Positive interactions and active mentalizing facilitate inter-brain synchronization in couples.

High-quality interactions rely on socio-cognitive abilities, which help partners understand and respond to one another. Hyperscanning, which measures brain activity in multiple individuals simultaneously, can examine inter-brain synchronization as a possible signature of social coordination. In this study, we explore this synchronization during natural, everyday interactions in romantic couples. Forty-two dyads participated in a naturalistic behavioral paradigm while EEG was recorded. We measured behavioral dyadic reciprocity and negative states, and computed inter-brain synchronization using the circular correlation coefficient (CCorr) across multiple frequency bands (Theta [4-8 Hz], Alpha [8-13 Hz], and Beta [14-31 Hz]). Positive interactions elicited greater dyadic reciprocity and fewer negative states compared to negative interactions. At the neural level, inter-brain synchronization in the beta band was significantly higher during positive compared to negative interactions. Mentalizing compared to counting elicited increased inter-brain synchronization in both alpha and beta bands, indicating that active sociocognitive processing enhances neural alignment between partners. These findings demonstrate that inter-brain dynamics provide a neural signature of both interaction valence and mentalizing engagement during real-world social interactions, putting forward inter-brain synchronization as a sensitive marker of sociocognitive coordination.