To date, gender differences in ethics have received little theoretical attention. We utilize a social-cognitive framework to explain why these differences emerge and when women engage in less unethical negotiating behavior than do men. We propose that, relative to men, women’s stronger moral identities suppress unethical negotiating behavior. Study 1 establishes a gender difference in moral identity strength through a meta-analysis of over 19,000 people. Study 2 observes gender differences in two aspects of negotiator ethics – moral disengagement and opportunism. Study 3 establishes moral identity strength as an antecedent to negotiator ethics. Finally, Studies 4 and 5 explore financial incentives as a situational moderator. Because financial incentives temporarily decrease the salience of moral identity, they could mitigate gender differences in negotiator ethics, leading women to act more like men. Across both studies, financial incentives impacted women’s (but not men’s) unethical negotiating behavior. Our findings help to explain why and when gender differences in ethics emerge.