The setup was such that a token could be traded for equal rewards for both partners, and a token that would give more goodies to the partner who made the choice.
In some trials, one partner proposes a reward division to the other via a token, and the receiver must accept the token in order for both parties to get rewards. In others, the partner's acceptance is not required.
The researchers found that chimpanzees and children both tended to make decisions about splitting rewards similarly to adult humans. In the situation where the responder could accept or reject the division of rewards, both chimpanzees and children tended to split the rewards with their partners. But when the partner was not given the opportunity to reject the proposal, chimps and kids tended to choose the selfish arrangement -- a token that favored the chooser.
So, does this mean that chimpanzees show the same sense of fairness as humans? Keith Jensen of the University of Manchester, who has conducted similar experiments in the past, isn't so sure. His results did not show that chimpanzees have a sense of fairness.
Jensen is concerned about the results of this new study because it's not clear that the responders knew that they could reject offers. None of the participants, human or chimp, ever rejected the offers of their partners.
"The fact that responders never rejected nonzero offers suggests that they were not sensitive to unfairness but were only motivated by getting food for themselves, regardless of the intentions of the proposers or the consequences for them," he said in an e-mail.
But de Waal said that responders did display negative reactions in response to some offers. Chimps would spit water and the children would say something like "You're getting more than me" in response to a selfish offer. "That indicates that they know what's going on," he said.
Jensen also criticized the design of the experiment because participants were primarily interacting with the researchers, not each other. Although one chimp had to pass a token to the other, this could be just a necessary step to get food, not a sign of agreement with the offer, he said. But de Waal stands by the study.
There are very few studies of this nature on chimpanzees compared to in humans, and more research should be done to explore the nature of the sense of fairness of human relatives.
The secret lives of primates
There's still a lot that humans don't know about their close relatives.
De Waal has made some fascinating inroads, however, including a study showing that chimpanzees can look at the behind of another chimpanzee and match it to the corresponding face, provided it's a chimp they know. This shows that the chimps have "whole-body knowledge," a concept that has not been rigorously tested in humans, he said. The research won him a 2012 Ig Nobel prize, honoring research that is both humorous and thought-provoking, shared with Jennifer Pokorny.
And he has also studied yawn contagion, the phenomenon of one person yawning in response to another person's yawn. Those who are sensitive to yawns tend to be more empathetic people, and friends and family members yawn more with each other than with strangers. This has also been shown in chimpanzees, who will yawn if another chimp they know yawns too.
But de Waal isn't sure, for instance, why three females were patrolling their compound when CNN visited in October. Males, though, have a clear purpose in patrolling: In the wild, they do it to protect their territory, de Waal said. Perhaps, he postulates, the females are mimicking the males.
Chimp males compete with each other regularly, but also come together to repair their relationships, de Waal said. This pattern of behavior is seen in human families and in the workplace -- these cycles of one-upmanship and reconciliation.
"There are many animals who are very good at cooperation, and I'm personally not convinced that we humans are necessarily best at that, but we are very good at it, that's for sure."
His next book, coming out this spring, is called "The Bonobo and the Atheist: In Search of Humanism Among the Primates," which brings together evidence that there are biological roots in human fairness and addresses the role of religion in society.