The examination of a test case, the popular movement to abolish slavery, demonstrates that the insights of recent psychological research about moral judgment and motivated reasoning can contribute to historians’ understanding of why large-scale shifts in cultural values occur. Moral psychology helps to answer the question of why the abolitionist movement arose and flourished when and where it did. Analysis of motivated reasoning and the just-world bias sheds light on the conditions that promoted recognition of the moral wrongfulness of chattel slavery, as well as on the conditions that promoted morally motivated social action. These findings reveal that residents of Great Britain and the northern United States in the eighteenth and nineteenth centuries were in an unusually good position to perceive, and to act on, the moral problems of slavery. Moral psychology is also applicable to other social issues, such as women’s liberation and egalitarianism.