Sean McWeeny ^1,2* Adam Luoma ¹ Yaseen Al-Saleem ¹ Laurel J Trainor ^1,2,3

• ¹ Department of Psychology, Neuroscience, & Behaviour, McMaster University, Hamilton, Canada
• ² Institute for Music and the Mind, Faculty of Science, McMaster University, Hamilton, Ontario, Canada
• ³ Rotman Research Institute (RRI), Toronto, Ontario, Canada

Purpose: Music performance facilitates prosociality across many cultures and contexts (Savage et al., 2021). Interestingly, the relationship between prosociality and sensorimotor synchronization (SMS) has so far primarily been demonstrated in the context of in-phase synchrony with only a few mixed results for anti-phase coordination. In anti-phase coordination, participants move at the same rate, at opposite phases, which also requires high levels of coordination and attention. This case is particularly relevant for music and prosociality, as music regularly involves naturalistic anti-phase coordination. We thus tested whether anti-phase synchronization is as effective as in-phase synchronization at eliciting prosocial behaviour. Methods: Dyads (N = 50 dyads) were randomly assigned to complete four trials of a drumming sensorimotor synchronization-continuation task (SCT) either alone, synchronously or in anti-phase. Before and after the drumming task, dyads completed a behavioural economics game involving trust. Additionally, a questionnaire about trust, cooperation, affect, and similarity was given after the drumming task. Results: Cooperation rates in the stag-hunt game were near ceiling (~87%) across all conditions pre-SCT, with negligible change after the drumming task. Questionnaire items were analyzed using Bayesian probit mixed effects models to account for dyadic sampling and ordinal data, and to provide evidence in favor of the null hypothesis. Models provided moderate to extremely strong evidence that the anti-phase and in-phase coordination conditions rated their affect, trust, similarity, and cooperation more strongly than dyads in the alone condition (all BF 10 > 3). When only comparing the anti-phase and in-phase conditions, moderate evidence in favor of the null (i.e., that phase does not affect ratings) was found for all questions (all BF 10 < .3). Descriptions of the posterior, as well as leave-one-out cross validation (LOO) results, were in general accordance with the Bayes Factor results. Conclusion: Evidence indicates anti-phase drumming coordination is as effective as in-phase in increasing perceived trust, cooperation, affect, and similarity. Future analyses will examine how other characteristics of the drumming coordination, such as the lag-1 autocorrelation and variability of the inter-tap interval time-series, relate to prosocial behaviour and ratings of trust and cooperation.