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Method

Participants and procedure Participants (n = 102, M_age = 20.5, SD = 3.91) were undergraduates who participated in exchange for course credit. The procedure consisted of three stages that were run on subgroups of participants in sessions [...]. In Stage 1 of each session (minimal group formation), each participant was randomly assigned to one of two groups and wore group-specific color-coded pinnies (red or blue). [...] The facilitator (research assistant) for each group instructed members to get acquainted by introducing themselves and disclosing one piece of personal information [...]. Each group then completed a cooperative team-building exercise. [...] In Stage 2 of each session (movement synchrony manipulation), the two minimal groups were brought together to form one aggregate group of six participants. [...] Next, participants were provided with personal headphones and asked to tap their hand on the table to the musical beat they heard. Each aggregate group was randomly assigned to one of three movement synchrony conditions: (a) intergroup synchrony (all individuals tapped to the same musical beat), (b) in the intragroup synchrony (each minimal group tapped to different musical beat), or (c) asynchrony (each individual tapped to a different musical beat). [...] In Stage 3 of each session (public goods game and questionnaire), participants played a public goods game to assess willingness to cooperate within and across group boundaries. Participants were instructed to divide 10 tokens among a private account, a minimal-group account, or an aggregate-group account. [...] The game measures how willing the participants are to cooperate with either the minimal group (within group boundaries) or the aggregate group (across group boundaries). [...] The fifth round was used as the outcome measure of cooperation. Participants were entered into a draw following each round. The winner of the draw received an amount in cash equivalent to their earnings from one trial. Following the public goods game, participants completed a questionnaire. [...]