human civilization, to explain how selfish individuals manage to cooperate sufficiently well to establish elaborate functioning societies. Smith’s basic answer was the existence of sympathy—the ability of one human to understand what another is feeling. Modern neuroscience has begun to show how sympathy works, by identifying “mirror neurons,” nerve cells in the brain that fire their signals both in performing an action and when viewing someone else performing that same action.

Other neuroscientific studies have identified the neural basis of both individual behavioral propensities and collective and cooperative human behavior. Scientists scanning the brains of players participating in a repeated Prisoner’s Dilemma game, for instance, have identified regions in the brain that are active in players who prefer cooperating rather than the “purely rational” choice to defect.14

Another study used a version of the trust game to examine the brains of people who punish those who play uncooperatively (by keeping all the money instead of returning a fair share). In this game, players who feel cheated may assess a fine on the defector (even though they must pay the price of reducing their own earnings by half the amount of the fine they impose). People who choose to fine the defector display extra activity in a brain region associated with the expectation of reward. That suggests that some people derive pleasure from punishing wrongdoers—the payoff is in personal satisfaction, not in money. In the early evolution of human society, such “punishers” would serve a useful purpose to the group by helping to ostracize the untrustworthy noncooperators, making life easier for the cooperators. (Since this punishment is costly to the individual but beneficial to the group as a whole, it is known as “altruistic punishment.”)15

Such studies highlight an essential aspect of human behavior that a universal Code of Nature must accommodate—namely that people do not all behave alike. Some players prefer to cooperate while others choose to defect, and some players show a stronger desire than others to inflict punishment. A Code of Nature must accommodate a mixture of individually different behavioral ten-

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