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The Evolution of Fairness

This View of Life is pleased to present excerpts from a new book entitled The Fairness Instinct: The Robin Hood Mentality and Our Biological Nature (Prometheus Books, 2013), by Lixing Sun. Professor Sun is a behavioral and evolutionary biologist at Central Washington University. His current research includes the evolution of moral behavior, social networks, and collective decision-making in primates. His book synthesizes researches in biology, psychology, economics, anthropology, and political science, and champions a new theoretical framework, showing how fairness emerged and evolved via cooperation and social hierarchy. He then probes into the motives that underlie such phenomena as envy, consumerism, anti-intellectualism, revenge, revolution, terrorism, marriage, democracy, and religion, to showcase how powerful the fairness instinct can be in our society and personal lives.

Everybody knows even young children can cry “That’s not fair!” when shortchanged. But when Sarah Brosnan and Frans de Waal published their article, “Monkeys Reject Unequal Pay,” in Nature in 2003, many were surprised.

In the study, Brosnan and de Waal trained capuchin monkeys to use rocks as tokens in exchange for food rewards from researchers. They paired up the monkeys and gave each a rock as token money. If a monkey returned the rock, it was rewarded with either a grape, a highly valued snack, or a slice of cucumber, which is not so desirable. The researchers found that when two monkeys in a pair were given the same rewards for rock tokens—either grapes or slices of cucumber—both were willing to participate in the exchange game. However, when the rewards were unequal—that is, one monkey got a grape while the other received a slice of cucumber—the shortchanged monkey often refused to cooperate in the exchange. Some even threw the cucumber slices back at the researchers in protest.

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(See short video below of monkeys vehemently rejecting unequal pay. This video should not be missed.)

According to the principle of parsimony, if a trait is seen in both humans and animals that are closely related to humans, say chimps, the most likely—most parsimonious—explanation is that the trait had already existed before humans and chimps parted in their evolutionary lineages. This is the logic behind the recent discoveries of behaviors such as consciousness, tool use, language, and culture in non-human primates, all of which were widely—but wrongly—conceived as uniquely human only a few decades ago.

Brosnan and de Waal’s finding of fairness in the capuchin monkey has settled the long-standing question of whether animals have a sense of fairness and further blurred the line between humans and other animals in a new territory: the sphere of morality.


The questions now are how fairness works and why it evolved. Part of the answers lies in monkeys like the capuchin. But monkeys are complex animals—not only are they highly intelligent; they are also highly social. Which of the two—mental capacity or close-knit social life—hosts the evolutionary wellspring of fairness?

Unfairness evokes strong and spontaneous emotional reactions—ranging from mild discontent and envy to resentment and outright hatred—in humans, and possibly in other primates as well. These emotions goad us to take action in an attempt to curb the perceived injustice. Fairness, in this chain of responses, involves complex cognitive processes. In Brosnan and de Waal’s study, a monkey has to be able to compare its reward with what its peer gets for the same “service”—exchanging rocks with humans. For the monkey, the reward of its peer sets the bar for its expected gain. The gap between the actual and expected is then used in judging whether the reward is acceptable or not—that is, fair or unfair. All of these steps are hinged on a basic sense of quantity to gauge the gap between the actual and expected rewards. Simply put, fairness judgment requires quantitative cognition.

This requirement poses little challenge for a wide variety of animals, including some that are not considered “brainy.” Many have a keen sense of quantity, or, in psychological lingo, mathematical cognitive ability. Birds, for instance, often have a clear numerical sense, likely related to the task of counting eggs in their nests. Several species of eagles, for instance, lay only one egg; pigeons, two; robins, three to four; starlings, five; mallards, typically seven to ten.

Many mammals have at least some basic numerical sense, often represented as mental magnitudes. They can comprehend what is more and what is less. Several species of monkeys even show rudimentary skills in simple arithmetic such as addition, subtraction, and putting numbers in order. This numerical sense appears relevant to survival. For example, chimps cue on pant-hoot vocalization to assess the size of an enemy troop. Only when they are sure they outnumber their opponents will they launch an attack. Apparently, mammalian brains are far more sophisticated than what is minimally required to perform simple quantitative comparisons.

