Neural geometry of social knowledge emerging from observed social interactions

Humans readily infer others' attributes and their complex web of social relationships from everyday social interactions. Yet, the neural mechanisms that transform these transient interactions into structured, multidimensional social knowledge remain unknown. Here, using a naturalistic fMRI paradigm, we develop a computational framework demonstrating how the human brain factorizes and integrates dynamic social interactions to construct multiplex social knowledge. This approach not only predicts neural responses during movie-viewing, but also enables the reconstruction of subjective social cognitive maps directly from brain activity. Crucially, the relational geometry of these reconstructed maps accurately predicts inferred personality traits, suggesting that relational and trait knowledge emerge from a shared neural representation reflecting interactional dynamics. Together, these findings provide a unifying computational framework for how the brain constructs multiplex social knowledge from observing naturalistic social interactions over time.