Friendship modulates physical effort and neural dynamics during group coordination in shared social networks

Human social interactions are embedded within social networks, yet it remains unclear how existing interpersonal relationships influence the behavioural and neural dynamics of collective action. Here, we combined social network mapping with three-person EEG-hyperscanning to investigate how friendships within shared networks modulate effort allocation during real-time coordination. We mapped social networks across seven undergraduate cohorts and recruited triads composed of two mutually close friends and one non-friend from the same network. Triads performed a novel joint motor task in which they produced forces towards a collective target. Non-friends consistently produced greater and more vigorous forces than friends during joint action, an effect strongest among non-friends embedded in smaller social networks. Non-friends also exhibited stronger alpha- and beta-band event-related desynchronization, inter-subject correlation, and inter-brain group phase synchrony, consistent with greater engagement of action-related and attentional processes. Taken together, these findings show that friendship and social network context influence how individuals allocate physical effort and engage during coordination, suggesting that non-friends invest more effort and attention when interacting in socially asymmetric group settings. By linking real-world friendships to behavioural and multi-brain neural dynamics, this work demonstrates that relational context within social networks is embodied in the mechanisms supporting human group coordination.