The neural processes of quantitative cognition and comparison are complex, involving dopamine neurons in the midbrain. A closer look shows that these neurons come in two types. One decides how to act. It gets excited by rewards and is inhibited by punishments. The other type makes no distinction between rewards and punishments; it responds only to the size of the stimulus—large reactions for large stimuli, small reactions for small stimuli. Putting the scenario in the context of Brosnan and de Waal’s monkeys, the first type of dopamine neuron prompts them to either accept or reject the rewards by comparing what their peers get—equal or better, accept; worse, reject. The second type determines how strongly they should react (accept or gladly accept; reject or angrily reject, for instance) according to what they expect and what they actually get.

This minimalist view of the two types of dopamine neurons is inadequate for a sense of fairness, however. Otherwise, many animals would act as if they all knew the fairness principle of “equal pay for equal work” in its primordial version—simply because they have similar dopamine neurons. If the experiment with capuchin monkeys tells us anything about their sense of fairness, it is in the tantrums they throw when shortchanged. Here, emotions are critical in signaling what they perceive as fair or unfair. But emotional signals like tantrums have to be understood by peers in context—for example, the context of carving up a hunting spoil. Otherwise, the barrage of negative sentiments unleashed by the feeling of injustice would be wasted and are harmful to the body, for which they would stand little chance to evolve. Therefore, an animal with a sense of fairness has to get its emotional messages across to its peers. In other words, an animal must be capable of experiencing the emotions of its social partners—empathy—to have a sense of fairness.

How does the brain handle empathy? In the 1990s, scientists discovered a special type, called mirror neurons, that underlie the ability to experience empathy. These neurons are activated when a person sees, and is triggered to imitate, the behavior of another. Mirror neurons allow us to feel other people’s emotions by swapping mental perspectives, for which they are dubbed as “mind-reading” cells. Mirror neurons were also found in monkey brains, allowing them to respond to the emotions and assess the intentions of their social partners.

Also important for empathy are spindle cells, located in the prefrontal cortex in primates. Not surprisingly, the brain region is directly wired to, and presumably receives information from, the dopamine neurons in the midbrain. In people with autism, spindle cells are found in a different location in the brain. Consequently, the lack of empathy—the most debilitating mental deficiency among autistic people—may be related to abnormal neural wiring.


If empathy itself is bound up with social life, how important is social living to the evolution of fairness? Before answering this question, let us ask a more basic one: Why are some animals social while others are solitary?

We know natural selection mandates that, all things considered, if the benefits of social living (including coordinated hunting, food sharing, joint defense, and mutual protection) are less than the costs (including higher level of disease transmission and competition for food and mates), animals should live alone. As the benefits pivot on cooperation among partners, cooperation is, therefore, the main drive of social living.

Social living brings conflict of interest between peers to the fore, however. If not properly resolved, conflict of interest can be the downfall of any society. It can derail cooperation and dissolve hard-won relationships. For this reason, conflict of interest may be the mother of all moral problems and the hub of the biopsychological quest for morality. To enjoy the “sweet spot” of cooperation, many social animals, humans included, have evolved a battery of strategies for conflict resolution. When physical disputes occur, they will often go to great lengths to repair wounded relationships. Wolves in a pack lick each other for reconciliation, and monkeys and apes engage in mutual grooming—much more than usual—after an outbreak of fights. Humans (and maybe other primates) are equipped with a special emotional tool—feelings of guilt—to help keep relationships in working order. Guilt motivates us to restore peace with our social partners when they are hurt or treated unfairly by us. These are just a few examples indicating the importance of affable relationships for lasting cooperation.

Cooperation is propelled by two biological engines: kin selection and reciprocity. While kin selection is an elegant explanation for altruism between genetic kin, it does not explain altruism between strangers in many societies. For this, biologist Robert Trivers proposed another evolutionary passage of cooperation: reciprocal altruism, which says, if two parties can each gain more by working together than by going it alone, cooperation will prevail. Indeed, reciprocity among social partners is common in primates, ranging from mutual grooming to long-term alliances. Free from blood ties, reciprocity is more ubiquitous than kin selection in promoting cooperation and is often the main driver for large-scale cooperation among humans such as teamwork, charity, patriotism, and international alliance. Accordingly, reciprocity should be given its due in our search for the connection between social living and fairness.


Reciprocity has a major loophole—it is extremely vulnerable to cheating. Reciprocity is supposed to be a mutual process, swapping favors between partners. But the time lapse between donating and receiving a favor opens an ominous window for recipients to shirk their obligations. If a favor is not returned, the original donor suffers a net loss. Clearly, cooperation without assessing the likelihood of reciprocation is, in ecologist Garrett Hardin’s words, “promiscuous altruism,” easily to be preyed on by cheaters.

