Interpersonal synchrony, such as dancing to a shared rhythm, elicits bonding and rewards, leading many to see it as a mechanism for group cohesion. Yet synchronized activities also reduce ingroup bias, rapidly bond strangers, flatten hierarchies and blur roles, and evoke trance states. Synchrony thus softens many boundaries that “groupishness” requires. Perhaps in consequence, cultural authorities frequently condemn expressive dance music rather than embracing it as a tool for social cohesion. Aiming to account for these multifarious features, this review integrates findings from cognitive motor science, biomusicology, and adjacent fields to synthesize the relaxed priors through synchrony (RePS) model. Highlighting synchrony’s effects on high-level social predictive processing, RePS depicts the sensorimotor signals associated with beat perception and synchrony as lending precision to bottom-up prediction error at temporally granular (< 2-second) scales. Precision for abstract social priors is thereby reduced, inverting the social brain’s characteristic “top-heaviness.” With categorical beliefs about roles and groups no longer scaffolding coordination and joint action, local-timescale planning and processing evoke low psychological construal. Tighter links between motivation and reward facilitate disinhibited enjoyment orthogonal to status and group boundaries. In the entropic cognitive state that results, relaxed social priors are more easily revisable, enabling coordinated change and updating, as in ritual celebrations. This model affords rich opportunities for empirical testing and interdisciplinary theory-building. Overall, RePS explains why authorities frequently dislike synchrony, why dance plays centrally in social transitions such as weddings, and why rhythmic music is associated with trance. More than a bonding device, synchrony facilitates change.