A new study reverses a decade of research claiming chimpanzee selfishness.

Charles Darwin had more in common with chimpanzees than even he realized. Before he was universally known for his theory of natural selection, the young naturalist was faced with one of the great moral choices in the history of science. The decision he made has long been hailed as the type of behavior that fundamentally separates humans from other apes. But a new study reveals for the first time that thinking of others unites humans and chimpanzees in a cooperative bond that reaches across two epochs to the very evolutionary ancestor Darwin predicted.

On the morning of June 18, 1858, a parcel arrived that threatened to undo the originality of Darwin’s masterwork. Alfred Russel Wallace, a friend of Darwin’s who was then conducting field research in Borneo, sent his colleague a theory of evolution that closely matched what Darwin had secretly been working on for more than two decades.

“Even his terms now stand as heads of my chapters,” Darwin wrote, almost in a panic. “So all my originality, whatever it may amount to, will be smashed.”

What should he do? Borneo was thousands of miles away, what if the package had never arrived? After all, mail was lost at sea all the time. Darwin could publish his theory immediately and take his chances with any awkwardness he might face with his friend down the road. But that’s not what he chose to do.

“I would far rather burn my whole book than that he or any man should think that I had behaved in a paltry spirit,” he wrote to Charles Lyell, his friend and mentor. "He does not say he wishes me to publish; but I shall, of course, at once write and offer to send to any journal.” And so Wallace's outline was included alongside an abstract of Darwin's theory and presented jointly before the Linnaen Society the following month. Rather than receive a reward all to himself, Darwin made a prosocial choice so that his colleague could receive one as well. On the Origin of Species was published just over a year later.

This kind of prosocial behavior, a form of altruism that seeks to benefit others and promote cooperation, has now been found in the species that Charles Darwin did more than any other human to connect us with. A paper released today in Proceedings of the National Academy of Sciences by Victoria Horner, J. Devyn Carter, Malini Suchak, and Frans de Waal of the Yerkes National Primate Research Center at Emory University in Atlanta, Georgia, is the first to document spontaneous prosocial choice by chimpanzees, a species that, until now, was thought to be “indifferent to the welfare" of others.

The researchers made their breakthrough by presenting chimpanzees with a simplified version of the same moral test Darwin faced 150 years ago. Pairs of chimpanzees were brought into the testing room where they faced each other separated only by a wire mesh (see Figure 1). On one side was a bucket containing 30 tokens that the chimpanzee could give to an experimenter for a food reward. Half of the tokens were of one color that resulted in a selfish outcome in which only the chimpanzee who gave the token received a reward. The other tokens were of a different color that resulted in a prosocial outcome in which both chimpanzees received a food reward. The individual making the selection was rewarded no matter what. Their only choice was whether or not their “colleague” would benefit as well.

Because the individual making the choice always received a reward there was no incentive in the test design to encourage the prosocial option. The choice was each individual's alone. Twenty-one pairs were tested in all, with each individual repeating the test on three different occasions and never sitting across from the same partner twice. If chimpanzees were indeed motivated only by selfish interests it would be expected that they would be more likely to choose a reward only for themselves (or it should be 50-50 if they were choosing randomly). But individuals were significantly more likely to choose the prosocial outcome compared to the no-partner control, indicating that Pan troglodytes clearly considered others when making their choice (see Figure 2).

“Offered a free choice between a prosocial and selfish option,” the scientists conclude, “chimpanzees overwhelmingly favored the former to the advantage of their partner.” Like Darwin, the chimpanzees in the study made a prosocial choice and decided to share the reward rather than enjoy it all for themselves.

This result should not be surprising to those who have followed De Waal’s research (see my in-depth interview about his work here) as well as similar studies concerning primate cooperative behavior. For example, bonobos (Pan paniscus) are as closely related to humans as chimpanzees are (sharing 99.3% and 99.2% of our DNA respectively) and have previously been shown to cooperatively share their food with others. Researchers Brian Hare and Suzy Kwetuenda working at the Lola ya Bonobo Sanctuary in the Democratic Republic of Congo reported last year that bonobos, when given the choice, "preferred to release a recipient from an adjacent room and feed together instead of eating all the food alone."

