Inequity aversion in animals

Inequity aversion in animals is a negative response some animal species sometimes display when rewards are not distributed equally. It has been observed in bonobos, chimpanzees, macaques, capuchin monkeys, tamarins, dogs, rats, and ravens, all species that cooperate naturally. Disadvantageous inequity aversion is most common, that is, the animal protests when it gets a lesser reward than another animal. But advantageous inequity aversion has been observed as well, in chimpanzees 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.

The first researcher to discover inequity aversion in animals was Sarah Brosnan. She got the idea for an experiment when during a normal feeding session, handing out peanuts to the lower-ranked capuchin monkeys, an alpha male named Ozzie offered her an orange to also get a peanut. Brosnan and de Waal published the results of their first study in 2003 in Nature. They described how capuchin monkeys refused to participate in a food-for-token exchange task once they saw another monkey get rewarded more desirable food for equal effort. This effect was even stronger if they observed the partner receiving such a reward without any effort at all.

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 human experimentator in exchange for a food reward. The animals need to have good visibility of their partner and their actions, preferably close and side-by-side. The Ultimatum game is a paradigm in which one animal chooses a token, hands it over to another animal who then can decide to exchange it for rewards. These rewards depend on the type of token and can be equal for both, more for one than the other, or nothing for one of them. Repetition of this task is key so the animals have an opportunity to base their actions on previous outcomes. Another paradigm is feeding without any task. This paradigm has so far not resulted in any evidence of inequity aversion in animals.

Background

The ability of humans to cooperate is well documented, but its origin is an open question. By researching aspects of cooperation in other species, evolutionary psychologists aim to pinpoint when and under which conditions cooperation emerges. 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.[1] Humans show a consistent preference for equal over unequal outcomes.[2] A full-blown concept of fairness is typically present in children aged 6, although 3-year-olds already prefer a giver who distributes rewards from cooperation fairly over one who does so unfairly.[3] When given the choice to accept an unfair reward, children rejected it if it was less valuable than the reward of their peer (disadvantageous inequity aversion), researchers Blake et al. found in a study across seven countries. Even if it was more valuable than the reward of their peer, children in three countries[upper-alpha 1] still on average rejected it (advantageous inequity aversion), albeit only older children did so.[5] Disadvantageous inequity aversion is considered a universal feature of human behavior,[6] whereas advantageous inequity aversion may be strongly influenced by cultural norms.[7]

Humans are not the only cooperative animals. Many species of animals cooperate in the wild.[8] Collaborative hunting has been observed in the air (e.g., among Aplomado falcons),[9] on land (e.g., among chimpanzees),[10] in the water (e.g., among killer whales),[11] and under the ground (e.g., among driver ants).[12] Further examples of cooperation include parents and others working together to raise young[8] (e.g., among African elephants),[13] and groups defending their territory, which has been studied in primates and other social species such as bottlenose dolphins, spotted hyenas, and common ravens.[14] Fairness in cooperative animals in the wild has also been observed. Chimpanzees are known to divide the carcass obtained during collective hunting partially based on each individual’s contribution to the hunt.[15]

With cooperation not being uniquely human, inequity aversion may not be uniquely human either.[16] 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.[1]

First research

The first researcher to discover inequity aversion in animals was Sarah Brosnan while she was at Emory University. She got the idea for an experiment during a feeding session, when, as she was handing out peanuts to the lower-ranked capuchin monkeys (Cebus apella), an alpha male named Ozzie offered her an orange, a higher value food, to also get a peanut.[upper-alpha 2] Under guidance of her professor, Frans de Waal, Brosnan set up an experiment to ascertain if capuchins' 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 cucumber as a reward, or one was given a cucumber and the other a grape, which was 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 in the two conditions.[18]

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 matters if the other monkey receives the 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 stone 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 others actions and, crucially, each other's rewards. A further control condition was to ascertain if the presence of the higher value reward mattered or the presence of another monkey. 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.[18]

