Inequity aversion in animals is the willingness to sacrifice material pay-offs for the sake of greater equality, something humans tend to do from early age. It manifests itself through negative responses when rewards are not distributed equally between animals. In controlled experiments it has been observed, to varying degrees, in capuchin monkeys, chimpanzees, macaques, marmosets, dogs, wolves, rats, crows and ravens. No evidence of the effect was found in tests with orangutans, owl monkeys, squirrel monkeys, tamarins, kea, and cleaner fish. Based on mixed results in experimental studies it may be concluded that some bonobos, baboons, gibbons, and gorillas are inequity averse. Disadvantageous inequity aversion, which occurs when the animal protests as it gets a lesser reward than another animal, is most common. But advantageous inequity aversion has been observed as well, in chimpanzees, baboons and capuchins: the animal protests when it gets a better reward. Scientists believe that sensitivity to inequity co-evolved with the ability to cooperate, as it helps to sustain benefitting from cooperation. There is little evidence for inequity aversion in non-cooperative species.
The first researchers to discover inequity aversion in animals were Sarah Brosnan and Frans de Waal, in an experiment with five capuchins, described in a 2003 article in Nature. The monkeys tended to refuse to participate in a food-for-token exchange task once they saw another monkey get rewarded more desirable food for equal effort. On some occasions they threw the food back at the experimenter.
Dozens of studies have been undertaken since. A few experimental paradigms have been used to test inequity aversion. The exchange is most common. Here animals need to hand over a token to the experimenter in exchange for a food reward. The results and findings are mixed. In terms of refusal rates being higher in inequity conditions than equity, there is substantial variation across species, across studies, and even across individuals within the same studies. Some researchers have argued that small differences in experimental setup can make the effect disappear. This is the case, for instance, if the animals are not side by side and do not have good visibility of their partner and their actions, or if there is no task and the animals are simply given food. In some species the females did not refuse inferior rewards but the males did; in some other species it was the other way around. Due to low sample sizes, not all studies controlled for sex and rank.
Background The ability of humans to cooperate is well documented, but its origin is an open question. One key aspect of cooperation is a sense of fairness: the reward an individual gets from cooperating should be fair compared to others or else future cooperation may break down. A full-blown concept of fairness is typically present in children aged six, although three-year-olds already prefer a person who distributes rewards from cooperation fairly over one who does so unfairly. When given the choice to accept an unfair reward, most children rejected it if it was less valuable than the reward of their peer (this is called disadvantageous inequity aversion, or sometimes, first-order inequity aversion), researchers Blake and colleagues found in a study across seven countries. Even if it was more valuable than the reward of their peer, older children in three countries still on average rejected it (advantageous inequity aversion or second-order inequity aversion). Disadvantageous inequity aversion is considered a universal feature of human behavior, whereas advantageous inequity aversion may be strongly influenced by cultural norms.
Humans are not the only cooperative animals. Many species of animals cooperate in the wild. Collaborative hunting has been observed in the air (e.g., among Aplomado falcons), on land (e.g., among chimpanzees), in the water (e.g., among killer whales), and under the ground (e.g., among driver ants). Further examples of cooperation include parents and others working together to raise young (e.g., among African elephants), and groups defending their territory, which has been studied in primates and other social species such as bottlenose dolphins, spotted hyenas, and common ravens. By researching aspects of cooperation in other species, evolutionary psychologists aim to pinpoint when and under which conditions cooperation emerges. With cooperation not being uniquely human, inequity aversion may not be uniquely human either. Fairness in cooperative animals in the wild has also been observed, in particular in primates. Chimpanzees are known to divide the carcass obtained during collective hunting partially based on each individual's contribution to the hunt. Through controlled experiments with animals, researchers look for this behavior and hope to be able to answer the questions of how and why inequity aversion, and cooperative behavior as a whole, evolved.
First research
The first researcher to test inequity aversion in animals was Sarah Brosnan. As a PhD student at Emory University in Atlanta, Georgia, the idea for an experiment came to her during a feeding session with capuchin monkeys. When she was handing out peanuts to the lower-ranked monkeys, an alpha male named Ozzie offered her an orange, a higher-value food, to also get a peanut. Under guidance of her professor, Frans de Waal, Brosnan set up an experiment to ascertain if a capuchin's behavior is influenced by rewards given to others. In a preliminary test with two conditions, capuchins were tested side by side and were either both given a piece of cucumber as a reward, or one was given cucumber and the other a grape (known to be perceived as a higher-value food). The results indicated that female capuchins might be sensitive to unequal distribution of rewards. Male capuchins did not show any different behavior between the two conditions.