Earlier studies have shown that chimpanzees engage in spontaneous altruism when witnessing an unfamiliar chimp trying to reach food on the other side of a door. As reported last year by Alicia Melis, Michael Tomasello, and colleagues, chimpanzees were much more likely than not to pull a chain that offered access to this food even though the altruist received nothing themselves. Chimpanzees have also been shown to provide assistance when presented with an unfamiliar human struggling to reach an object just out of reach. In 2007 Felix Warneken and colleagues found that chimps behaved identically to human toddlers under this scenario and were only too willing to help a stranger in need, even if that meant climbing over a series of obstacles in order to do so. Offering a reward for their assistance had no effect on the display of generosity in either study. Service, it seems, was its own reward.

The current study by Horner and De Waal now joins this chorus of research that reverses a decade of scholarship claiming that humans are profoundly different where it comes to regarding others. According to a highly reported 2005 study in Nature "chimpanzee behaviour is not motivated by other-regarding preferences" because individuals did not share food with others under a specific testing procedure. A 2006 study in Proceedings of the Royal Society insisted that another failed experiment meant that "chimpanzees made their choices based solely on personal gain." Scientists are usually trained to avoid citing negative results as evidence that something does not exist, because a null hypothesis cannot be proven. Unfortunately, this is often forgotten.

Based on these kinds of negative findings researchers in the social sciences and humanities boldly concluded that humans were the only species capable of engaging in altruistic or prosocial behavior. Just this year, famed cultural anthropologist Marshall Sahlins wrote that, "non-human primates live by themselves and for themselves" while the economists Ernst Fehr and Urs Fischbacher wrote in a review article for Nature that prosocial behavior in nonhuman animals is "largely restricted to kin groups" making human societies "a huge anomaly in the animal world." The economists Samuel Bowles and Herbert Gintis even went so far as to claim "human cooperation is the result of human capacities that are unique to our species."

As Carl Sagan famously wrote, "Absence of evidence is not evidence of absence." In a clear illustration of this dictum, Horner and De Waal determined that it was not that chimpanzees are motivated only by selfishness as these earlier reports contended. The tests that were designed to measure chimpanzee behavior were overly complicated and resulted in false conclusions.

"The chimps had to understand a complex food-delivery system," De Waal wrote to me via e-mail, "and were often placed so far apart that they may not have realized how their actions benefited others. Our experiment is the first to avoid an apparatus altogether. Of course, our study also provides a warning against negative findings."

In order to preempt the standard criticisms, their study also controlled for variables such as dominance rank, kinship, and the potential effects of punishment. Low-ranking individuals were not found to share more often with high-ranking individuals and there was no difference found between relatives and non-relatives. Likewise, when observing individuals together following the test, there was no correlation between their choice and the level of grooming or contact directed towards them afterwards, showing that chimpanzees were not punished for making the wrong choices.

“Intimidation behavior evidently did not help the partner’s cause, thus contradicting suggestions in the literature that chimpanzees share only under pressure,” the researchers conclude. “Nevertheless, we cannot rule out the possibility that chimpanzees in our study were influenced by reciprocal exchanges outside the experimental setting such as food sharing, increased grooming, or agonistic support.”

It is this latter possibility that shows exciting research opportunities for the future. Chimpanzee society, like the greater scientific community who studies them, is largely built around such reciprocal exchanges. Science is a social activity and sharing the rewards from one another's research is what allows scientists to improve their work over time. Charles Darwin understood this and built a network of collaborators that allowed his theory to be as solid as possible. It simply wouldn't have occurred to him to take full credit for the work that relied on the assistance of so many others.

"My Book, if it will ever have any value, will not be deteriorated," Darwin wrote as he considered no longer being the only one to reap the rewards of his evolutionary research, "as all the labour consists in the application of the theory." Darwin, like the chimpanzees he would bond us with, recognized the utility of sharing the rewards with others. Behaving in a "paltry spirit" was not the proper choice for a cooperative ape.


Victoria Horner, J. Devyn Cartera, Malini Suchaka, and Frans B. M. de Waal (2011). Spontaneous prosocial choice by chimpanzees Proceedings of the National Academy of Sciences : 10.1073/pnas.1111088108