Philosophers as diverse as Plato, Immanuel Kant and John Stuart Mill tried hard to argue that there is a rational basis for fair and just behavior. However, the best philosophy in the world is only worth so much when there is the chance to make bucket-loads of cash.

And when Darwinism got twisted out of shape by nonscientists, it seemed, wrongly, that there was a biological imperative to selfish behavior. After all, wasn’t life all about the “survival of the fittest?” (Herbert Spencer’s phrase, not Darwin’s).

It is a view that has been hard to kill, and the Gordon Gecko mentality of the 1980s didn’t help. But evolutionary psychologists have long recognized that humans have a sense of fairness, even an inbuilt sense of fairness.

Last year, John Tooby of the University of California, Santa Barbara, published work suggesting that human brains have neural circuits dedicated to detecting cheaters. His research team also found that people from different cultures are equally good at spotting unfair behavior.

Obviously, the ability to detect cheating and unfairness is important in humans: social animals who regularly work and trade together. Human brains have a separate mental component dedicated to the detection of cheating, Tooby concluded. But what another research group has now found is that nonhuman primates seem to have a sense of justice, too.

Sarah Brosnan and Frans de Waal, of the Yerkes National Primate Research Center of Emory University, Atlanta, Ga., have shown that brown capuchin monkeys respond negatively if they see other monkeys getting a greater reward for doing the same job. Such a negative reaction is often seen in humans and is apparently based on our universal sense of fairness, but the new study is the first to experimentally confirm this trait in nonhuman primates.

The findings were published last week in Nature. The paper carried a title more similar to a newspaper headline about a union dispute than to one normally found on scientific reports: “Monkeys reject unequal pay.”

Brosnan and de Waal taught capuchins to swap tokens for food. Normally, monkeys were happy to swap their tokens for cucumber. Capuchins happily eat chunks of cucumber, but they much prefer juicy grapes. What the researchers found is if the monkeys saw another monkey getting a piece of grape, they took offense. Some refused to pay, others took the cucumber, but disdained to eat it. The animal’s umbrage was even greater if the other monkey was rewarded for doing nothing.

Previous experiments with humans have shown that they become less cooperative if treated unfairly, and punish the uncooperative — even if their own pay-off declines as a result. This latest result suggests that this sense of equality may be common among social primates. Tantalizingly, it suggests that a sense of justice might have evolved before humans did.

“People often forgo an available reward because it is not what they expect or think is fair,” said Brosnan. “Such irrational behavior has baffled scientists and economists, who traditionally have argued all economic decisions are rational. Our findings in nonhuman primates indicate the emotional sense of fairness plays a key role in such decision-making.”

Brosnan and de Waal conducted four tests, each including two sessions of 25 trials, on pairs of female capuchins. The researchers first gave study monkeys the low-value rewards of cucumbers in exchange for tokens. Then, they measured the study subjects’ responses when the higher-value reward — grapes — were given to their partners for exerting varying levels of work effort.

“We showed the subjects compared their rewards with those of their partners and refused to accept a lower-value reward if their partners received a higher-value reward,” says Brosnan, “This effect is amplified when the partner does not have to work for the reward.”

The researchers recorded a 95 percent completed exchange rate with the subject monkeys during the equity test, in which both subject and partner received cucumber as the reward for the same amount of work. The completed exchange rate fell to 60 percent during the inequity test, in which subjects observed their partners receiving grapes for completing the same amount of work. A further decrease to 20 percent of completed exchanges occurred in the effort-control test, when partners received the higher-value reward for less work.

Cooperation and altruism are often favored by natural selection, despite the widespread feeling among nonbiologists that “selfish genes” run a ruthless, uncooperative show. It’s the reason why social insect colonies work. It pays individual worker bees, for example, to cooperate to raise the eggs and larvae of their mother, the queen bee, because in doing so they are passing on their own genes.

And workers that try to “cheat” are punished. If worker bees lay eggs, other workers destroy them.

Mathematical models, too, predict the evolution of cooperation on purely financial, numerical grounds.

But the work of Brosnan and de Waal supports a different argument. It says (for social primates, at least), that economic decision-making is based as much on an emotional sense of fairness as on rational considerations.

Moreover, they give experimental, biological backing to the famous comment of the British lawyer Gordon Hewart. In a case report in 1923, Hewart wrote that it “is of fundamental importance that justice not only be done, but should manifestly and undoubtedly be seen to be done.”

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