Social relationship is associated with distinct patterns of inter-brain synchrony and directed connectivity during interpersonal coordination

How social relationship and effector pairing relate to behavioral coordination and inter-brain interaction during joint timing remains unclear. Using functional near-infrared spectroscopy hyperscanning during a dyadic synchronization-continuation task, we examined 21 romantic couples and 23 stranger dyads performing hand-hand and hand-foot coordination. Romantic partners showed smaller tapping deviations than strangers during continuation, while hand-foot coordination was associated with greater tapping variability. Motor-region inter-brain synchrony (IBS) was greater in romantic partners than in strangers. True-dyad IBS exceeded the pseudo-dyad baseline in the right dorsolateral prefrontal cortex, corresponding differences were not detected in the motor cortex or left temporoparietal junction. Dynamic IBS analysis identified two recurrent configurations differing in synchrony strength and global efficiency, but state-transition frequency did not differ significantly between relationship groups. Granger-causality-based directed connectivity was stronger overall in stranger dyads, with female-to-male values exceeding those in the reverse direction. Together, these findings indicate that social relationship is associated with contrasting patterns of motor-region synchrony and directed connectivity, while effector pairing is associated with behavioral variability.