Neural and Facial Dynamics of Intergroup Bias in ToM: An fNIRS Study of Smile Discrimination

Intergroup bias modulates both Theory of Mind (ToM) during social judgments, yet behavioural measures alone cannot fully elucidate the underlying mechanisms. Multimodal approaches are therefore needed to uncover the corresponding neural correlates. Here, we used functional near-infrared spectroscopy (fNIRS) to investigate how intergroup bias shapes neural responses during smile discrimination–a proxy for ToM. Thirty-three adults viewed videos of ingroup or outgroup members displaying genuine or posed smiles, judging smile authenticity (ToM) and smiler gender (Gender), with facial expressions recorded. Behaviourally, no intergroup bias emerged; however, neuroimaging data revealed distinct pathways: ingroup smiles engaged the middle frontal gyrus (MFG) and dorsolateral prefrontal cortex (dlPFC), whereas outgroup smiles activated the inferior frontal gyrus (IFG). ToM conditions elicited stronger IFG and superior temporal cortex (STC) responses for outgroup members compared with Gender conditions. Although facial mimicry was absent, integrated fNIRS and facial analyses demonstrated that intergroup bias differentially recruits neural systems during ToM-driven smile processing. These findings indicate that social affiliation and cognitive control networks are sensitive to ingroup cues, while outgroup cues elicit greater mentalizing, potentially reflecting perceptions of outgroup members as less trustworthy and requiring more effortful interpretation.