An amygdala-mediated simulation-to-decision cascade aligns interbrain dynamics to solve social dilemmas.

Mutual cooperation (MC) requires independent decision-makers to coordinate under uncertainty about each other's intentions. Using simultaneous intracranial electroencephalography from epilepsy-patient dyads playing a real-time iterated prisoner's dilemma, we identified neurocognitive mechanisms that enable interacting brains to align during mutual cooperation. Amygdala interbrain representational alignment (IBRA), similarity in fine-grained neural spectral patterns across partners, emerged selectively during MC and predicted trial-by-trial outcomes. Early θ-suppression-characterized IBRA reflected rapid, history-sensitive alignment, whereas later IBRA, characterized by low-frequency suppression and γ increases, supported coordinated commitment and action. Furthermore, mutual cooperation relied on an amygdala-mediated, simulation-to-decision cascade, operationalized here as a model-based prospective estimate of partner cooperation that guided self-choice. Amygdala low-frequency suppression tracked this prospective estimate of partner cooperation and θ-γ coupling linked this partner estimate to self-decision processes. Conditional cooperators strategically relied on partner tracking, whereas unconditional cooperators leveraged dispositional tolerance of uncertainty to transition into anticipatory neural alignment. These findings reveal the neural interdependency underpinning MC, redefining cooperation not as isolated decision-making but as a dynamic, interbrain, cognitive alignment.