Participant dyads making group decisions in social uncertainty adopt the exploitative strategy and show enhanced...

People in socially uncertain circumstances often polarize their decision-making behaviors when making decisions as a group rather than as individuals. One may think the group tends to explore new possibilities in social uncertainties, while the other believes they are more adept at exploiting the successful experience acquired before. To test these hypotheses, we recruited participants of dyads to perform a trust game while their brain activities were recorded using the functional near-infrared spectroscopy (fNIRS) hyper-scanning technique. Participant dyads playing the game as a group versus as individuals demonstrated the increased investments under gain conditions but not under loss conditions. Computational modeling attributed the increased investment to the exploitation strategy, namely, relying more on experience from positive feedback. Brain-to-brain analysis revealed that the strategy was associated with the dyads' enhanced interpersonal brain synchrony (IBS) in the right temporo-parietal junction (rTPJ), which was also correlated with the increased group investment. Further, both the learned strategy and the enhanced IBS occurred from the middle to the end of playing the trust game. In sum, to make group decisions in social uncertainty, participant dyads adopt an exploitation strategy by leveraging past successful experiences, along with the enhanced interpersonal rTPJ synchrony between them. Our study provides new insight into cognitive and inter-brain processes associated with of group decision-making.