sounds a little far-fetched. Nowak, though, had explored the idea of indirect reciprocity in detail with the mathematician Karl Sigmund in Vienna. They had recently published a paper showing how indirect reciprocity might actually work, using the mathematics of game theory (in the form of the Prisoner’s Dilemma) to make the point. The secret to altruism, Nowak suggested, is the power of reputation. “By helping someone we can increase our reputation,” he said, “and to have a higher reputation in the group increases the chance that someone will help you.”

The importance of reputation explains why human language became important—so people could gossip. Gossip spreads reputation, making altruistic behavior based on reputation more likely. “It’s interesting how much time humans spend talking about other people, as though they were constantly evaluating the reputations of other people,” Nowak said. “Language helped the evolution of cooperation and vice versa. A cooperative population makes language more important…. With indirect reciprocity you can either observe the person, you can look at how he behaves, or more efficiently you can just talk to people…. Language is essential for this.”¹⁴

Reputation breeds cooperation because it permits players in the game of life to better predict the actions of others. In the Prisoner’s Dilemma game, for instance, both players come out ahead if they cooperate. But if you suspect your opponent won’t cooperate, you’re better off defecting. In a one-shot game against an unknown opponent, the smart play is to defect. If, however, your opponent has a well-known reputation as a cooperator, it’s a better idea to cooperate also, so both of you are better off. In situations where the game is played repeatedly, cooperation offers the added benefit of enhancing your reputation.

Gossip about reputations may not be enough to create a cooperative society, though. Working out the math to prove that indirect reciprocity can infuse a large society with altruistic behavior turned