What if participants in a one-trial prisoner's dilemma know before making their decision that the other person has already defected? From the perspective of classic game theory, a dilemma no longer exists. It is clearly in their best interest to defect too. The empathy-altruism hypothesis predicts, however, that if they feel empathy for the other, then a dilemma remains: self-interest counsels defection; empathy-induced altruism counsels not. This motivational conflict should lead at least some empathically aroused individuals not to defect. To test this prediction, we placed 60 undergraduate women in a one-trial prisoner's dilemma in which they knew the other had already defected. Among those not induced to feel empathy, very few (0.05) did not defect in return. Among those induced to feel empathy for the other, almost half (0.45) did not defect. These results underscore the power of empathy-induced altruism to affect responses in a prisoner's dilemma. Copyright © 2001 John Wiley & Sons, Ltd.