Classical economical theory sees rejections of unfair offers by people playing the Ultimatum Game (UG) as “irrational”. Recent studies suggested that these are triggered by negative emotions, such as frustration (Sanfey et al., 2003; van’t Wout et al., 2006) and by the urge to punish those who made the offers (Fehr & Gachter, 2002). Another account postulates that rejections are instead “rational” according to the rules of social exchange reasoning, in that they will increase the chance of future players to receive fair offers (Zamir, 2001). We tested these two accounts by employing healthy participants in modifi ed version of the UG in which players knew that their putative rejections were not harming those who made offers. The analysis of skin conductance responses shows that this task was signifi cantly less emotionally arousing than the traditional UG game. However, unfair offers were rejected at a comparable rate in both the classical and modifi ed versions of the Ultimatum Game. In light of these results, theories holding rejections as triggered by emotional arousal and by the urge to punish who made the offers should be re-discussed; in fact, our data suggest that the emotional response might be triggered whenever one’s own interest is at stake, and is not the ultimate cause of this behavior. We believe instead that any offer leading to an unfair distribution of money within the group is suffi cient to trigger a rejection and, therefore, that psychological mechanisms which account for social exchanges dynamics might be better candidates for explaining this behavior.