Abstract It is argued that moral ambivalence is best explained through a naturalistic approach that construes morality as a social invention for promoting and regulating social cooperation. Morality accomplishes this function through the shaping not only of behavior but also of motivational structures in human beings. Biological and cultural evolutionary theories identify plausible bases for the emergence of such an invention (e.g., the strength of self-concern combined with capacities to develop other concern, reliance on cultural norms to regulate and direct behavior and motivation). Such bases, together with the common conditions of social cooperation constrain the variety of ways that the function of morality could be fulfilled (e.g., norms of reciprocity are required, and justifications for the subordination of the interests of some to that of others). Within these constraints a plurality of moralities can be true. It is explained how the conditions for what counts as a true morality can vary with the meaning of moral concepts.