Although scientific naturalistic philosophers have been concerned with the role of scientific psychology in illuminating problems in moral psychology, they have paid less attention to the contributions that it might make to issues of moral ontology. In this paper, I illustrate how findings in moral developmental psychology illuminate and advance the discussion of a long-standing issue in moral ontology, that of moral realism. To do this, I examine Gilbert Harman and Nicholas Sturgeon's discussion of that issue. I contend that their explorations leave the issue unresolved. To break the stalemate, I appeal to empirical psychological findings about moral internalization—the process by which children acquire the capacity to act in terms of moral norms. I contend that these findings illuminate the issue, suggest a way to advance it, and tend to support a moral realist position.