In economic experiments decisions often differ fromgame-theoretic predictions. Why are people generousin one-shot ultimatum games with strangers? Is therea benefit to generosity toward strangers? Research onthe neural substrates of decisions suggests that somechoices are hormone-dependent. By artificiallystimulating subjects with neuroactive hormones, wecan identify which hormones and which brain regionsparticipate in the decision-making, to what degreeand in what direction. Does one hormone make subjectsgenerous and another stingy? In this paper, twolaboratory experiments are described using thehormones oxytocin and arginine vasopressin to see ifoxytocin may make subjects more generous and isarginine vasopressin may do the opposite, make peoplestingy. Since these hormones naturally release duringthe day in response to the environment, it makessense to test what happens when they change.