Behavioral contagion leads the brain functional network towards a more balanced state.

Behavioral contagion is defined as the tendency of individuals to conform with others' behavior after observing them. Despite the important role that behavioral contagion plays in societies, its brain mechanism is still not fully understood. In this study, we hypothesized that the brain functional network rearranges its regional associations in a way to achieve a more stable state after behavioral contagion. Therefore, the stability of the brain network before and after observing others' choices (preferences) was assessed using structural balance theory (SBT) on the fMRI data. For this purpose, we developed a version of the dictator game as a task, and recorded participants' brain responses using fMRI. The participants were classified into Contagion and No-Contagion groups based on significant changes in their preferences after observing others' choices. In both groups, changes in SBT parameters of the brain functional network were statistically compared before and after the observation. The results indicated that behavioral contagion is accompanied with a rearrangement of the links in the network to transform imbalanced triads into balanced triads. This process lowers the balance energy of the brain functional network and pushes it to a more stable state. Interestingly, for the No-Contagion group, no significant changes in stability (balance energy level) of the brain functional network was observed. Based on these findings, we believe that if the brain network goes to a more stable state by changing the topology of negative links (anti-correlated regional associations) after observation of others' preference, the contagion may occur.