The results showed a clear effect of other capuchins' rewards influencing their own acceptance of rewards. Whereas in the equity condition cucumbers were happily accepted as reward for handing back the rock, in the inequity condition cucumbers were rejected one in three times. Rejection sometimes took the shape of actively throwing the piece of cucumber back at the experimenter, and sometimes as violently pulling the dividing screen. In addition, one in six times the capuchins did not even return the stone 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.[18]

Each run of tests consisted of 25 trails 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 condition, 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 one had received. In the food control however, where grapes were visible but without any other monkey present, 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.[19]

Brosnan and de Waal published the results of their first study in 2003 in the science journal Nature.[20] The study has since been cited over a thousand times.[21]

Subsequent studies

A few researchers have criticized Brosnan and de Waal's conclusion that the capuchins avoided inequity. The data could also be explained by other driving forces.[22] To get to the bottom of this, the original Brosnan and de Waal study has been replicated many times, and alternative experimental designs have been used as well.[23]

Alternative hypotheses

Various alternative explanations have been provided by other researchers:

  • Perhaps the animals refuse food because getting less preferred than expected. An expectation set either by what is visible or in previous trials. Perhaps this has nothing to do with a social context.[24]
  • Perhaps refusal is caused by a social factor but not the partner animal but the human experimenter.[25]

Controlled experiments with various species have been performed to test these hypotheses.[25]


Subjects

Researchers have selected various species as subjects of their inequity aversion experiments.[23] Within the group of species closely related to humans, researchers have chosen both social primates (chimpanzees, bonobos, owl monkeys,[26] marmosets,[26] and tamarins[27]), and ones living solitary or just with their offspring (orangutans,[28] gorillas, squirrel monkeys[29]). Given its connection to cooperation, researchers have also looked for animals that displayed any capability of cooperation in experiments, such as corvids (crows, ravens) and dogs.[30] Brosnan and de Waal have called for experiments with elephants, dolphins, and domestic cats to further the understanding of the evolution of fairness.[31]

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 controlled for.[32]

Conditions

To control for factors that may or may not influence inequity aversion researchers have used various conditions in their experiments.

Effort control

Does it matter if the food is handed out as a reward for effort or simply as a gift? Most commonly used is an exchange task: the experimenter hands the subject a token, and this or something equivalent needs be given back to the experimenter, for which as reward a food item is given. In a task called 'target' subjects must hold on to a token for a specified period of time.[23]

Contrast control

Is any aversion to inequity 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[33] and involve a series of higher-value rewards, followed by a low-value reward. Subjects can be tested alone[34] or side-by-side.[35]

Proximity control

Does it matter if the animals are side-by-side with full visibility of each other's actions and rewards, or far apart or across from each other.[32]

Ultimatum game

A few studies have deviated from the exchange paradigm and followed a paradigm used in inequity experiments with humans, the Ultimatum game.[36][37] In this game, one individual, the proposer, has to pick a token that either represents a fair division of rewards or an unfair division. The other individual, the responder, then needs to decide to either accept this token, in which case rewards are given as per token value, or reject the token, in which both proposer and responder do not get any reward.[32] [upper-alpha 3]

Cooperative pulling

Within the cooperative pulling paradigm (an experimental design in which two or more animals pull rewards towards themselves via an apparatus that they can not successfully operate alone) researchers have varied the rewards. They compare the likelihood of animals cooperating again after both received the same reward for jointly pulling to that of one receiving more than to other, or in extremis, one receiving all and the other none. This division can be done by the experimenter (one bowl each) or by the animals (one bowl).[30]

Findings

Overview

The findings are mixed. There is substantial variation across species, across studies, and even across individuals within the same studies.[38] In studies that did find an overall inequity aversion effect, there were always some individuals that do not display any effect at all.[39] Researchers found that chimpanzees, long-tail macaques, rhesus macaques, capuchins, domestic dogs, carrion crows and ravens tend to avoid disadvantageous inequity in exchange tasks.[30][40] They refuse rewards if their partner receives a better one or they stop performing after multiple exposures to such inequity. No species displayed these behaviors when food was gifted without any effort.[30]

