Moral Lessons, Down Aisle 9

a) Move to a village in the Amazon and go foraging with the indigenous Tsimane people.

b) Move to a Dolgan and Nganasan settlement on the Siberian tundra, herd reindeer and join the Russian Orthodox Church.

c) Visit a Himalayan monastery and follow instructions to “gaze within” and “follow your bliss.”

d) Join a camp of nomadic Hadza hunter-gatherers sharing giraffe meat and honey on the Serengeti savanna.

e) Join a throng of Wal-Mart shoppers buying groceries on the Missouri prairie.

Well, the Siberian church might impart some moral lessons, but your best bet is to go shopping, at least by my reading of the experiments reported in the current issue of Science. It doesn’t have to be Wal-Mart, by the way — any kind of grocery store seems to have an effect. Wal-Mart just happens to be popular with the exceptionally fair-minded residents of Hamilton, a small rural town in northwestern Missouri. They scored higher in a test of fairness toward strangers than did any of the less-modern communities in Fiji, Papua New Guinea, Africa, Asia and Latin America.

The study doesn’t prove the moral superiority of Missourians, because traditional societies emphasize different virtues, like providing food and comfort to relatives. But the results do help explain a central mystery of civilization: How did small family clans evolve into large cities of cooperative strangers? Why are New Yorkers sometimes nice even to tourists?

Being nice made evolutionary sense when we lived in small bands surrounded by relatives, because helping them helped our genes survive. And we had a direct incentive to be fair to people who would later reciprocate kindness or punish selfishness. But why even consider returning a stranger’s wallet you find in a taxicab? Why leave a tip in a restaurant you’ll never visit again?

Some evolutionary psychologists have suggested that we have an innate sense of fairness left over from our days of living in small clans. According to this theory, our inherited instincts cause us to be nice to strangers even when we’re hurting our interests, just as our ancient taste for fat and sugar causes us now to eat more calories than are good for us.

But there’s more to it than just inherited instinct, says Joseph Henrich of the University of British Columbia, who led the study’s team of anthropologists, psychologists and economists. They found wide cultural variations by observing more than 2,000 people in 15 small communities participate in a two-player game, called Dictator, with a prize equal to the local pay for a day’s work.

One player, the dictator, was given the authority to keep the entire prize or share part of it with the other, unseen player, whose identity remained secret. Along with this power came the assurance that the dictator’s identity would also remain secret, so that no one except the researcher would ever know how selfish the dictator had been.

The most lucrative option, of course, was to keep the whole prize and stiff the anonymous partner. But the Missourians on average shared more than 45 percent of the prize, and some other societies were nearly as generous, like the Ghanians living in the city of Accra and the Sanquianga fishermen on the coast of Colombia.

But most of the hunter-gatherers, foragers and subsistence farmers were less inclined to share. The Hadza nomads in the Serengeti and the Tsimane Indians in the Amazon gave away only a quarter of the prize. They also reacted differently when given a chance, in variations of the game, to punish another player for hogging the prize.

Selfishness offended the Missourians so much that they would punish the player even though it cost them money. But the members of traditional societies showed little inclination to punish others at their own expense. “There are lots of norms in these small-scale societies for how to treat one another and share food,” says Dr. Henrich. “But these rules don’t apply in unusual situations when you don’t know anything about the kinship or status of the other person. You don’t feel the same sense of responsibility, and you act more out of self-interest.”

The researchers found that people in small communities like the Hadza camp (population about 50) were less willing to inflict punishment than people in larger communities like Hamilton (about 1,800). That makes practical sense: the more strangers there are, the more need to keep them from exploiting one another. But what enabled those larger societies to grow in the first place?

Dr. Henrich and his colleagues identified two distinguishing factors.

People belonging to a modern “world religion,” like the Islamic faith of the Orma cattle herders in Kenya or the Christian faith of the Dolgan reindeer herders in Siberia, tended to share more of their prize than did adherents of local religions. As larger communities became possible after the invention of agriculture, the researchers write in Science, “intersocietal competition may have favored those religious systems that galvanize pro-social behavior in broader communities, perhaps using both supernatural incentives (for example, hell) and recurrent rituals that intensify group solidarity.”

But a second factor seemed even more important. In explaining attitudes toward fairness, Dr. Henrich and his colleagues found that the strongest predictor was the community’s level of “market integration,” which was measured by the percentage of the diet that was purchased. The people who got all or most of their food by hunting, fishing, foraging or growing it themselves were less inclined to share a prize equally.

Grocery shopping may seem an unlikely form of moral education, but the researchers argue in Science that the development of “market norms” promotes general levels of “trust, fairness and cooperation” with strangers. (You can debate that point at nytimes.com/tierneylab.)

“Markets don’t work very efficiently if everyone acts selfishly and believes everyone else will do the same,” Dr. Henrich says. “You end up with high transaction costs because you have to have all these protections to cover every loophole. But if you develop norms to be fair and trusting with people beyond your social sphere, that provides enormous economic advantages and allows a society to grow.”

One such dynamic society was ancient Greece, whose ethical norms spread as it grew, widely, and perhaps it was no coincidence that those ethics were developed by philosophers debating alongside merchants at the central marketplace called the agora. In retrospect, maybe Diogenes and his lamp didn’t really have all that far to go.