Learning morality from monkeys

"I'm very interested in the origin of morality," says psychologist Frans de Waal, director of the Yerkes Living Links Center, who studies primates for clues to human evolution. De Waal advocates the theory of Scottish philosopher David Hume, that human morality grew out of moral sentiments, instead of a rational process.

"I believe very strongly that actually (our morality) came out of our psychology, and our psychology is a primate psychology," de Waal says. "In primates you can see many of these same tendencies, you see empathy and sympathy. You see reciprocity and retribution. You see rules that are reinforced, and they beat up someone who doesn't follow the rules."