Orangutans,[28] squirrel monkeys,[29] owl monkeys,[26] marmosets,[26] tamarins[27] and New Caledonian crows[1] are not sensitive to inequity.[30] Gorillas have only been tested in the no-effort condition and, like all other species, did not object to being gifted lower-value food.[30] Pet dogs behave differently from pack dogs and wolves. Pet dogs only object to the other receiving a reward while they themselves do not get anything, but when the difference is in quality, they do not seem to mind. Pack dogs and wolves were sensitive to reward quality.[41]

The first follow-up studies that failed to replicate the original Brosnan and de Waal conclusions subsequently were shown to have a crucial difference in experimental setup. They involved animals getting food for no effort. Later studies have shown that the effect disappears in this context.[22]

Studies that found evidence for inequity aversion and controlled for contrast effects ruled out that the aversion was due to mere visibility of better rewards is not the issue. This is aligned with findings animals reliably performing tasks for lesser rewards regardless of whether or not better ones are immediately in front of them.[34] Only Engelmann et al. did not explain reward refusal as inequity aversion. But they did not attribute the refusals to the contrast effect either. Instead, from their experiments with chimpanzees, they concluded that refusal is due to disappointment in the behavior of the human experimenter.[25] Sheskin et al. however found no evidence that capuchins differentiated between experimenters who either distribute equal rewards or unequal ones.[42] A few species (e.g. squirrel monkeys) respond more strongly to contrast effects than inequity; some respond to both (rhesus macaques), some seem indifferent to either condition (orangutans),[34] and some respond more strongly to inequity.[43]

In cooperative pulling tasks, individuals who are the victims of reward-monopolizing individuals punish this behavior by refusing to cooperate subsequently.[30] In a bar-pulling apparatus with unequal rewards capuchins still achieved success. Pairs that tended to alternate which monkey received the higher-value food were more than twice as successful in obtaining rewards than pairs in which one monkey dominated the higher-value food.[44] In an experiment with chimpanzees, on almost half the trials the pair negotiated to work for the equal division.[45]

Physical proximity is an essential ingredient for inequity aversion to appear in effortful tasks. Across species the effect virtually disappears if the animals are not side-by-side with full visibility of the action.[34]

Dominance rank, sex, the quality of the relationship, and reward characteristics all also influence the presence and/or strength of the reaction.[32] For example, within a well-established group of chimpanzees inequity aversion was less pronounced than in a newly-formed group.[46] And the difference in value between the higher and lower reward matters for capuchins, as does the quality of the reward but not the quantity.[47] Having a barrier between the capuchins or not did not make any difference.[48]

Only three studies have found evidence for advantageous inequity aversion.[30] Although in previous studies with chimpanzees it had never been observed, in a 2010 study with 16 captive adult chimpanzees, males and females, it was found chimpanzees that received a higher-value grape refused to participate more often when the other chimpanzee received an inferior carrot than they did when the other chimpanzee also received a grape.[49]

In another study with chimpanzees and very young children, a modified version of the Ultimatum Game, researchers gave chimpanzees a choice between a token representing five bananas for them and one for their partner, and a token representing an equal split of three bananas each. They had to pass this token to their partner who could either accept it and have it exchanged into the appropriate reward distribution, or reject it, resulting in nothing for both. Two of the four chimpanzees chose the equity token significantly more often than chance. When comparing their choices to those in a control condition, in which their partner had no option to reject the token, all four chimpanzees chose the equity token significantly more often.[50] Respondents never rejected an offer, but they did sometimes protest, for instance by spitting water at the selfish proposer.[31]

