This paper discusses recent neuroeconomic evidence related to other-regarding behaviors and the decision to trust in other people’s other-regarding behavior. This evidencensupports the view that people derive nonpecuniary utility (i) from mutual cooperation in socialndilemma (SD) games and (ii) from punishing unfair behavior. Thus, mutual cooperation and the punishment of free riders in SD games is not irrational, but better understood as rationalnbehavior of people with corresponding social preferences. We also report the results of anrecent study that examines the impact of the neuropeptide Oxytocin (OT) on trusting andntrustworthy behavior in a sequential SD. Animal studies have identified Oxytocin as anhormone that induces prosocial approach behavior, suggesting that it may also affect prosocialnbehavior in humans. Indeed, the study shows that subjects given Oxytocin exhibit much morentrusting behavior, suggesting that OT has a direct impact on certain aspects of subjects’ social preferences. Interestingly, however, although Oxytocin affects trusting behavior, it has no effect on subjects’ trustworthiness.