In this particular version of the game, the researchers got people to play sequentially: one would go and then the other, fully aware of what the first had done. In theory, anyone thinking only of their own personal gain would always cheat, as this pays more than cooperating. But in the experiments, although many of players who went first did cheat, others cooperated, despite knowing that the second player could sucker them by cheating. What's more, roughly half those who went second rewarded cooperation by treating their opponent fairly, even though that meant forgoing an easy pay-off for themselves (Human Nature, vol 13, p 1). "The facts are clear," Fehr says. "Many people are willing to cooperate and to punish those who don't, even when no gain is possible."
This tendency - which researchers call "strong reciprocity" - throws into question the assumption that apparently selfless behaviour must have some selfish explanation. Across disciplines, researchers now agree that people often act against their own self-interest. "This is the most important empirical work on the human sense of justice in many years," says evolutionary biologist Robert Trivers of Rutgers University in New Jersey.