Talbot, Parrish, Watzek, Essler, Leverett, Paukner, and Brosnan argue that it is likely that the results are mixed because of small procedural differences.[47] They acknowledge that controlling for this is hard, especially across species as procedures often have to be tailored to the species in question, for instance in terms of their size and natural behaviour.[39]

Brosnan and de Waal summarized the findings as inequity aversion being most pronounced in animals that cooperate outside of the bonds of mating and kinship.[34] Chimpanzees, bonobos, capuchins, macaques, dogs and corvids are all highly cooperative in nature and show inequity aversion; orangutans, owl and squirrel monkeys are not cooperative outside kin and do not show inequity aversion.[51]

Method Evidence
Species Task Side-by-side Disadvantageous Advantageous Comments
BonoboExchange[52]NoMaybe–
BonoboUltimatum game[53]NoNoNo
BonoboNone[54]NoNo–
CapuchinExchange[20]YesYes–
CapuchinExchange[55]YesYes–
CapuchinPulling[56]YesYes–
CapuchinPulling[57]NoYesYes
CapuchinExchange[58]YesNo–
CapuchinExchange[59]NoNo–
CapuchinNone[60]YesNo–
CapuchinNone[61]YesNo–
CapuchinNone[59]YesNo–
CapuchinExchange[62]YesYes–
ChimpanzeeExchange[46]YesYes–
ChimpanzeeExchange[63]YesYesYes
ChimpanzeeExchange[64]YesYes–
ChimpanzeeExchange[52]NoNo–
ChimpanzeeUltimatum game[65]No–No
ChimpanzeeUltimatum game[53]NoNoNo
ChimpanzeeUltimatum game[36]Yes–Yes
ChimpanzeeNone[54]NoNo–
ChimpanzeePulling[66]NoNoNo
ChimpanzeeNone[67]YesYes–
ChimpanzeeChoice[25]YesYes–
CrowExchange[40]YesYes–
CrowExchange[1]YesNo–
DogAction[68]YesYes–
DogAction[69]YesYes–
DogAction[70]YesYes/NoNo
Dog (pack)Action[71]YesYesNo
GorillaNone[54]NoNo–
KeaNone[72]NoNo–
MacaquePulling[73]YesYesNo
MacaqueTarget[74]YesYes–
MarmosetTarget[26]YesNoNo
MarmosetTarget[75]YesYes–
MarmosetPulling[76]YesYes–
OrangutanExchange[52]NoNo–
OrangutanExchange[28]YesNoNo
OrangutanNone[67]YesNo–
OrangutanNone[54]NoNo–
Owl monkeyTarget[26]YesNoNo
RatChoice[77]YesYes–
RavenExchange[40]YesYes–
Squirrel monkeyExchange[29]YesNoNo
Squirrel monkeyTarget[26]YesNoNo
TamarinExchange[27]YesNoNo
TamarinPulling[78]YesNoNo
WolfAction[71]YesYesNo

Primates

Bonobos

Bonobos (Pan paniscus) are social animals that live in less hierarchical structures than chimpanzees.[79] Researchers Bräuer, Call, and Tomasello tested bonobos, together with other great apes, twice and twice failed to find evidence of inequity aversion.[54][52] In the first study the apes were simply given food.[80] Three years later they made a few procedural changes, critically now using the token exchange paradigm.[81] Bonobos still did not refuse lower-value food when seeing that a partner got a better reward. The researchers argued that because in their method apes in the equity condition were shown the higher-value reward prior to receiving the lower-value reward, only their method allowed for proper comparison between inequity and equity conditions.[82] Kaiser, Jensen, Call, and Tomasello designed a variant of the Ultimatum game that involved inequity being created by the proposer stealing a portion of the responder's share. They did not find any bonobo refusing any food, and proposers consistently stole food from responders, seemingly oblivious to the effect their action would have on their partner.[83]

Capuchins

Roughly half of the studies with capuchins (Sapajus apella) found evidence of inequity aversion.

