Abstract
<p>When people talk, their bodies often move together, usually unnoticed. Researchers have studied this synchrony as one behavior. We show that it is several, and that they resolve into two distinct regimes: one for friends and one for strangers. To do so, we analyzed 387 conversations between friends and strangers using markerless pose estimation. We identify three separable forms of body motion synchrony (quantity, directional, and pseudo-synchrony) and largely explain them through four mechanisms (turn-taking, conservation of motion, entrainment, and station keeping). Both the forms and mechanisms of synchrony differ between friends and strangers. Friends synchronize in strong, simple way, moving as a single embodied system (“co-enaction"). Strangers synchronize in a more heterogenous regime, navigating a set of norms (“regulation”). These findings suggest that body motion synchrony is best understood not as single behavior but as paralinguistic scaffolding, its form shaped by the conversation – and relationship – it supports.</p>