You might recognize prominent primatologist Frans de Waal from lectures he has given about his research on primate behavior, which have been popularized on YouTube.
His face is familiar to chimpanzees, too; some chimps that he knew as babies still recognize him even after decades apart, he said.
"Chimpanzees have the advantage that you cannot ask them questions, so you have to watch (their) behavior to see what they do," says de Waal, director of Emory University's Living Links Center, in his Dutch-accented voice that is both gentle and authoritative.
He adds, with dry humor: "With humans, you can ask questions and you get all sorts of answers I don't trust, so I prefer to work with chimpanzees for that reason."
Living Links is part of the oldest and largest primate center in the United States: The Yerkes National Primate Research Center, a secluded grassy area in suburban Atlanta where humans work in office trailers and other animals play in open-air compounds.
De Waal, who has been at the center for more than 20 years, has made a career out of finding links between primate and human behavior, particularly in the areas of morality and empathy.
You might think of "morality" as special for humans, but there are elements of it that are found in the animal kingdom, says de Waal -- namely, fairness and reciprocity. His latest study, published this week in the journal Proceedings of the National Academy of Sciences, suggests that chimpanzees may show some of the same sensibility about fairness that humans do.
The popular belief that the natural world is based on competition is a simplification, de Waal says. The strength of one's immune system, and the ability to find food, are also crucial. And many animals survive by cooperating.
"The struggle for life is not necessarily literally a struggle," he said. "Humans are a highly cooperative species, and we can see in our close relatives where that comes from."
Mammals such as wolves, orcas and elephants need their groups to survive, and empathy and cooperation are survival mechanisms. De Waal discusses these mechanisms in his 2009 book "The Age of Empathy: Nature's Lessons for a Kinder Society."
"We think that empathy evolved to take care of others that you need to take care of, especially, of course, between mother and offspring, which is universal in all the mammals," de Waal said.
What it means to be fair
De Waal isn't sure that his monkeys have what a philosopher would call a "concept of justice" in an intellectual sense. But the emotional reactions researchers have observed indicates that there is, at a more basic level, a sense of justice among them.
Among the questions he investigates: If an animal gets more than another, is there is a feeling that this is somehow unjust? And if one shares food with another, is there an expectation of returning the favor?
In a 2008 study, de Waal and colleagues put two capuchin monkeys side by side and gave them a simple task to complete: Giving a rock to the experimenter. They were given cucumbers as a reward for executing the task, and the monkeys obliged. But if one of the monkeys was given grapes, something interesting happened:
As observed in a popular video that de Waal showed in his TED talk, after receiving the first piece of cucumber, the capuchin monkey gives the experimenter a rock as expected. But upon seeing that the other monkey has grapes, the capuchin monkey throws the next piece of cucumber that it is given back at the researcher.
Like children, the monkeys feel they "need to get the same thing as somebody else," de Waal said.
Based on experiments such as these, de Waal came to believe that the sense of fairness observed in monkeys is egocentric. The capuchin monkeys were upset, selfishly, when they didn't get the grapes that their neighbors received. De Waal believed this model of fairness would apply to chimpanzees also. Chimpanzees are so closely related to us that they share 99% of their DNA with humans.
But the new study, which compares chimpanzees to young children, makes de Waal rethink that view.
"Now with this experiment, we are thinking that they have a higher level, where they worry about reward division in general," he said, "and it's now unclear how they differ from humans."
The new study: A human sense of fairness?
In the new study, de Waal and colleagues had chimpanzees and, separately, young children, play an "ultimatum game." This is "the gold standard of fairness for humans" because it has been played all over the world, by people in different cultures, to show that, universally, humans appear to have a sense of fairness.
The basic structure of an ultimatum game is that there are rewards that can be divided between two individuals. One proposes how to divide them and the other accepts or rejects this offer. If the receiver rejects, no rewards are given out.
Human trials have shown that people usually propose a generous division of the goodies, such as half and half or 60% and 40%, de Waal said.
In the version used in the new experiment, six adult chimpanzees and 20 human children, between ages 2 and 7, participated.