Chimpanzees

Chimpanzees (Pan troglodytes) are smart, social animals.[84] In the wild they cooperate to hunt, dominate rival groups, and defend their territory.[85] Most studies did not find any evidence of chimpanzees rejecting lesser rewards when partners received better, but some did. One study found evidence of chimps rejecting better rewards.



Gorillas

In an experiment with three other great ape species, Bräuer, Call and Tomasello subjected six gorillas to the inequity task. Apes were given food without having to perform a task.[80] The researchers did not report results specifically for gorillas, but overall for all four species the apes did not refuse food more often when a partner got better food.[86] From their controlled experiments they concluded that any food refusal is not due to inequity aversion but, most likely, their expectation. This food expectation hypothesis says that subjects have an expectation to receive the preferred food in some conditions but not others. Seeing the experimenter give favored food to a conspecific, not just being placed into an empty cage, may have created the expectation that they will get some of the favored food soon.[86] In speculating why their findings are different from Brosnan and de Waal's, Bräuer, Call and Tomasello point to procedural differences, especially giving food versus token exchange.[87][upper-alpha 4]

Macaques

Macaques (Macaca spp.) are small monkeys that live in hierarchical groups. They do not habitually use tools, hunt cooperatively or share food.[89] Two out of two studies did find evidence for equity aversion in macaques.[73][74] Massen, Van Den Berg, Spruijt, and Sterck tested 12 long-tailed macaques in equity and inequity conditions with strangers and "friends". The researchers hypothesized that friends pay less attention to equity than strangers and thus they predicted no or only a small effect in the friends condition. Contrary to their predictions, the macaques responded to inequity in largely the same way with a friend as with a stranger. The monkeys only refused food significantly more when their effort was moderate. No effort or a lot of effort (the tray the subjects had to pull towards themselves was counterweighted with 2.3 kilograms) did not result in inequity aversion.[90] The researchers ruled out that refusal rates were higher due to a frustration effect because each individual monkey only ever received one type of reward. No evidence for advantageous inequity aversion was found.[91]

Hopper, Lambeth, Schapiro, Bernacky, and Brosnan were the first researchers to study the development of inequity aversion in animals. They first tested 20 young rhesus macaques (on average 17 months old) and found no difference in refusal rate between inequity and equity conditions. A year later they tested eight of them again and now found them refusing rewards more often in the inequity condition.[74]

Marmosets

Marmosets (Callithrix spp.) are small monkeys that form long-term parental pair bonds.[92] In two of three experiments with marmosets evidence was found of inequity aversion.[26][75][76] Freeman, Sullivan, Hopper, Talbot, Holmes, Schultz-Darken, Williams, and Brosnan found that none of the ten marmosets they tested differed significantly in their rate of refusals among the three task conditions.[93] In contrast, Yasue, Nakagami, Nakagaki, Ichinohe, and Kawai did find a difference in their test of six marmosets required to hold a spoon for two seconds to receive a reward. The monkeys almost always successfully performed the task when they observed a partner receive the same reward, but only in 70% of trials when they witnessed the partner receiving a more attractive reward.[94] This high refusal rate was not present in another condition where five other marmosets had been exposed to valproic acid, leading the researchers to conclude that inequity aversion stems from weak social motivation.[95]

Mustoe, Harnisch, Hochfelder, Cavanaugh, and French tested eight marmosets in a tray pulling experiment, where subjects would pull a tray with food towards themselves and partners.They found evidence for inequity aversion in the four male marmosets towards their pairmates. The males did not avert inequity when paired with strangers. The neurohypophysial hormone oxytocin, which has been found to modulate social behavior in primates, did not influence inequity aversion.[96]

Orangutans

Four inequity aversion studies involving orangutans (Pongo pygmaeus) have been published. No study found evidence of inequity aversion.

