complex social interactions among humans.”


Title of original paper:Medial prefrontal cortical thinning mediates shifts in other-regarding preferences during adolescence

Name of author:



Sunhae Sul

Department of Psychology, Pusan National University, Busan, Republic of Korea

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