of players. Further studies suggest why the human race might have evolved to include punishers.

In one such test of a public goods game,18 most players began by giving up an average of half their points. After several rounds, though, contributions dropped off. In one test, nearly three-fourths of the players donated nothing by round 10. It appeared to the researchers that people became angry at others who donated too little at the beginning, and retaliated by lowering their own donations—punishing everybody. That is to say, more of the players became reciprocators.

But in another version of the game, a researcher announced each player’s contribution after every round and solicited comments from the rest of the group. When low-amount donors were ridiculed, the cheapskates coughed up more generous contributions in later rounds. When nobody criticized the low donors, later contributions dropped. Shame, apparently, induced improved behavior.

Other experiments consistently show that noncooperators risk punishment. So it may have been in the evolutionary past that groups containing punishers—and thus more incentive for cooperation—outsurvived groups that did not practice punishment. The tendency to punish may therefore have become ingrained in surviving human populations, even though the punishers do so at a cost to themselves. (“Ingrained” might not be just in the genes, though—many experts believe that culture transmits the punishment attitude down through the generations.)

Of course, it’s not so obvious what form that punishment might have taken back in the human evolutionary past. Bowles and Gintis have suggested that the punishment might have consisted of ostracism, making the cost to the punisher relatively low but still inflicting a significant cost on the noncooperator. They show how game theory interactions would naturally lead societies to develop with some proportion of all three types— noncooperators (free riders), cooperators, and punishers (reciprocators)—just as other computer simulations have shown. The human race plays a mixed strategy.

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