Interbrain Synchrony Mitigates Extremism Within Echo Chambers.

People tend to engage with content that aligns with their pre-existing attitudes, forming echo chambers that reinforce biases and may amplify extremism. Here, we investigate whether discussions within homogeneous groups drive attitudinal extremity and whether interbrain synchronized activity between the executive control brain regions of group members can moderate this relationship between homogeneity and increased extremity. One hundred and eighty-eight participants were randomly divided into groups of four individuals. They then engaged in a moral judgment task in which they privately rated and then discussed the appropriateness of actions taken to resolve moral dilemmas, while their brain activity was scanned using functional near-infrared spectroscopy (fNIRS). Group homogeneity was evaluated using participants' pre-discussion private ratings, while extremism was measured based on how extreme their post-discussion private ratings were compared to their pre-discussion ratings. Our results show that discussions within homogeneous, compared to heterogeneous groups, led to adopting more extreme views. Critically, we found that higher interbrain synchrony between group members' dorsolateral prefrontal cortex (DLPFC) during discussions diminishes this effect of homogeneity on extremism. We propose that interbrain synchrony in the DLPFC can counter harmful interpersonal mechanisms that take place within an echo chamber environment.