Moving in synchrony with others encourages prosocial behavior. Adults who walk, sing, or tap together are later more likely to be cooperative, helpful, and rate each other as likeable. Our previous studies demonstrated that interpersonal synchrony encourages helpfulness even in 14-month-old infants. However, in those studies, infants always experienced interpersonal synchrony in a musical context. Here we investigated whether synchronous movement in a nonmusical context has similar effects on infant helpfulness. Fourteen-month-olds were bounced gently while the experimenter faced the infant and bounced with them either in- or out-of-synchrony. In contrast to our previous studies, instead of listening to music during this interpersonal movement phase while being bounced, infants listened to nonrhythmic nature sounds. We then tested infant prosociality directed toward the experimenter. Results showed that synchronous bouncing still encouraged more prosociality than asynchronous bouncing, despite the absence of music. However, helping was more delayed and fussiness rates were much higher than in our previous studies with music. Thus music may not be necessary for interpersonal synchrony to influence infant helpfulness, but the presence of music may act as a mood regulator or distractor to help keep infants happy and allow them to fully experience the effects of synchronous movement.

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