Chimpanzees Prefer Fair Play To Reaping An Unjust Reward

i-4e8219519783fcbb3664b4829a3ef258-Chimpanzee Close Up.jpg
A new study shows that chimps sacrifice their own advantage if they earned it unfairly.
Image: Owen Booth / Creative Commons

This post was chosen as an Editor's Selection for ResearchBlogging.orgFairness is the basis of the social contract. As citizens we expect that when we contribute our fair share we should receive our just reward. When social benefits are handed out unequally or when prior agreements are not honored it represents a breach of trust. Based on this, Americans were justifiably outraged when, not just one, but two administrations bailed out the wealthiest institutions in the country while tens of thousands of homeowners (many of whom were victims of these same institutions) were evicted and left stranded. It smacked of favoritism, the corruption of politics by corporate money, and it was also just plain unfair. But isn't that the way the world works? Isn't it true, as we were so often told as children, that life is unfair?

The American financial tycoon Andrew Carnegie certainly thought so and today's economic elite have followed his example. In 1889 he used a perverted form of Darwinism to argue for a "law of competition" that became the cornerstone of his economic vision. His was a world in which might made right and where being too big to fail wasn't a liability, it was the key to success. In his Gospel of Wealth, Carnegie wrote that this natural law might be hard for the least among us but "it ensures the survival of the fittest in every department."

We accept and welcome therefore, as conditions to which we must accommodate ourselves, great inequality of environment, the concentration of business, industrial and commercial, in the hands of a few, and the law of competition between these, as being not only beneficial, but essential for the future progress of the race.

In other words, his answer was yes. Life is unfair and we'd better get used to it, social contract or no social contract.

While this perspective may be common among those primates who live in the concrete jungle of Wall Street, it doesn't hold true for the natural world more generally. Darwin understood that competition was an important factor in evolution, but it wasn't the only factor. Cooperation, sympathy, and fairness were equally important features in his vision for the evolution of life. In The Descent of Man he wrote that "Those communities which included the greatest number of the most sympathetic members would flourish best, and rear the greatest number of offspring." By working cooperatively, by sharing resources fairly, and by ensuring that all members of society benefited, Darwin argued that early human societies would be more "fit" than those societies where members only cared about themselves. The Russian naturalist Peter Kropotkin championed this aspect of Darwin's work and argued that mutual aid was essential for understanding the evolution of social mammals as a whole. In the time of Darwin and Kropotkin the research needed to verify these claims was in its infancy, but recent work has supported this vision of the natural world. Now, a new study has added one more plank to this growing edifice of knowledge, and the view from on top suggests that life, in contrast to what Carnegie believed, may not be so unfair after all.

According to research due to be published in the journal Animal Behaviour, fairness is not only essential to the human social contract, it also plays an important role in the lives of nonhuman primates more generally. Sarah F. Brosnan and colleagues conducted a series of behavioral tests with a colony of chimpanzees housed at the University of Texas in order to find out how they would respond when faced with an unfair distribution of resources. A previous study in the journal Nature by Brosnan and Frans de Waal found that capuchin monkeys would refuse a food item when they saw that another member of their group had received a more desired item at the same time (a grape instead of a slice of cucumber). Some individuals not only rejected the food, they even threw it back into the researchers face. The monkeys seemed to recognize that something was unfair and they responded accordingly. This raised the provocative question: can the basis of the social contract be found in our evolutionary cousins?

Chimpanzees are known to be highly individualistic where food is concerned, so Brosnan and colleagues sought to determine whether these earlier results could be replicated in a more competitive species. The researchers first trained all 16 chimpanzees to exchange an inedible token for a food reward and then assessed their food preferences (it turns out that chimpanzees always prefer grapes to a similarly sized piece of carrot). In this simple cash economy they came to understand that each token was worth one reward and they eagerly handed it over to the researchers in expectation. Once all chimps had made the association individually they were brought into the testing area in pairs where they were allowed to exchange their tokens for food so that researchers could gauge their responses when in the company of a group mate.

In the first trial both chimpanzees were given the same food reward when they exchanged their token (sometimes the high-value grape, other times the low-value piece of carrot). This served as the control test and was used for comparison in the trials that followed. In the second trial, what the researchers called the Inequity Test, only one member of the pair was given a grape while the other received a carrot. In a third variation, both individuals were shown a grape at first, but were then given carrots once they handed over their token. In each trial the researchers recorded the number of times that chimpanzees refused a food item and then compared this with the control test to determine if they behaved differently when receiving a different reward.

