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
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