Synchrony as a measure of conversation difficulty: Movement coherence increases with background noise level and complexity in dyads and triads

When people interact, they fall into synchrony. This synchrony has been demonstrated in a range of contexts, from walking or playing music together to holding conversation, and linked prosocial outcomes such as development rapport efficiency cooperation. While the basis remains unclear, several studies have found increase when an interaction is made challenging, potentially providing means facilitating interaction. Here we focus on head movement during free conversation. As verbal information obscured conversing over background noise, investigate whether greater high vs low levels well addressing effect noise complexity. Participants held series conversations with unfamiliar interlocutors while seated lab, level changed every 15-30s between 54, 60, 66, 72, 78 dB. We report measures recorded via high-resolution motion tracking at extreme (i.e., 54 dB) dyads (n = 15) triads 11). In both triads, increased coherence compared speech-shaped noise. Furthermore, compare behaviour multi-talker babble, find more complex babble condition. Key differences 0.2–0.5 Hz frequency bands, are discussed terms their correspondence talkers’ average utterance durations. Additional occur higher frequencies only >5 Hz), which may relate backchannel cues (as multiple individuals were listening responding same talker). Not do these replicate prior work indicating interlocutors’ reliance behavioural task difficulty increases, but demonstrate effects using manipulations across different sized groups.