Perhaps unsurprisingly, chimpanzees behaved the same way that capuchins did and objected if they only received a carrot when their group mate was given a delicious grape for the same price. Out of 76 trials the chimpanzees were significantly more likely to refuse a carrot in these tests compared to times where both received the same low-value food reward (p = 0.004). Likewise, when both individuals received a carrot after first being shown a grape, they were significantly more likely to refuse than in cases where no expectation of a better reward had been presented. The bottom line was that if things weren't fair a tantrum would ensue.

If this sounds eerily familiar, you're right on the mark. Parents will testify to how careful they must be to make sure that siblings are always treated equally and fairly, and chimpanzees are known to have the cognitive abilities of three-year-old children. What these results suggest then is that chimpanzees have an expectation of fairness and will protest in cases where this expectation is not met. This existed both in cases where rewards were handed out unequally and when a prior agreement was not honored.

However, chimpanzees in this study went beyond the basic tenets of the social contract and demonstrated what could be considered the foundation of social solidarity. In 95 trials chimpanzees that received a grape were significantly more likely to refuse the high-value reward when their group mate only received a carrot (p = 0.008). Even those who benefitted from inequality recognized that the situation was unfair and they refused to enjoy their own reward if it meant someone else had to suffer. As the authors reported:

We unexpectedly found that chimpanzees were more likely to refuse a high-value grape when the other chimpanzee got a lower-value carrot than when the other chimpanzee also received a grape. . . This reaction was not seen in previous studies of inequity in primates, either among chimpanzees or among capuchin monkeys.

But in comparing this simple behavior in chimpanzees to the complexities of human ethics aren't we really talking apples and oranges (or, perhaps more appropriately, carrots and grapes)? I don't think so. When we were children we wouldn't have understood that using financial derivatives to repackage subprime loans in order to resell them as AAA-rated securities was an unfair thing to do. Few of us today (including members of the commission charged with overseeing the financial services industry) can even understand that now. But we did know it was unfair when our sibling got a bigger piece of pie than we did. We began life with a general moral sense of what was fair and equitable and we built onto the framework from there. Chimpanzees, according to this study, appear to have a similar moral sense. The intricacies of what we judge to be fair or unfair would seem to have more to do with human cognitive complexity than anything intrinsically unique to our species. In other words, what we're witnessing here is a difference of degree rather than kind.

What this also suggests is that we've been swindled. The Andrew Carnegies of the world have led us to believe that they are an exception to the social contract; fairness and equality may be fine for the little people, but for masters of industry it is best to leave such quaint ideas by the wayside. But he was as wrong about this as he was about the way that evolution operates. As we move to regulate financial markets it might be wise to consider Darwin's understanding of human society and follow the lead of our ape cousins. By emphasizing cooperation and sympathy with other members of our society we stand a better chance of success than each of us working alone. But if the situation is unfair we should refuse to perpetuate it, even if that means giving up a larger share of the pie for ourselves.

For more on this topic see:

The Sacrifice of Admetus: How the Evolution of Altruism Reveals Our Noblest Qualities
Punishing Cheaters Promotes the Evolution of Cooperation
The Bonding Brain: Primate Sociality is Linked to Brain Networks for Pair Bonds
Why Chimpanzees Make Bad Suicide Bombers
The Struggle for Coexistence: Individuals, Systems and the Emergence of Cooperation


Brosnan, S., Talbot, C., Ahlgren, M., Lambeth, S., & Schapiro, S. (2010). Mechanisms underlying responses to inequitable outcomes in chimpanzees, Pan troglodytes Animal Behaviour DOI: 10.1016/j.anbehav.2010.02.019


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I think the problem with this research and this approach to the balance of "social fairness" vs "personal greed" is that the balance point is likely contingent on the physiological state of the individual at the time of determination.

Under tremendous stress or deprivation, or the "fight or flight" state, the balance is likely to be shifted toward the "personal greed" axis.

I'd be curious to know how the chimps social status affected the grape rejection rates. Would a lower-ranking chimp who got the grape rarely accept it when a higher-ranking chimp got the carrot, or vise versa?

