Relationship between platelet serotonin content and rejections of unfair offers in the ultimatum game

The ultimatum game (UG), a well-studied decision task used in experimental neuroeconomics, represents a simple two-person bargaining between a proposer and a responder. The proposer offers the responder how to split a sum of money. The responder decides whether to accept or reject the offer. When the responder accepts it, each player earns money according to the proposer's offer. If the offer is rejected, neither player gets anything. Rejections of “free” money in the UG represent a deviation from the standard economic model of rationality. This behaviour could be linked to adverse psychological reactions to unfair offers, including anger, hostility and impulsiveness. Currently, it is believed that the most plausible biological system related to anger and impulsivity is the serotonergic system. We hypothesize that serotonergic activity, as measured by platelet serotonin levels, will differentiate subjects who either reject or accept low UG offers. A sample of 60 economy students (31 males and 29 females, mean age: 24.4 ± 2.3 years) was investigated. As predicted, the mean platelet serotonin level was significantly lower in participants who reject unfair offers (€1 out of €10) than in those who accept (2.86 ± 0.13 versus 3.48 ± 0.11 nmol/109 platelets, respectively, p < 0.001). We conclude that low platelet serotonin may serve as a reliable biomarker to identify people who are more likely to reject unfair ultimatum offers in an experimental neuroeconomic setting. Our pilot data seem to indicate that the serotonergic system may play an important role in the UG rejection behaviour.