Respiration and individual differences shape behavioral and inter-brain synchronization

Humans coordinate their actions while their bodies and brains synchronize in social interactions, yet how behavioral, physiological, and neural dynamics interact during interpersonal coordination remains largely unknown. We investigated how respiration and individual differences influence behavioral and inter-brain synchrony in a musical synchronization task, which requires precise temporal alignment. Eighty-eight individuals first produced a familiar melody at their most comfortable rate to measure their spontaneous production rate (SPR). They then synchronized the melody with a randomly assigned unfamiliar partner under natural (unpaced), instructed in-phase, and instructed anti-phase breathing conditions, while hyperscanning 64-channel electroencephalography (EEG), respiration, auditory-motor timing, and perceived social connectedness were recorded. Relative to natural respiration, both in-phase and anti-phase breathing increased inter-brain phase-locking and perceived social closeness but reduced behavioral synchrony; the instructed respiratory phase relationship was mirrored in their inter-brain relative phase. Differences in phase of respiratory cycles (inhalation and exhalation) also influenced coordination: Inhalation was associated with stronger inter-brain synchronization but less precise and stable behavioral coordination than was exhalation. Moreover, Individual differences influenced coordination: Partners with the faster SPR consistently preceded the slower partners and exhibited an alpha-band inter-brain phase advance in synchronization, whereas dyads with similar SPR showed greater behavioral synchrony and smaller inter-brain phase difference. Our findings identify respiration as a previously overlooked factor that organizes behavioral and neural synchronization, while individual intrinsic rhythms constrain how interacting partners align in time. Together, these findings reveal a dynamic interplay between the rhythms of the body and brain during human social interactions.