In economic experiments decisions often differ from game-theoretic predictions. Why are people generous in one-shot ultimatum games with strangers? Is there a benefit to generosity toward strangers? Research on the neural substrates of decisions suggests that some choices are hormone-dependent. By artificially stimulating subjects with neuroactive hormones, we can identify which hormones and which brain regions participate in the decision-making, to what degree and in what direction. Does one hormone make subjects generous and another stingy? In this paper, two laboratory experiments are...