John MacNamara once proposed that children come to learn about right and wrong, good and evil, in much the same way that they learn about geometry and mathematics. Moral development is not merely cultural learning, and it does not arise from innate principles that have evolved through natural selection. It is not like the development of language or sexual preference or taste in food.
Instead, moral development involves the construction of a intricate formal system that makes contact with the external world in a significant way. This cannot be entirely right. We know that gut-feelings, such as reactions of empathy or disgust, have a major influence on how children and adults reason about morality. And no serious theory of moral development can ignore the role of natural selection in shaping our moral intuitions. But what I like about Macnamara's proposal is that it allows for moral realism. It allows for the existence of moral truths that people come to discover, just as we come to discover truths of mathematics. We can reject the nihilistic position (help by many researchers) that our moral intuitions are nothing more than accidents of biology or culture.
And so I believe (though I cannot prove it) that the development of moral reasoning is the same sort of process as the development of mathematical reasoning.