Brosnan subsequently tested five female capuchins in different conditions. As before, the rewards were either equal or inferior to what the other monkey received. Brosnan also tested if it mattered if the other monkey received food as reward for effort or for not doing anything at all. The task the capuchins had to perform was a common exchange task: the experimenter handed the monkey a pebble which simply had to be handed back. If done so, the experimenter would give the food reward. In the side-by-side setting the capuchins could see each other's actions and, crucially, each other's rewards. A further control condition was to ascertain if the behavior was caused merely by the presence of the higher-value reward, since primates have long been known to show a contrast effect. In this condition there was only one capuchin and the experimenter first placed a grape in front of the empty place where the other monkey would have been, before starting the exchange task with the test subject and a piece of cucumber.
The results showed a clear effect of others' rewards influencing capuchins' acceptance of rewards. Whereas in the equity condition cucumber was happily accepted as reward for handing back the pebble, in the inequity condition cucumber was rejected one in three times. Rejection sometimes took the form of throwing the piece of cucumber back at the experimenter, and sometimes as violently pulling at the dividing screen. One in six times the capuchins did not even return the pebble in the inequity condition. The failure rate to exchange was even higher in the effort control, where the other capuchin got a grape for not doing anything at all: three out of four times there was no successful exchange. In the food control, where grapes were visible but without any other monkey present, the monkeys were also more likely to refuse than in the equity condition.
Each run of tests consisted of 25 trials in the same condition. The researchers compared the results of the first 15 against the last 10. They found that in the inequity condition and in the effort control, the failure rate in the last 10 was higher than in the first 15, suggesting that it may have taken a few trials before the monkeys noticed what reward the other had received. In the food control however, the failure rate in the last 10 was lower than in the first 15, suggesting that expectations are based on seeing a partner receive high-value rewards rather than the mere presence of such rewards. The researchers concluded that female capuchin monkeys are inequity averse.
Brosnan and de Waal published the results of their study in 2003 in the science journal Nature. The study has since been cited over two thousand times in other scientific articles.
Subsequent studies The original Brosnan and de Waal study has been replicated many times, with various variations of the experimental design and involving a diverse set of species as subjects.
Subjects
Researchers have selected various species as subjects of their inequity aversion experiments. Within the group of species closely related to humans, researchers have chosen both social primates (chimpanzees, bonobos, owl monkeys, marmosets, baboons, gorillas, and tamarins), and ones living solitary or just with their offspring (orangutans, gibbons, and squirrel monkeys). Researchers have also tested non-primates that display any capability of cooperation: birds (crows, ravens, kea), canines (dogs, wolves), rodents (mice and rats), and cleaner fish. Brosnan and de Waal have called for experiments with elephants, dolphins, and domestic cats to further the understanding of the evolution of fairness.
Within their experiments researchers have controlled for various characteristics of subjects, just like Brosnan and de Waal did by using only female capuchins. A common factor is relationship: whether or not the two animals in the experiment have a genetic relationship or not. Dominance rank in social animals has also been known to play a role in cooperation experiments and is thus often controlled for. The number of subjects frequently is limited, making robust statistical conclusions challenging.
Conditions To control for factors that may or may not influence inequity aversion, researchers have used various conditions in their experiments. A food-contrast control is common. Is reward refusal due to the social aspect of the experiment or due to the animal expecting a higher-value reward? Contrast studies date back to the 1920s and involve a series of higher-value rewards, followed by a low-value reward. Subjects can be tested alone or side by side. Some researchers have questioned Brosnan and De Waal's use of the last 10 trials in their food control to arrive at the inequity aversion conclusion. These researchers have designed various ways to control for food expectation, for example hiding it after having shown it, or putting it in another cage.
Another common control is effort control. Does it matter if the food is handed out as a reward for effort or simply as a gift? Most commonly used for effort is Brosnan and De Waal's token-exchange task. Sometimes subjects must hold on to a token for a specified period of time (a task referred to as "target"). For no effort there are two cases: neither animal has to do anything, or the partner gets a free gift but the subject has to complete a task.
Ultimatum Game A few studies have deviated from the token-exchange paradigm and followed a paradigm used in inequity experiments with humans, the Ultimatum Game. In this game, one individual, the proposer, has to choose between two tokens, one representing a fair division of rewards, the other an unfair division. The other individual, the responder, then needs to decide to either accept the chosen token, in which case rewards are given as per token value, or reject the token, in which case both proposer and responder do not get any reward.
Choice