I'd be curious to know how the chimps social status affected the grape rejection rates.

High-ranking individuals were more likely to refuse a low-value reward and males were more likely to react to inequity than females were. But there was no significant difference reported for individuals that rejected a high-value reward unfairly.

The authors state in their study that it's unclear what the true motivations for refusing a greater reward may be and suggest that it could have been based on prosocial motivations or because they were concerned about any social repercussions that would come from accepting an unfair reward in front of their group mate. I think either option (or even both) is consistent with other studies looking at chimpanzee sociality. When we're talking about fairness in humans both certainly play a role as well. Public displays of morality often serve a social function, even though we like to flatter ourselves by saying that virtue is its own reward.

I'm not entirely convinced - the key piece of this study is that the chimps were necessarily aware that their behaviors were being watched by their partners.

We know for example that a lower ranking monkey is happy to take food when it is hidden from a higher ranking monkey behind an occluder.

We also know (e.g. Stammbach, 1988 Behavior) that a higher ranking monkey will only share food equally with a lower ranking monkey when the lower ranked individual has privileged information about how to access the food.

I'm happy to know that the authors acknowledge this limitation at the end of the paper, but I would love to see a study done where the partners don't get to watch the responses of the test subject. I bet that the effect completely goes away.

Uh oh... I feel a blog post coming on.

The search for the missing link is over. It is us.

By Benedict@Large (not verified) on 23 Apr 2010 #permalink

The evolution of Communism?

In responce to Jason's comment "the key piece of this study is that the chimps were necessarily aware that their behaviors were being watched by their partners", I don't think this matters a great deal.

Certainly it would be interesting to see if this was a contributing factor, but the fact that they even feign solidarity is remarkable enough. It implies a much more sophisticated moral code, even if they do break it when the others' backs are turned. But then to be honest, that's a behavioural trait we share as well.

By Adrian Blake (not verified) on 25 Apr 2010 #permalink

"In responce to Jason's comment "the key piece of this study is that the chimps were necessarily aware that their behaviors were being watched by their partners", I don't think this matters a great deal."

In my opinion, it matters: the one who received more might have attracted hostility of its pair, if this, the non-favored one, were to know of the advantage acquired by the other. This specially in cases where a chimpanzee of low ranking would receive a better reward than other of high ranking. As EMJ's post shows, the rejection of food has at least some correlation with the social ranking of individuals within pairs.

What I found most noteworthy in this study is this: other studies in this same model have already been published, and in them, as the authors admit, the finding of greatest importance in their paper - that even those who were favored had some tendency to reject the better reward when their pairs had not been treated in likemanner - was not verified, either in studies with chimpanzees or in those with capuchin monkeys. What is the reason for this: what distinguishes these chimpanzees from those of other studies? I think in two alternatives in this respect: either there was a greater variability of ranking within pairs in this study, or a higher degree of relatedness.

The best way to control for these factors is that of making pairs with unrelated individuals of equal social standing. But not even this would solve the problem entirely: the fact that an individual rejected a better reward could still be explained by making reference to the fear of hostile envy.

By Luis Fernando (not verified) on 26 Apr 2010 #permalink

Instead of 'pair', I meant 'peer' in some instances of the
post above.

By Luis Fernando (not verified) on 26 Apr 2010 #permalink

What one can see reading you guys are who wants to see chimpanzees in a 'only behavioral' perspective and those that expect a greater span of ideas and ideals hiding under what we can see in those studys. Those that see it behaviorally will also be the ones only granting domestic animals the minimum allowance of their needs going all the way back to those arguing that animals have no soul and therefore also (logic?) can feel no pain :)

I had a friend describing this later approach to Darwinism as vulgar-Darwinism, being right too, as I see it. And I wonder about your sense of ethics also reading you. Do they only apply to humans? In which strata then? Objectifying what you study to distance you work both ways. It might make you able to see yourself as a 'scientist' doing it for the good of all, but it will also 'steal' some of that humanity from you, as we can see in all small arms wars today. Yeah, I do mean this. Without working ethics you're just a kid, pulling the wings of a fly to see what happens.

Why is there not a PRINT command on this and other articles????

By Stacy Culpepper (not verified) on 07 Jul 2010 #permalink