Inequity aversion in animals
[16] Further examples of cooperation include parents and others working together to raise young[12] (e.g., among African elephants),[17] and groups defending their territory, which has been studied in primates and other social species such as bottlenose dolphins, spotted hyenas, and common ravens.
[28] 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.
[35] Researchers have also tested non-primates that display any capability of cooperation: birds (crows, ravens, kea), canines (dogs, wolves),[36][37][38] rodents (mice and rats),[39][40] and cleaner fish.
[36] The first follow-up studies that failed to replicate the original Brosnan and de Waal findings subsequently were shown to have a crucial difference in experimental setup.
[29] 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.
[53][47] McAuliffe, Chang, Leimgruber, Spaulding, Blake, and Santos, for example, found no evidence of either disadvantageous or advantageous inequity aversion in a choice experiment often used with humans.
[55] Talbot, Parrish, Watzek, Essler, Leverett, Paukner, and Brosnan argued that the results had been mixed because the experimental setups differ and even small details might influence the capuchins' behavior.
From the higher rejection rates in the second round with grapes than in the first, and little difference between the social and non-social settings, the researchers concluded capuchins are not inequity averse, but that instead their behavior is the result of a contrast effect.
[123] On the other hand, Brosnan, Talbot, Ahlgren, Lambeth, and Schapiro administered a similar test to 16 chimpanzees and found that males were inequity averse but females were not.
[75][87][56][76][90] Brosnan, Talbot, Ahlgren, Lambeth, and Schapiro attributed overall variability in results to differences in procedures and small sample sizes making it hard to reliably control for factors such as rank and sex.
[127] Feller tested two white-cheeked gibbons (Nomascus leucogenys) in a target-holding experimental paradigm, predicting that they would not respond negatively to receiving inferior rewards than their partner.
[101][65] Massen, Van Den Berg, Spruijt, and Sterck tested 12 long-tailed macaques (Macaca fascicularis) in a tray-pulling experiment with strangers and "friends".
[30][103][102] 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.
[140] 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.
[140] 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.
[151] Talbot, Freeman, Williams, and Brosnan tested squirrel monkeys in a token-for-food exchange experiment, with a free food and contrast conditions as controls.
[152] Freeman, Sullivan, Hopper, Talbot, Holmes, Schultz-Darken, Williams, and Brosnan found no evidence of inequity aversion in their test with squirrel monkeys either.
[140] Bucher, Bourgeois, Anderson, Kuroshima and Fujita used an experimental set-up in which squirrel monkeys had to pull a platform with trays, with different quantities and qualities of food, to themselves and a partner.
[106] Vale, Williams, Webb, and Schapiro tested female squirrel monkeys in a group setting, questioning the results of previous experiments with just pairs.
[157] Range, Horn, Virányi, and Huber found a negative reaction to an all-or-nothing distribution of rewards, in an experimental setup similar to Brosnan and de Waal's original work, with the task of giving a paw.
[99] Also following the 2018 review, McGetrick, Ausserwöger, Leidinger, Attar, and Range tested the hypothesis that a shared food source is necessary to elicit inequity aversion in dogs, but found that not to be the case.
[95][158] Brucks, Marshall-Pescini, Essler, McGetrick, Huber, and Range tested the willingness of 44 dogs to press a buzzer with their paw to get a food reward inferior to their partner's.
[40] Hernandez-Lallement, van Wingerden, Marx, Srejic, and Kalenscher tested 68 male rats in a series of maze experiments where the animals could choose between a path that led to rewards just for them or for a partner as well.
[165] In a similar experiment with 23 rats, Oberliessen, Hernandez-Lallement, Schable, van Wingerden, Seinstra, and Kalenscher found supporting evidence of inequity aversion.
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.
[180] 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.
[181] Basing their argument partly on the facts that female chimpanzees often range solitarily and avert inequity less than males, Brosnan, Flemming, Talbot, Mayo, and Stoinski state the most likely hypothesis is that natural selection favours those that care how their outcomes compare to others.
[183] In their review of studies into the cognitive abilities of parrots and corvids, Lambert, Jacobs, Osvath and von Bayern conclude it is simply too early to make any conclusions about the relationship between inequity aversion and cooperation.
[5] Debove, Baumard, and André ran computer simulations of individuals of different rank cooperating with equal and unequal reward distributions and concluded that when partner choice is a characteristic of the setup, fairness emerges.
