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Pupillary correlates of computations underlying human responder behaviour in the Ultimatum Game

Negotiating with others about how finite resources (e.g. achievement-based rewards) should be distributed is an important aspect of human social life. However, little is known about the mechanisms underlying value computations during goal-driven social interactions. Here, we report pupillary correlates of computations underlying human responder behaviour in a novel Ultimatum Game (UG) task, in which the opponent’s facial emotions and offer amounts were drawn probabilistically based on the participant’s (acting as the responder) decisions, creating a gradually evolving social interactive decision-making environment. Our model-free results confirm the prediction that both the opponent’s facial emotions and the offer amount significantly influence participant choice behaviour. These main effects clearly demonstrate that human behaviour violates the assumptions of the Rational Actor model (the optimal decision strategy for the UG) which posits that in order to maximise the reward rate all offers should be accepted irrespective of the offer size and the opponent’s facial emotions. Model-based results extend these findings and demonstrate that participants use an inequality aversion model while computing decision values in the UG. We show that the opponent’s facial responses to participant decisions influence how people perceive inequality, relaxing its otherwise negative influence on decision values. This cognitive model underlies how offers initially rejected can gradually become more acceptable under affective load. Our pupillary results highlight that negative affective reactions from the proposer, in response to participant decisions, lead to significant pupil dilation. We further demonstrate that a model-based surprise response is encoded in pupil size and also influence participants’ acceptance probability negatively. Our results highlight that surprising offers which violate participant’s expectations, for better or for worse, are less likely to be accepted.