Inducing Interpersonal Closeness: Effects on Inter-Brain Synchrony and Collaborative Performance.

Feeling close to another person is widely assumed to facilitate collaboration. However, causal evidence that interpersonal closeness improves collaborative behavior remains limited, and it is unclear whether any such influence is accompanied by increased synchrony at the neural level. Previous studies comparing inter-brain synchrony (IBS) and collaborative performance across relationship types (e.g., friends vs. strangers) have reported better collaboration and higher IBS in closer relationships, but these effects are confounded by familiarity and shared interaction history. To address these limitations, we experimentally manipulated interpersonal closeness in 123 dyads of strangers (N = 246) to test its causal influence on IBS and collaborative performance. Dyads were randomly assigned to one of three conditions: No Interaction, Small Talk, or a closeness-generating conversation. After the manipulation, dyads completed a collaborative task while undergoing mobile EEG hyperscanning. The manipulation was effective: perceived closeness increased significantly between the No-Interaction, Small-Talk, and closeness-generating conversation conditions. However, induced closeness did not affect collaborative performance or IBS during collaboration. These findings suggest that the better collaboration and higher IBS often reported for pre-existing relationships in previous studies may originate from factors beyond momentary feelings of closeness, such as familiarity derived from prior interaction. Interestingly, across all dyads, we observed a negative association between central delta IBS and collaborative performance, with IBS explaining unique variance beyond self-reported measures. This negative link is consistent with the idea that, in this context, IBS reflects speech-tracking and increased cognitive demands during coordination, rather than coordination efficiency.