Relational context shapes interpersonal coordination in naturalistic interaction.

Interpersonal coordination - the temporal coupling of movement between interactants - is widely considered to support social interaction, with laboratory studies demonstrating that greater coordination is associated with increased rapport, bonding, and positive affect. However, most of the evidence for these psychosocial benefits comes from tightly controlled interactions involving unfamiliar partners. Whether these findings generalise to naturalistic, real-world interaction remains largely unexplored. The present study examined interpersonal coordination and its psychosocial correlates in friends and strangers. Sixty-two dyads (30 friend pairs, 32 stranger pairs) completed a cooperative scavenger hunt task across a university campus. Movement was captured using motion tracking and analysed via cross-recurrence quantification analysis to characterise coordination dynamics across the interaction. Psychosocial measures - including rapport, connectedness, and affect - were assessed before and after the task. Friends exhibited higher overall levels of coordination than strangers, while strangers showed a significantly faster increase in coordination over time. Coordination metrics, however, showed limited associations with psychosocial outcomes. Although some measures predicted momentary affect in strangers, coordination did not predict rapport change in either group. Instead, psychosocial outcomes were primarily predicted by relationship type, with strangers showing greater increases in rapport and connectedness than friends. Together, findings suggest that the social consequences of interpersonal coordination are contingent on relational context and underscore the importance of studying interpersonal coordination within naturalistic settings.