Mutual Moral Prediction and Brain-to-Brain Synchrony in Dyadic Decision-Making

Moral decision-making often occurs in social settings, requiring individuals to consider others’ values, beliefs, and judgments. This study examined how people adjust their moral choices when deciding together, and whether such adjustments are guided by predictions about a partner’s moral orientation. Using a dyadic EEG hyperscanning design, four pairs of participants (N = 8) completed a series of moral dilemmas while their brain activity was recorded simultaneously. At two points (before and midway through the task) participants predicted whether their partner would respond in a more utilitarian or deontological way. These predictions were compared with the partner’s actual choices to measure prediction accuracy, belief updating, and behavioral alignment over time. Behavioral results showed greater alignment during joint than individual decisions, indicating that social interaction promotes convergence in moral decision making. Predictions of deontological responses were generally more accurate, reflecting a shared bias toward harm avoidance and moral norm adherence. Confidence did not reliably predict accuracy, suggesting limited metacognitive awareness of predictive success. Neural findings, although preliminary, were consistent: inaccurate predictions elicited stronger feedback-related negativity (FRN) amplitudes, while the error-related negativity (ERN) showed a small reversed trend, possibly reflecting cooperative monitoring rather than self-focused error detection. Mid-frontal theta power was slightly higher during accurate predictions, and inter-brain synchrony analyses revealed weak but measurable frontal coupling. Together, these exploratory findings suggest that mutual moral prediction fosters behavioral convergence and subtle neural alignment, highlighting the predictive and social nature of moral decision-making.