Owl monkeys

Owl monkeys (Aotus) live in small groups of up to five individuals, usually a male and female and their offspring. Both parents look after the children.[97] Freeman, Sullivan, Hopper, Talbot, Holmes, Schultz-Darken, Williams, and Brosnan tested three monkey species, including owl monkeys, in a variation of Brosnan and de Waal's original experiment. Instead of a token exchange the monkeys had to reach out of their cage, pick a token and hold on to it. Apart from the non-social condition with high-value food visible but not given, another control condition was added in which there was no task at all but the animals were simply given food. The researchers found that owl monkeys did not differ in their rate of refusals among the four conditions.[93] They hypothesized that for species that provide bi-parental care the cost of having conflict with their reproductive partner may be too high to warrant a refusal reaction over a small amount of inequity.[98]

Squirrel monkeys

In the wild, squirrel monkeys (Saimiri sciureus) do not regularly cooperate.[98] Talbot, Freeman, Williams, and Brosnan tested squirrel monkeys in a token-for-food exchange experiment, with a free food and contrast conditions as controls. The monkeys did not refuse food more often in the inequity condition than in the equity condition. They refused food far less often in the free food condition than in the token exchange conditions. The male squirrel monkeys refused food the most in the contrast condition, when given inferior food after initially having been shown better food. The females refused food less often in all conditions, the least in the free-food condition. The researchers concluded that squirrel monkeys are not averse to inequity.[99] Freeman, Sullivan, Hopper, Talbot, Holmes, Schultz-Darken, Williams, and Brosnan found no evidence of inequity aversion in squirrel monkeys either. They too observed a strong contrast effect: when there was no partner but a better reward present than given the monkeys refused food the most by far.[93]

Tamarins

Cotton-top tamarins (Saguinus oedipus) are New World monkeys that breed cooperatively, cooperative on tasks and tolerate food sharing.[24] Neiworth, Johnson, Whillock, and Greenberg tested 11 tamarins in six conditions. In one of the conditions, the effort+food inequity condition, the subjects were given the less preferred food as reward for a token exchange, whereas the partners were just handed the more preferred food for no effort. The highest rejection rates were in the food control condition, with low-value food given while preferred food was present. The researchers did find some evidence of inequity aversion, but only by comparing results from the first set of trials to the last in the food inequity condition of the token exchange task. In the non-social conditions this significant rise in rejections was not found. The researchers equated this finding to the original Brosnan and de Waal study and attributed it to an increasing aversion to the perceived inequity. Curiously, in the effort+food inequity condition the rejection rates did not rise significantly. The researchers theorised that the animals judged the situation differently as the partner did not have to act, not triggering the equity comparison scheme.[100]

McAuliffe, Shelton, Stone tested the reaction of 12 cotton-top tamarins to inequity in a personalised handle pulling task. The researchers suspected that the amount of effort involved is a key factor in inequity aversion and therefore designed a tray pulling task with weights, after having calibrated how much weight each subject was willing to pull for food. In the inequity conditions the subjects had to exert a lot of effort for little food while their partners received more food for no effort. The researchers found weak support for the influence of effort on inequity aversion, largely driven by one female.[78]

Other mammals

Birds

Evolution

Almost all researchers explain the findings of animals refusing less-preferred food while others receive more-preferred food in terms of inequity aversion and a sense of fairness. The cost of foregoing a low-value food when the partner gets a high-value reward is low. It's worth to reject, protest and possibly get something better. The cost of foregoing a high-value food when the partner gets an even higher-value reward is high, not worth protesting about and risk being left empty-handed.[101] Comparing one's gains to those of others makes evolutionary sense. If individuals were satisfied with any absolute benefit, they might still face negative fitness consequences if they were doing less well than competing others.[102] But this applies only in the context of extensive cooperation outside of kinship relationships.[23]

