A four-level meta-analysis and review of the joint Simon effect

For two decades, the joint Simon effect (JSE) has been a cornerstone of joint action research. Initially presented as evidence that humans spontaneously represent others’ actions in a functionally equivalent way to their own, the JSE now serves as an interdisciplinary measure of self-other integration across research on social cognition, human-robot interaction, and animal cognition. However, whether the JSE is better explained by non-social mechanisms remains an open question. The present meta-analytic review sought to provide an overview of the JSE literature and to evaluate its suitability as a measure of self-other integration. Our meta-analysis of 274 JSEs from 80 studies (7042 participants across 186 samples) revealed a significant overall effect of gD = .60. Analysis of 52 canonical JSEs (reflecting the seminal study) revealed a larger JSE of gD =.72. Finally, moderation by study characteristics suggested that JSEs at least partly reflect social representations of one’s co-actor. Importantly, our findings lend support to the continued use of the JSE as a measure of self-other integration.