PROJECT SUMMARY The vertebrate brain has evolved to enable complex social interactions, essential for survival. Brains of animals engaged in a shared social interaction exhibit inter-brain synchronization of neural activity, detectable at several levels of analysis. It remains unclear what aspects of social behavior are driven by these intriguing inter-brain dynamics. We propose to develop and apply a revolutionary set of molecular-genetic and optical tools to record and manipulate neural activity wirelessly and simultaneously in multiple interacting animals from distantly related rodent species across a broad spectrum of sociality. These studies include evaluation of synchrony parameters, along with causal imposition of synchrony to drive social behaviors and attachment. We aim to leverage novel functional genetic approaches to identify the cellular and subcellular basis and neuromodulatory mechanisms that underlie the emergence and strengthening of synchrony. This project will revolutionize our concept of the social brain, placing a multi-dimensional emphasis on neural activity within and across interacting brains, linked by the non-corporeal bonds of shared life. This is critical for understanding the full breadth of human sociality, identity, neurotypical and atypical behavior, as well as mental and physical health.