Babies, that is. They seem to have a grasp VERY early on of good and bad actions towards others. At least, they much prefer to be with creatures that are helpful to others.
The research is done in Paul Bloom’s lab at Yale. Below is a clip of nine month olds, but the NY Times has one that shows 3 month olds.
In his Times article, Bloom says:
A growing body of evidence, though, suggests that humans do have a rudimentary moral sense from the very start of life. With the help of well-designed experiments, you can see glimmers of moral thought, moral judgment and moral feeling even in the first year of life. Some sense of good and evil seems to be bred in the bone. … Socialization is critically important. But this is not because babies and young children lack a sense of right and wrong; it’s because the sense of right and wrong that they naturally possess diverges in important ways from what we adults would want it to be.
Smart Babies … For many years the conventional view was that young humans take a surprisingly long time to learn basic facts about the physical world (like that objects continue to exist once they are out of sight) and basic facts about people (like that they have beliefs and desires and goals) — let alone how long it takes them to learn about morality.
I am admittedly biased, but I think one of the great discoveries in modern psychology is that this view of babies is mistaken.
I really can’t remember from my son’s childhood how obvious it was that he had such a sense. It may be that the behavior which would show this doesn’t get much manifestation in anything like clear ways. I saw Hamlin present this work and, if I am remembering correctly, they will say that they cannot really be sure it is a moral sense, but it does seem at least to be triggered by what happens to others. It seems possible, then, that it might be the foundations of a moral sense. (As Hume so clearly saw, though he might be surprised at the children having some feeling for the toys.)