One explanation of the findings so far is that inequity aversion evolves in order to foster long-term cooperation between unrelated individuals. In particular, Brosnan suggests that responding to inequity facilitates partner choice. This increases an individual’s fitness by enabling them to reject partnerships which repeatedly lead to unequal outcomes. In support of this, inequity aversion is found in highly-cooperative capuchins, but not in the closely related, less cooperative squirrel monkey; and in cooperative chimpanzees, but not in typically less cooperative orangutans.[103] This theory predicts that domestic cats are far less sensitive to inequity than dogs.[31]

The main explanation for disadvantageous inequity aversion is anticipatory conflict resolution. The animal anticipates their partner reacting negatively to disadvantageous inequity and thus rejects the better reward, or in the case of the Ultimatum Game, favours the equity token over the favorable one. Researchers have speculated that the reason why it is limited to chimpanzees and capuchins is that it requires the cognitive capability of planning, anticipating their partner's disadvantageous inequity aversion. Few species have this capacity. Chimpanzees have shown their ability to plan ahead in other contexts, for instance in tool use. Advantageous inequity aversion may also directly benefit an individual by enhancing its reputation, which may increase that individual’s long-term access to beneficial relationships.[31]

Including evidence from canines, Essler, Marshall-Pescini, and Range conclude that it is possible that sensitivity to inequity was already present in an earlier common ancestor with primates. Alternatively, convergent evolution may be at play: under similar conditions the same behavior has emerged multiple times in evolution.[104] Kim, Choe, Jeong, and Kim state it is an open question whether orangutans have lost or chimpanzees have acquired a sense of fairness in the hominid lineage.[67]

While the controlled experiments have advanced the understanding of inequity aversion, their context cannot include all possible outcomes that exist in natural social interactions. In the standard inequity task, refusals only hurt the actor, whereas in a natural social context, protest against inequity may lead to the actor either receiving a larger share or seeking out a better partner to work with.[32]

Footnotes

  1. ↑ Advantageous inequity aversion was found in USA, Canada, and Uganda, but not in Senegal, Peru, Mexico, and India.[4]
  2. ↑ Brosnan's proposed experiment to better understand this socio-economic behavior nearly was rejected by her PhD committee, because she had already planned six other experiments. But because the committee could not agree on the expected outcome of her proposed experiment, her advisor, Frans de Waal, told her she obviously had to do it.[17]
  3. ↑ The Impunity game is a variant of the Ultimatum game with a small twist: if the responder rejects the proposal, the proposer gets to keep the reward and the responder gets nothing.[32] This game has not been tested with animals. The challenge is to ensure the animals understand that the proposer had a choice.[35]
  4. ↑ Researchers van Leeuwen, Zimmermann, and Ross analyzed video of play fights in captive gorillas. Seven out of eight gorillas would hit another ape and then run away. The researchers interpreted this as the hitter getting an advantage and wanted to keep this advantage hence the running. The "victim" wants to avoid this inequity and thus chases.[88]

