Many heads are different from one: neural basis of group vs individual decisions

We examine neural mechanisms that drive people’s decisions when they are alone compared to when they are part of a group.

In a modified dictator game, subjects behave pro-socially if they decide to allocate more money to the other player than to themselves (giving), whilst anti-social behaviour refers to decisions to subtract the other player’s income at no benefit or cost to themselves (burning).

We find that overall, when subjects make pro-social decisions (compared to anti-social) both alone and in a group, the ventral striatum and anterior insula are activated, implying that they feel 'rewarded' when they behave pro-socially.

When subjects are in a group and are able to observe the group’s choice before making their own decision, the TPJ is activated, which supports the theory of mind, i.e. they think about what others might do. Pro-social decisions made alone activate the dorsal PFC and bilateral anterior insula, compared to anti-social decisions.

In addition, we observe an increase in activity in the lateral PFC if both the group and the subjects behave pro-socially. But when the subjects’ behaviours are incongruent with the group’s behaviour (when the group behaves anti-socially), we observe the dorsal PFC being activated – similar to when they make pro-social choices alone – as well as the TPJ and ventral striatum.

Our results suggest that when everyone else behaves pro-socially, it is less taxing for an individual to behave pro-socially as well (the behavior is not driven by the emotional/cognitive part of the brain i.e. the dorsal PFC). However, when everyone else behaves anti-socially, it takes more effort to be pro-social.