That first swift limbic kick proved key. When given a mild anti-anxiety drug that suppressed the amygdala response, subjects still said they viewed an 80-20 split as unjust, but their willingness to reject it outright dropped in half. “This indicates that the act of treating people fairly and implementing justice in society has evolutionary roots,“ Dr. Gospic said. “It increases our survival.”

David Sloan Wilson, an evolutionary theorist at the State University of New York at Binghamton, sees the onset of humanity’s cooperative, fair-and-square spirit as one of the major transitions in the history of life on earth, moments when individual organisms or selection units band together and stake their future fitness on each other. A larger bacterial cell engulfs a smaller bacterial cell to form the first complex eukaryotic cell. Single cells merge into multicellular organisms of specialized parts. Ants and bees become hive-minded superorganisms and push all other insects aside.

“A major transition occurs when you have mechanisms for suppressing fitness differences and establishing equality within groups, so that it is no longer possible to succeed at the expense of your group,” Dr. Wilson said. “It’s a rare event, and it’s hard to get started, but when it does you can quickly dominate the earth.” Human evolution, he said, “clearly falls into this paradigm.”

Our rise to global dominance began, paradoxically enough, when we set rigid dominance hierarchies aside. “In a typical primate group, the toughest individuals can have their way and dominate everybody else in the group,” said Dr. Wilson. “Chimps are very smart, but their intelligence is predicated on distrust.”

Our ancestors had to learn to trust their neighbors, and the seeds of our mutuality can be seen in our simplest gestures, like the willingness to point out a hidden object to another, as even toddlers will do. Early humans also needed ways to control would-be bullies, and our exceptional pitching skills — which researchers speculate originally arose to help us ward off predators — probably helped. “We can throw much better than any other primate,” Dr. Wilson said, “and once we could throw things at a distance, all of a sudden the alpha male is vulnerable to being dispatched with stones. Stoning might have been one of our first adaptations.”

Low hierarchy does not mean no hierarchy. Through ethnographic and cross-cultural studies, researchers have concluded that the basic template for human social groups is moderately but not unerringly egalitarian. They have found gradients of wealth and power among even the most nomadic groups, but such gradients tend to be mild. In a recent analysis of five hunter-gatherer populations, Eric Aiden Smith of the University of Washington and his colleagues found the average degree of income inequality to be roughly half that seen in the United States, and close to the wealth distribution of Denmark.

Interestingly, another recent study found that when Americans were given the chance to construct their version of the optimal wealth gradient for America, both Republicans and Democrats came up with a chart that looked like Sweden’s. There’s no need to insult the meat in the land of lutefisk.