EEG synchronisation reveals the impact of group identity and membership duration on social cognitive bias.

Group identity induces social cognitive biases, and membership duration may amplify these effects. This study aimed to examine such bias by analysing similarities in neural processing among individuals in competitive scenarios. The fans of two Japanese baseball teams, the Hanshin Tigers and Orix Buffaloes, watched baseball matches between the teams, and EEG synchronisation was analysed for in-group (same team) and out-group (different team) pairs, considering fan history as a factor representing membership duration. The results revealed that in-group pairs showed stronger centroparietal alpha-phase synchronisation than out-group pairs, suggesting that top-down spatial attention modulated early visual processing in a similar way among in-group members. Furthermore, in-group pairs with longer fan histories exhibited higher parietal alpha power synchronisation, probably reflecting shared engagement and emotional responses, whereas this effect was absent in out-group pairs. Interestingly, longer fan histories were associated with reduced parietal delta and theta phase synchronisations, possibly due to diverse late-stage attentional processes among experienced fans. Additionally, frontal alpha-phase synchronisation increased with fan history, indicating enhanced auditory attention in long-term fans. These findings highlight how group identity and membership duration shape neural processing, and EEG synchronisation analysis provides a robust method for examining biases.