Human Nature - from the New Scientist:

Take the classic "ultimatum" game. Player 1 is given 10, part of which he has to share with Player 2. He can offer to hand over as much or as little as he likes, and Player 2 can accept what is offered or not. But if Player 2 turns the sum down, neither player gets anything. They play only once, and have no opportunity to get to know one another.

In this situation, Player 2 loses out by turning down any offer, no matter how small. Yet in reality, Player 2 frequently does reject "unfair" offers - on average anything less that 2.59 is rejected and both lose out.

These and a host of similar laboratory games suggest that humans have a strong sense of fairness. They will, for example, punish cheats in more extended games even if doing so leaves them out of pocket. An ultimatum game reported this year in Nature (vol 422, p 137) by Ernst Fehr of the Institute for Empirical Research in Economics in Zurich shows that a desire for fairness extends further. When one player is given powers to fine the other, unfair use of the fine to enforce cooperation can turn out to damage it. On the other hand, cooperation can be enhanced if a player voluntarily refrains from using the fine.

Robert Frank of Cornell University has provided a broader argument that many human emotions are designed to lead people away from pure "calculative rationality". What better way to keep reciprocity going than for it to be known how angry you get if you are cheated? Rage deters cheats, guilt makes cheaters feel bad, compassion produces compassion and loyalty makes people keep agreements. In essence, emotions may be the brain's way of making us do things - keeping our word, refusing to be cheated, cooperating - that will pay off for individuals long-term and stop us being what the economist Amartya Sen calls "rational fools".

Evidence from individuals with brain damage supports this view. One of neurologist Antonio Damasio 's well-known cases was of a man with damage to the prefrontal cortex who appeared to have lost connection to his emotions. In a specially designed gambling game he would inevitably pursue immediate short-term gain, even though it would lead to ultimate loss. The patient never seemed to be able to develop the "gut feel" that told him to steer away from this strategy, and unlike normal people playing the same game successfully he failed to show the galvanic skin responses that indicate emotional arousal.

People are not, morality-wise, blank slates. We have inborn notions of right and wrong.