Cheating, like a blood-sucking parasite, saps the vitality of cooperation. Reciprocity, to stand any chance of survival, needs some potent antidotes, which, in Trivers’s prescription, include long lifespans and stable group memberships. A long lifespan can provide ample opportunities for social animals to interact and reciprocate; stable group membership helps build trust and deter cheating. Not surprisingly, people tend to be more trustful of one another and behave more honestly in villages and towns than in large cities.

Humans are not alone in these considerations. Vampire bats, for example, also show ample reciprocity within trusted circles. Biologist Gerald Wilkinson found that if a bat does not suck blood for seventy-two hours after a big meal, or after just three nights in a row of failed hunting, it will starve to death. To prevent starvation by spreading the risk, cavemates that roost in close vicinity operate a food-sharing system. Satiated bats from successful hunting regurgitate blood to the famished. This system protects the cooperating bats from cheaters—those who might come to share blood, but either do not return the favor at all, or return it less often than they should.

In vampire bat communities, cheaters, if detected, may suffer a dire consequence—death by starvation. Thus, cheating is easily deterred by simple rejections of food-sharing requests. In most situations, however, stronger measures are needed to reinforce the trust between cooperating partners. Just as penalties—such as higher interest rates and potential legal actions—are vital for the smooth running of our credit system, some basic behavioral rules are essential to safeguard cooperation in animal societies. Transgressions against these rules must incur penalties to the transgressors, such as physical retaliation and rejection of future cooperation. Here we can see, how, through reward and punishment, fairness can arise to be central among such behavioral rules to resolve conflicts of interest in cooperation.


As we have seen throughout the animal kingdom, social living occurs when the benefits outweigh the costs for all members of the group over their lifetimes. Yet a social group is made up of many individuals, each bidding for its own highest net gain, as mandated by natural selection. To realize the best outcome of social living, an animal has to strive for its best to gain an edge over rivals when interests conflict. Reciprocity, too, can be rife with antagonism.

In such a system, cooperative partners may be torn amid the conflicting motives of gaining more now, on one hand, and losing potential partners for future cooperation on the other. In the long run, fairness in reciprocal exchanges can satisfy both parties. As a result, this happy medium has naturally evolved to be an equilibrium point. This may explain why fairness is vital for keeping trading—the central activity in the economic market—going in human societies.

The reciprocal nature of trading is even more obvious in bartering where goods and services are directly swapped without the mediation of money. Biologists Ronald Noë and Peter Hammerstein believe that reciprocal exchanges seen in many animals are indeed trading, governed by the market forces of supply and demand. This biological market idea has generated new insights into reciprocal exchanges in animal societies. Grooming in primates, for example, besides the obvious benefit of ridding them of parasites and soothing itches, can lower stress hormone levels in the body and boost endorphins (a family of happiness hormones) in the brain. Thus, the need for grooming is a service in high demand. An animal that provides the service can use grooming as an all-purpose currency to barter for a variety of other goods: obtaining consolations, being scratched and comforted, mating opportunities, gaining permission to hold infants, reconciling wounded relationships, building or consolidating alliances, and currying favors from higher-ups. In Tibetan macaques, females of equal status trade grooming in equal amounts. But when such trading occurs between females of unequal status, the lower-down receives much less from the higher-up. Apparently, the lower-ranking female trades grooming for something else, such as favorable treatments from the higher-up. In sifaka lemurs, males usually only groom themselves. But when the mating season sets in, they begin to barter grooming for sex with females. This seasonal shift in the grooming pattern of males reflects the oscillation in the price of sex governed by supply and demand, a distinctive feature of markets.

Other services and commodities can be traded as well. For instance, trading sex for resources is seen in animals as simple as flies and crickets. In the hangingfly, a female will accept a courting male that offers a blowfly as a nutritious nuptial gift. But the female “charges” by the minute: the larger the blowfly, the longer the female allows the male to mate with her, quid pro quo. This shows that sex can be a highly valuable commodity in the biological market.

Trading is cooperation writ large. Extensive trading would be unthinkable if the exchanges were not perceived as fair by market participants. The reason is simple: trade between willing parties results in a win-win outcome. Since consistently unfair trades will eventually put one out of business, trading, as a form of cooperation, is unlikely to evolve without fairness.

In Brosnan and de Waal’s study, capuchin monkeys do not have to know the face value of a token used for trading—though with patient coaching they can learn some human-like tricks of trade for money or sex. They simply need to make the comparison between what they get and what their social partners get for the same trade, because the “price” of their token money is set by the “market value” at the moment. It’s not difficult to see that trading in the biological market is a major force behind the evolution of fairness in social animals, whose brains have some basic capacity of quantitative cognition (for judging what is fair and what is not) and a certain level of empathy (for understanding the emotions of peers).


