In nature, synchrony may emerge spontaneously among entities. Similarly, interacting individuals can align in a form of biological and behavioral synchrony. In this context, inter-brain synchrony (IBS) has been proposed as a mechanism of functional integration between brains, observed in the prefrontal cortex (PFC) and temporoparietal junction (TPJ)—regions critical to Theory of Mind and mentalization—during cooperation, and influenced by relationship closeness. While IBS holds promises for advancing our understanding of social functioning and models of psychopathology, the field still lacks beyond-correlation evidence and mechanistic explanations. This project addresses these gaps through three work packages (WP): WP1 uses a triple-blind randomized controlled trial (RCT) to test how induced relational closeness affects IBS at the electroencephalographic (EEG) level; WP2 applies Kuramoto models to mechanistically describe the non-linear dynamics underlying IBS in both simple and complex social interactions; lastly, WP3 employs a triple-blind RCT to test the causal roles of the TPJ and PFC in generating EEG IBS, using personalized direct and alternating transcranial electrical stimulation techniques calibrated through frequency peaks and topography analysis with generalized eigendecomposition. By integrating experimental and computational methods, this project will contribute to the neural decoding of social interactions, ubiquitous, yet profoundly complex.