Beyond perfect synchrony: shared interpersonal rhythmic timing enhances self-other merging judgements.

Perfect synchrony is highly prosocial, yet interpersonal rhythms globally exhibit rich variation. In two online experiments, we tested the effect of varying interpersonal rhythms on self-other merging. First, we hypothesized that shared temporal features, acting as attentional frameworks to track and integrate self-other actions, would drive combined representations. Participants viewed and rated self-other pairs producing simple rhythms, polyrhythms and irregular rhythms, at three complexity levels. Merging was unsurprisingly highest for perfect synchrony and declined with other rhythmic ratios. Crucially, simpler polyrhythms were rated higher than irregular rhythms, supporting our tracking-and-integration hypothesis. Second, we tested whether interpersonal rhythmic variation specifically affected self-other merging versus aesthetic judgements, by collecting liking ratings for the identical stimuli. We hypothesized that liking would be driven by overall perceptual features versus interpersonal features. While ratings were unaffected by simple rhythms' ratios, polyrhythms showed a sharp decrease, suggesting that social individuation inherent in polyrhythms additionally affected aesthetic judgements. The distinct liking pattern suggested that self-other merging judgements were specifically linked to the interpersonal nature of rhythmic variation, and not mere aesthetic preferences. Our data are consistent with the hypothesis that interpersonal rhythmic variation evolved to support prosocial bonds by signalling shared intentions and aiding clear self-other distinctions.