My Fair Monkey.

Justice would seem a human concept if ever there was one. Its allure has stirred souls as diverse as American and French revolutionaries, British abolitionists and sign-wavers at the World Trade Organization talks last week in Cancun. Many of the world's religions think of justice in sacred terms, as something ultimately meted out by God. But among more secular thinkers the only question is whether it's an inalienable part of who we are, or--as many economists seem to believe- -a patina of culture slapped onto the human animal, who is forever on the verge of reverting to his natural state of brute selfishness. Human motives, the latter argue, come down to one thing: self-interest.

The latest salvo in this longstanding debate comes not from philosophers, economists or priests, but from the lab. After a four- month study of capuchin monkeys, Frans de Waal and Sarah Brosnan of Emory University came to a startling conclusion: these primates have a rudimentary sense of justice, and in some cases are willing to take a stand against injustice even at the cost of their own well-being. The findings, published last week in the scientific journal Nature, suggest that justice is not so much a product of nurture, a lesson taught by parents and community, as an evolved trait. For humans, the implications are clear: if lower primates have an inborn sense of justice, humans probably do, too. Does this mean that Thomas Hobbes and Adam Smith were wrong about our selfish natures? "Economists have always disregarded the sense of fairness because it's not considered to be a rational behavior," says Brosnan. "Now it looks like it was there long before advanced cultural institutions came about."

So how did Brosnan and de Waal find the sense of fairness in primates? They began with a pair of brown capuchin monkeys, chosen precisely because they're extremely interdependent creatures known to be good at cooperating with one another. They gave the monkeys tokens, and then, with outstretched hand, palm up, asked for the token in exchange for a slice of cucumber. The monkeys at first seemed happy with this barter arrangement, willingly giving up a token for a reward.

Things really got interesting when the scientists began treating the monkeys unfairly. They gave one monkey, in full view of her partner, a sweet, juicy grape. Then they turned to the partner and offered not a grape but a plain old cucumber. Often, the reaction was emotional and decisive: she spurned the cucumber--even though she had no reason to expect that her action would prod ...