References

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  26. 1 2 3 4 5 6 7 8 Freeman et al. 2013.
  27. 1 2 3 Neiworth et al. 2009.
  28. 1 2 3 Brosnan et al. 2011.
  29. 1 2 3 Talbot et al. 2011.
  30. 1 2 3 4 5 6 7 8 Brosnan & de Waal 2014, p. 1251776-4.
  31. 1 2 3 4 Brosnan & de Waal 2014, p. 1251776-5.
  32. 1 2 3 4 5 6 Brosnan & de Waal 2014, p. 1251776-2.
  33. ↑ Tinklepaugh 1928.
  34. 1 2 3 4 5 Brosnan & de Waal 2014, p. 1251776-1.
  35. 1 2 Brosnan 2013, p. 10418.
  36. 1 2 Proctor et al. 2013.
  37. ↑ Yamagishi et al. 2009.
  38. ↑ Talbot, Price & Brosnan 2016.
  39. 1 2 Talbot et al. 2018, p. 76.
  40. 1 2 3 Wascher & Bugnyar 2013.
  41. ↑ Essler, Marshall-Pescini & Range 2017, p. 1861.
  42. ↑ Sheskin et al. 2014.
  43. ↑ Talbot et al. 2018, p. 80.
  44. ↑ Brosnan, Freeman & de Waal 2006, p. 713.
  45. ↑ Melis, Hare & Tomasello 2009.
  46. 1 2 Brosnan, Schiff & de Waal 2005.
  47. 1 2 Talbot et al. 2018, p. 85.
  48. ↑ Talbot et al. 2018, p. 84.
  49. ↑ Brosnan et al. 2010, p. 1235.
  50. ↑ Proctor et al. 2013, p. 2071.
  51. ↑ Brosnan & de Waal 2014, p. 1251776-4,5.
  52. 1 2 3 4 Bräuer, Call & Tomasello 2009.
  53. 1 2 Kaiser et al. 2012.
  54. 1 2 3 4 5 Bräuer, Call & Tomasello 2006.
  55. ↑ van Wolkenten, Brosnan & de Waal 2007.
  56. ↑ Fletcher 2008.
  57. ↑ Takimoto, Kuroshima & Fujita 2010.
  58. ↑ Silberberg et al. 2009.
  59. 1 2 Fontenot et al. 2007.
  60. ↑ Roma et al. 2006.
  61. ↑ Dindo & de Waal 2007.
  62. ↑ Talbot et al. 2018.
  63. ↑ Brosnan et al. 2010.
  64. ↑ Hopper et al. 2014.
  65. ↑ Jensen, Call & Tomasello 2007.
  66. ↑ Ulber, Hamann & Tomasello 2017.
  67. 1 2 3 Kim et al. 2018.
  68. ↑ Range et al. 2009.
  69. ↑ Range, Leitner & Virányi 2012.
  70. ↑ Horowitz 2012.
  71. 1 2 Essler, Marshall-Pescini & Range 2017.
  72. ↑ Heaney, Gray & Taylor 2017.
  73. 1 2 Massen et al. 2012.
  74. 1 2 3 Hopper et al. 2013.
  75. 1 2 Yasue et al. 2018.
  76. 1 2 Mustoe et al. 2016.
  77. ↑ Oberliessen et al. 2016.
  78. 1 2 McAuliffe, Shelton & Stone 2014.
  79. ↑ Hare et al. 2007, p. 619.
  80. 1 2 Bräuer, Call & Tomasello 2006, p. 3124.
  81. ↑ Bräuer, Call & Tomasello 2009, p. 176.
  82. ↑ Bräuer, Call & Tomasello 2009, p. 179.
  83. ↑ Kaiser et al. 2012, p. 943.
  84. ↑ Rekers, Haun & Tomasello 2011, p. 1756.
  85. ↑ Melis, Hare & Tomasello 2006, p. 275.
  86. 1 2 Bräuer, Call & Tomasello 2006, p. 3126.
  87. ↑ Bräuer, Call & Tomasello 2006, p. 3127.
  88. ↑ van Leeuwen, Zimmermann & Ross 2011.
  89. ↑ Massen et al. 2012, p. 154-155.
  90. ↑ Massen et al. 2012, p. 145;149.
  91. ↑ Massen et al. 2012, p. 151.
  92. ↑ Mustoe et al. 2016, p. 70.
  93. 1 2 3 Freeman et al. 2013, p. 4.
  94. ↑ Yasue et al. 2018, p. 38.
  95. ↑ Yasue et al. 2018, p. 36.
  96. ↑ Mustoe et al. 2016, p. 69.
  97. ↑ Freeman et al. 2013, p. 2.
  98. 1 2 Freeman et al. 2013, p. 6.
  99. ↑ Talbot et al. 2011, p. 681.
  100. ↑ Neiworth et al. 2009, p. 15-16.
  101. ↑ Talbot et al. 2018, p. 83.
  102. ↑ Brosnan 2006.
  103. ↑ Jelbert et al. 2015, p. 2.
  104. ↑ Essler, Marshall-Pescini & Range 2017, p. 1864.

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