Expectancy effects threaten the inferential validity of synchrony-prosociality research

A large body of theoretical and experimental work has argued that synchronized movement among people increases prosocial attitudes and behavior. Here we review prior and new evidence that reported effects of synchrony may be driven by experimenter expectancy, leading to experimenter bias; and participant expectancy, otherwise known as placebo effects. To test for the possibility of placebo effects, we asked whether participants have a priori expectations about synchrony and prosociality that match the findings in published literature. In a preregistered experiment, we asked undergraduates (N = 216) to imagine they participated in a synchrony experiment and then predict how they would feel and act afterward. The imagined experiment and the measures of their feelings and actions were taken from a highly cited experiment on synchrony. Even without experiencing an actual synchrony manipulation, participants’ expectations about the effects of synchrony on prosocial attitudes closely matched the actual effects of synchrony reported in the original experiment. The participants who imagined synchronizing expected to feel greater levels of connection, trust, same team feeling, and similarity to their partner, but not greater happiness, than the participants who imagined action that was not synchronized. These expectations (both positive and null) directly mirror reported effects of synchrony, raising the possibility that the synchrony-prosociality literature is vulnerable to placebo effects. Previously reported effects may reflect participants’ top-down expectations about synchrony, rather than the impact of experience with synchrony itself.