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Survival of the Fairest: Understanding Justice

by Belinda Recio

Experiments show that monkeys have an innate sense of fairness

The next time you feel irritated about getting the short end of the stick, and wonder why it’s so hard to let go of your indignation, watch the YouTube video called “Two Monkeys Were Paid Unequally.” The popular video is an excerpt from primatologist Frans de Waal’s “Moral Behavior in Animals” TED talk. With over 10 million views, the video presents an experiment featuring two capuchin monkeys whose behaviors demonstrate that the drive for fairness is more universal than we thought, extending beyond the human species.

The idea for the now-famous experiment arose from the observation of one of de Waal’s students, Sarah Brosnan, while she was working with captive capuchins. She noticed that one of the monkeys would become agitated when it saw another monkey get a better reward for performing the same task. Known as “inequity aversion” in human economics, this behavior prompted Brosnan and de Waal to design an experiment to explore what appeared to be a sense of fairness in these diminutive primates.

The capuchins that participated were previously trained to give researchers small stones in exchange for food. During the experiment, the trained monkeys were paired and placed in adjacent cages, so they could see each other. The TED talk video excerpt shows a few rounds of stone-for-food exchanges between the monkeys and a researcher. During the first round, the monkey on the left gives the researcher a stone and in return receives a piece of cucumber, which it readily eats. The monkey on the right then hands over its stone and is given a grape—a food that capuchins greatly prefer to cucumbers—and it happily eats its reward. The interesting thing is that the monkey on the left immediately notices—with a calm curiosity—that its counterpart has received a much better reward for the exact same trade.

In the second round, the monkey on the left is once again given a piece of cucumber in exchange for a stone. Even before the monkey on the right makes its exchange for a grape, the left monkey starts to throw a tantrum. It tastes the cucumber, then takes it out of its mouth and throws it at the researcher. It pounds the floor with its fists and rattles its cage. It repeats this same performance in the third round—clearly outraged that it is not getting a grape like the monkey on the right.

At this point, most people viewing the video burst into empathic laughter because the monkey’s reaction is so incredibly humanlike. The monkey on the left is clearly getting shortchanged. It is performing the same work as the other monkey—exchanging a stone for food—but isn’t getting the same reward. When it throws the cucumber at the researcher, viewers can’t help but imagine it shouting, “You can take this job and...”

Brosnan and de Waal’s experiment showed that monkeys’ sensitivity to fairness was linked to effort. If they did not work (make an exchange) but were given unequal food portions, the monkeys didn’t have negative reactions to any difference in the rations. However, if treats were used as payment for work—for the exchange of the stone—then the capuchins did notice inequities. In fact, the more effort it took to receive the reward, the more sensitive a monkey was to seeing another get something better.

Researchers are starting to believe that a sense of fairness is most likely an evolutionary adaptation that developed in certain social species. When discussing the capuchin fairness experiments, for example, de Waal compared the similarity between the responses of the monkeys and chimps to the protesters in the Occupy Wall Street movement: all were objecting to the idea of economic injustice. As de Waal pointed out, if eating were the only thing that mattered, the monkeys would have as readily accepted the cucumber as the grape. But fairness matters, too. Over time, if inequities are allowed to perpetuate they become exploitative and have a detrimental impact on the health of individuals, groups, and perhaps even entire species.

So, getting miffed over inequities may be a survival strategy. Perhaps evolution has favored emotions like indignation because it invokes behaviors that foster fairness. And when a species behaves fairly, not only is each individual better off, but the species as a whole is, too.

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