Using a behavioral research technique called the Ultimatum Game, researchers from the Yerkes National Primate Research Center at Emory University have demonstrated that chimpanzees have a sense of fair play– a trait previously thought to exist only in humans.
“We used the Ultimatum Game because it is the gold standard to determine the human sense of fairness,” says first author Darby Proctor. ”In the game, one individual needs to propose a reward division to another individual and then have that individual accept the proposition before both can obtain the rewards. Humans typically offer generous portions, such as 50 percent of the reward, to their partners, and that’s exactly what we recorded in our study with chimpanzees.”
The researchers tested six adult chimpanzees and 20 human children, divided into pairs and playing for bananas and stickers, respectively. One subject, the “proposer,” offers a blue or white token to their partner, the “receiver.” Both are made to understand that the white token represents an equal split of the reward, while the blue token represents an unfair split in the proposer’s favor. If the receiver doesn’t approve of the offer, they are free to veto it by refusing to hand the token to the researcher, in which case neither gets a reward.
While both subjects are technically better off in absolute terms with either token, research has demonstrated recipients will forego their own smaller share for the sake of fairness. There is also an overriding tendency for human “proposers” to propose the fair option, an inclination now observed in our evolutionary ancestors.
“Both apes and children responded like humans typically do. If their partner’s cooperation was required, they split the rewards equally. However, with passive partners—a situation akin to the so-called dictator game—they preferred the selfish option. Thus, humans and chimpanzees show similar preferences regarding reward division, suggesting a long evolutionary history to the human sense of fairness,” write the authors in their study’s abstract.
The following is a transcript of an exclusive interview with Darby Proctor, Postdoctoral Fellow at the Yerkes National Primate Research Center of Emory University and lead author of the study.
Science Recorder (SR): How old were the children participating in the study?
Darby Proctor (DP): The average age was 3.5 years.
SR: Out of the six chimps and twenty children, how many of each group chose the fair option?
DP: All of the chimps increased their fair behavior in the UG (Ultimatum Game) versus the preference test. So they all became more fair. With the children we used different cohorts of children, so can only look at their data at the group level. This was due to our limited access to the children in the preschools.
SR: How did you go about communicating the rules of the game to the chimpanzees?
DP: We did extensive training prior to the task including: passing tokens to other chimpanzees, quantity preference tests, and training of the meaning of tokens. Because we did all of this prior to the ultimatum game we believe the chimpanzees understood the contingencies of the task.
SR: In your abstract you mention that with ‘passive partners’ the choosers preferred the selfish option. What exactly was a passive partner in the context of the game?
DP: It was a similar setup to the ultimatum game, but with one key difference. Instead of proposers handing the token/offer to the partner, they returned it to the experimenter. Thus, the partner was not actively involved – but they were rewarded based on the offer.
SR: In your study you mention that there may have been an issue surrounding the recipient’s understanding that they had the power to veto an offer. Were you confident that the offerer also understood this veto power existed? How can you tell?
DP: We did not explicitly train the chimpanzees (nor the children) that they could refuse an offer. But, we did see interactions between the chimps that suggest they understood the role of their partner. Several times a partner would do something like spit water at the proposer after an unfair offer was made. While they did not refuse the offer, they did at times express their dissatisfaction with the offer.
SR: Is it possible to play the Ultimatum Game with any other animal species? What do you think the outcome might be?
DP: It is possible, as long as it is adapted in a species suitable way. I would expect that we may see signs of something like a sense of fairness in other cooperative species. During the evolution of cooperation it was probably very important to be able to compare your rewards with others to ensure that you had an optimal cooperation partner.
SR: What implications does your study have for our understanding of human evolution and basic human nature?
DP: It seems to us that the evolutionary origins of a sense of fairness predate humans. That is, something like a sense of fairness was probably present in the last common ancestor between humans and chimpanzees.
SR: Do you plan to repeat the experiment with larger sample sizes of both groups?
DP: At this time, we do not plan to repeat the UG but are going to look into this idea of fairness more in the future.
SR: What will you focus on next in your research?
DP: I am continuing to explore nonhuman primate reactions to economic situations.
Study co-author Frans de Waal also lent us his insights on the study, quoted below.
“The new study … is on chimpanzees, who seem to go much further than the monkeys, more like humans (chimpanzees are of course also more closely related to us). The monkeys reacted negatively to getting less than others (known as “inequity aversion”), but never corrected the situation by sharing.
The new study is more complex in that it looks at fair distribution when the agreement of a partner is required as in the Ultimatum Game, which has been played with humans all over the world. Humans make fair offers if the partner’s agreement is required and selfish offers if they can decide all by themselves.
There have been previous attempts to play the UG with primates, but none has been conclusive. Most of the time the primates in those other studies had no idea, it seems to us, how the game worked. It involved a complex apparatus. We decided on a simpler, more intuitive procedure, see the below video:
Chimpanzees, like children (we tested children with the same UG set up), make strikingly fair offers if they need their partner’s cooperation to obtain rewards. Until now this tendency was considered uniquely human, part of a unique human nature around fairness principles.
Token exchange is very well understood by many primates, they even distinguish tokens of high value from those with low value, the way we do with money. This has all been tested out long ago.
We have now reached the point that in fact it is unclear if and how the human sense of fairness differs from that in our close relatives. The preferences of the two are very similar.
The critical behavior is the Proposer, who makes the offer (equal split or selfish). In humans, this game is usually played only once between two given individuals, so the partner hardly gets a chance to reject the offer, and in fact rarely does. This is similar to what we found in both children and chimpanzees.
We did notice behavioral reposes indicating that the partner got upset about selfish offers. These behavioral responses (spitting, intimidation) are important as they indicate that a) the Responder understood how the game worked, and b) these reactions put pressure on the Proposer, which may have triggered his or her more generous behavior.
Inequity aversion has also been tested in capuchin monkeys, other monkeys, and even dogs. We think the reactions are fairly widespread. With dogs anyone can try it at home: reward one dog for his tricks but ask another one to do the same tricks without rewards. This study was done by Frederieke Range in Vienna.
We think the outcome of our study fits the idea that cooperation requires that you pay attention to who gets what in the end. You cannot cooperate and let the partner take everything. What would be the point? You need to be sensitive to reward division.
The idea is that the more highly cooperative an animal is the more you will get these reactions so long as the cooperation is based on partner choice. So, ants and bees, may not show these reactions, but all cooperative mammals and birds should watch what they get out of joint efforts.
We have a long history of working on fairness reactions in primates, the most popular video being the one from my TED talk, which shows capuchin monkeys in a study from 10 years ago (Brosnan & de Waal, 2003, Nature):
The study’s findings were recently published in the journal Proceedings of the National Academy of Sciences.