Title: You Are What You Sync: Neural Synchronization Predicts Religious Affiliation

Existing research has found that shared neural responses to naturalistic narratives are associated with shared understanding, especially among members of the same social group. In this fMRI study, we tested whether in-group neural synchronization solely reflects explicit shared understandings, or relates to other group characteristics. To do so, we compared the neural synchronization patterns among two distinct social groups: Religious (N = 21) and Secular (N = 21). Participants were scanned while watching short video clips containing religious or neutral content and then answered questions regarding their reactions to- and interpretation of the videos. Behavioral results did not reveal group differences in responses homogeneity: neither in the engagement and agreement while watching the videos, nor in the emotions and verbal reactions elicited by it. However, neuroimaging results revealed that religious participants exhibited strikingly higher in-group synchronization than secular, both for religious and neutral narratives. Remarkably, it was possible to predict individuals religious affiliation solely based on their in-group neural synchrony, with accuracy scores of up to 92%. This pattern was consistent across all neural networks, most prominently in the Default Mode Network (DMN), Control and Attention networks. It also emerged in the Salience network and Somatomotor regions, hinting at neural processes that may foster group cohesion and identification. We propose that increased neural synchrony in the Religious group was driven mainly by greater homogenous social structure, suggesting a novel perspective for interpreting the role of neural synchrony in group dynamics.