Market economic theory and simple models of evolution suggest individuals (or their genes) act to maximise their own benefit. But altruism is common, as apparently witnessed by festive gift-giving. The standard evolutionary explanation is "reciprocal altruism" (give me a gift and I'll give you one just as nice). One of the most widely used experimental setups to investigate the origins of altruism is the "ultimatum game". Two subjects are asked to share a cash sum of say £100. One of them (the proposer) decides the cut - who gets what. The other (the responder) can either accept the share offered or toss the money back in the proposer's face, in which case, neither of them takes any of it away.
They play the game only once, so there's no opportunity to develop reciprocal altruism. If the responder behaves entirely and rationally selfishly, he or she should accept whatever the proposer is prepared to give. But if the proposer offers less than £25, the other player tends to refuse the share and both leave empty handed. Most people are prepared to forsake personal benefit to punish selfishness. In the language of evolutionary psychologists, we are spiteful.
Evolutionary psychologists recognise four basic forms of social behaviour: mutualism, selfishness, altruism and spite. Altruism has been a focus of attention over the last few decades. But spite, altruism's evil twin, is puzzling. Whereas lots of examples of altruism can be found in the animal kingdom, spite is rarer and may be uniquely human.
Keith Jensen and colleagues at the Max Planck Institute for Evolutionary Anthropology in Leipzig sought to investigate whether chimpanzees are capable of spite. In an ingenious experiment they persuaded chimps to play the ultimatum game. A pair of chimps is encouraged to cooperate to receive a reward: a quantity of raisins. The proposer is able to choose between trays of raisins that are divided either fairly (50:50), selfishly (80:20) or altruistically (20:80); and makes an offer by pulling the chosen tray forward. However, the trays are still out of reach. For either chimp to reach the food, the responder has to play his part by pulling on a rod that brings both shares within reach of both chimps. The questions are what kind of offers is the proposer likely to make and what will the responder chimp be willing to accept?
If the players were human, the researchers would expect proposers to make mostly fair offers and the responders to refuse selfish divisions. But chimpanzees were selfish proposers (they mostly made unfair offers) and hardly ever refused, no matter how unfair the division. Unlike humans, chimpanzees do not seem to be sensitive to fairness and behave according to traditional economic models of self-interest.
The research suggests that spite is indeed a uniquely human trait. Jensen believes that it might play a crucial role in human society by protecting cooperative behaviour from cheats. "In the absence of punishment," Jensen says, "cooperation falls apart, because it only takes a few selfish individuals ... to ruin everything for everybody. But if you give people the opportunity to punish free riders, they stop cheating." Whatever its origins, we should all be wary of this uniquely human trait when wrapping that pack of assorted socks.
· Johnjoe McFadden is professor of genetics at the University of Surrey