Oxytocin promotes advantageous inequity preference by modulating the fairness norm enforcement system

Aversion to inequity is innate in humans and constitutes a major contributor to social tension. Such aversion can be triggered either when one receives more (advantageous inequity-AI) or less (disadvantageous inequity-DI) than others. However, it remains unclear whether oxytocin can modulate how humans respond to inequity. The present pre-registered study combined behavioral measures, computational modelling, and fMRI in a novel monetary allocation evaluation paradigm to examine oxytocins modulation of behavioral and neural responses to inequity. Results showed that oxytocin increased perceived fairness and AI allocation choices via enhancing AI preference independent of gain vs. loss contexts, which was further confirmed by model-based results of oxytocin decreasing AI aversion. In parallel at the neural level, oxytocin increased fairness rating-related activity in regions within fairness-related norm enforcement systems involved in norm violation detection, salience processing, and cognitive control. Multivariate-based representational similarity analyses provided further evidence that oxytocin induced distinct representational patterns of inequity in these core regions. These results suggest that oxytocin promoted AI preference possibly by strengthening regulatory control to support the pursuit of economic self-interest, while simultaneously enhancing the salience of AI allocations. Our study provides new insights into behavioral and neural mechanisms through which oxytocin modulates fairness-related behavior.