Sam Harris: Good Biological Reasons.

There are obvious biological reasons why people tend to treat their parents well, and to think badly of murderers, adulterers, thieves, and liars. It is a scientific fact that moral emotions—like a sense of fair play or an abhorrence of cruelty—precede any exposure to scripture. Indeed, studies of primate behavior reveal that these emotions (in some form) precede humanity itself. All of our primate cousins are partial to their own kin and generally intolerant of murder and theft. They tend not to like deception or sexual betrayal much, either. Chimpanzees, especially, display many of the complex social concerns that you would expect to see in our closest relatives in the natural world.