Testing the causal relationship between interpersonal closeness and inter-brain synchrony.

Previous research has suggested a relationship between interpersonal closeness and inter-brain synchrony (IBS), but evidence is correlational, and the behavioural mechanisms involved remain unclear. In this study, we experimentally manipulated closeness in 123 pairs of strangers to test its causal effects on IBS and motor synchrony. Using mobile EEG hyperscanning and video recordings, we measured IBS and motor synchrony as pairs engaged in three between-subject conditions: no-interaction control, small-talk, or closeness-generating conversation. The manipulation successfully increased perceived closeness: the closeness-generating conversation led to significantly greater closeness than small talk. However, IBS and motor synchrony did not differ between the two conditions, nor were they associated with self-reported closeness. In contrast, social interaction strongly influenced synchrony: interacting pairs showed greater delta (1–4 Hz) IBS and higher motor synchrony compared with non-interacting pairs. Importantly, although both delta IBS and motor synchrony were higher during social interaction, motor synchrony did not account for the IBS increase, suggesting that other aspects of social interaction drove the observed neural coupling. Although we found no causal effect of closeness on IBS, these findings suggest that delta-band IBS could be a marker of social interaction.