Why are we so generous?

1. JUST FOR KICKS

In 2002, a team of researchers led by psychiatrist Gregory Berns from Emory University in Atlanta, Georgia, used brain imaging to find out what is going on inside our heads when we cooperate. They discovered that when players work together in the prisoner's dilemma game (see Diagram), the active parts of their brain include the orbitofrontal cortex and the striatum - areas associated with processing reward (Neuron, vol 35, p 395). And, last year, economist Ernst Fehr and psychologist Dominique de Quervain of the University of Zurich discovered that we get a similar mental buzz when we punish cheats, even when it means incurring a personal monetary cost (Science, vol 305, p 1254).

2. IT's GOOD FOR THE IMAGE

Punishing others who don't toe the line can boost your reputation, as a recent study by anthropologists Rob Boyd and Karthik Panchanathan of the University of California at Los Angeles shows. Using computer simulations, they explored the benefits of a strategy of punishment that entails simply shunning others with a bad reputation and helping those with a good reputation. By doing this, individuals can enhance their own standing, they found. What's more, by altering their behaviour according to people's reputations, these individuals minimise the cost of meting out punishment and gain the edge over indiscriminate cooperators who help anyone regardless of reputation (Nature, vol 432, p 499).

3. TO PLEASE TEACHERS (AND GODS)

Despite our altruism, generosity may not be in our genes. If true altruism has evolved through competition between groups, as some researchers maintain (see main story), then it is more likely to be the product of cultural evolution. Genetic evolution works by selecting individuals with traits that are well adapted to their environment, but it has a far weaker grip on traits that benefit the group. So altruism is more likely to be learned. After all, every human culture invests considerable effort in instilling children with moral norms that help further cooperation. Often these are enshrined in powerful religious beliefs and reinforced by promises of salvation and threats of eternal damnation.

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