There is another twist in the link between social living and fairness. We know that a social life is also a hierarchical life. No society in the animal kingdom is entirely equal for all members. Among humans, high or low status is a significant factor in even the most egalitarian societies. Hierarchy, in essence, represents inequality. Studies of wolves, monkeys, and apes consistently produce results showing that higher ranking individuals have better access to food, water, shelter, and mates, which translates to a higher level of fitness than that of lower ranking group members.

For the rank and file, it behooves them to reject the ordained fate of being taken advantage of in life, on one hand. Natural selection mandates them to fight back in order to advance their own status. Otherwise, their genes may face elimination from the gene pool over time. Indeed, as studies in many social animals show, lower ranking individuals do sneak feeding and mating opportunities against the interests of higher ranking members in the group. And, when the time is right, they challenge or revolt against higher-ups. Conflicts such as these can send the social hierarchy into a state of flux. A wobbly or chaotic hierarchy typically hurts higher-ups more than lower-downs because the former have more to lose than the latter. In lions and langurs, for instance, dethroned alpha males lose their harems and their young offspring to usurpers. In ancient human societies, when monarchies and dynasties were overthrown, kings and emperors were often killed, together with their children, relatives, and, if not taken by the victors as reproductive trophies, wives and concubines.

On the other hand, even though conflict of interest is a recurring factor of social life, an unstable hierarchy, one rife with frequent and intense strife, invokes a high cost for social animals, as is often seen during leadership turnover in primates. In humans, an unstable or transitional period in organizations—schools, companies, local governments—will likely be accompanied by uncertainty, anxiety, and even chaos. At the national level, in extreme cases, it may lead to mass violence. A stable hierarchy, even though it calls up the distasteful feeling of inequality, is not all that bad for the majority of the members of the group. Peace and stability are usually in the collective interest of most members, enabling them to conduct their basic biological business—that of realizing their potential fitness.

A social hierarchy thus implies a truce between the two opposing selection forces in resolving the perpetual quandary of conflicting interests. Each and every member in the group wants to be the king of the mountain, yet each has to make necessary concessions to others in order for peace to prevail. To arrive at a compromise under varying socio-ecological conditions, different species, different populations of the same species, or even different groups of the same population may take vastly different routes. Consequently, nature exhibits a large spectrum of hierarchical patterns, from complete dictatorships to highly egalitarian societies.

To understand how fairness evolves under the context of social hierarchy, we can imagine ourselves as males in a gorilla society. In our world, there is only one alpha male, the silverback. For the rest, all but one—the wretched omega—are both victims and victimizers, being exploited by those who sit above and, at the same time, exploiting those who sit below. So, except the alpha male, everybody else fights for a higher status. This “gorilla confession” illustrates that fairness emerges as a bottom-up leveling force, pushing toward equality in resource distribution in the hierarchy. We can expect that the steeper and more rigid the hierarchy is, the stronger the selection for fairness becomes.

What works for gorillas is, by and large, what also works for humans. Our perception of fairness tends to vary with our social status. Business leaders often prefer large pay disparities and low taxes; blue-collar workers, meanwhile, are inclined to support more equal pay scales and taxing the rich. On the issue of allocating pay raises, people, when assuming the role of a supervisor, tend to emphasize fairness in the procedure, so that they have an advantage in the outcome over the rank and file. But when put in the shoes of a subordinate, they care more about fairness in the outcome, that is, even division of resources. Even in the global arena among nations, the situation is similar. Winston Churchill, when reflecting on the link between national strength and the international pecking order, was brutally honest: “The whole history of the world is summed up in the fact that, when nations are strong, they are not always just, and when they wish to be just, they are no longer strong.” Here one can hardly miss the ubiquitous bottom-up push—selection—for equality in social hierarchies. Even for higher-ups, the very prospect that they, too, can fall to the lower rungs of the hierarchy one day makes them prefer a certain level of fairness. After all, what goes around comes around.

By now, we have traveled a considerable distance in tracing the evolutionary genesis of fairness. In conclusion, fairness, as a mental instinct and behavioral rule for solving conflicts of interest, is spawned from social living and social hierarchy. Behind it are two major selection forces, reciprocity for mutual benefit and compromise for social harmony, both of which are critical for maximizing the net benefit of cooperation.


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