There are four ways that one individual can interact with another based on fitness gains and losses for both individuals: altruism, mutualism, selfishness, and spite (Hamilton 1964
)—see . Of these, altruism and spite are the most puzzling because of their costs to the actor. Darwin (1871)
recognized that for altruism to evolve there must be benefits to the altruist, and Hamilton (1964)
and Trivers (1971)
suggested how inclusive fitness and reciprocity, respectively, can provide these benefits. Even so, the existence of altruism in animals remains controversial (e.g. Hammerstein 2003
). Spite has received far less attention than altruism, and though theoretically plausible (Hamilton 1970
; Wilson 1975
), its existence in animals is also debated (e.g. Foster et al. 2001
Table 1 Payoff matrix for costs and benefits to two individuals (actor and recipient) as a result of the actor's actions (after Hamilton 1964).
It has been suggested by Brosnan et al. (2005)
that one of humans' closest living relatives, the chimpanzee (Pan troglodytes
), has a sense of fairness in that they are sensitive and averse to disadvantageous inequity. In their study, captive chimpanzees exchanged PVC tubes with a human experimenter for food. If another chimpanzee received a higher quality piece of food for equal effort, the focal subject would be more likely to reject the human experimenter's offer. These results require some qualification, however, as chimpanzees living in long-term groups almost never rejected food, and overall, chimpanzees did not show sensitivity to unfair offers based on differences in effort, whereas brown capuchins (Cebus apella
) did (Brosnan & de Waal 2003
). More critically, the application of the results of these studies to inequity aversion is confounded by the fact that the individuals could not directly correct inequitable outcomes; in fact, by rejecting ‘unfair’ offers, they were actually increasing disadvantageous inequity.
Silk et al. (2005)
did allow chimpanzees to control outcomes for conspecifics and found that they did not take others' outcomes into consideration (i.e. they were not other regarding). Chimpanzees from two research centres were given two different apparatuses that allowed them to pull food towards themselves. When paired with conspecifics, the chimpanzees could choose between mutualism (1/1 payoff) and selfishness (1/0 payoff). Chimpanzees in this study were found to be not averse to inequity—they were just as likely to make a mutually beneficial choice as a selfish one. While this is a valuable contribution to the study of the evolution of fairness, Silk et al.
tested for other-regarding behaviour in only one context (mutualism versus selfishness), and there was no direct demonstration that the chimpanzees understood or attended to the distal consequences of their choices.
We gave captive chimpanzees the opportunity to control outcomes for both themselves and conspecifics in three food-acquisition tasks designed to probe both advantageous and disadvantageous inequity aversion using the Hamilton payoff matrix, and we examined their understanding of the tasks. We used a modified version of a food-pulling procedure that Hauser et al. (2003)
used on cotton-top tamarins (Saguinus oedipus
). To make out-of-reach food accessible by pulling on a rope, the chimpanzees also made another piece of food move further away; they could, therefore, pull food towards or
away from another individual. In the first study, chimpanzees could choose between mutualism and selfishness. If averse to advantageous inequity, chimpanzees should choose mutualism over selfishness; if averse to disadvantageous inequity, they should choose selfishly to prevent free-rider benefits to the other; if not averse to inequity, or if not other-regarding, then chimpanzees should show no preference. In the second study, chimpanzees were given a choice between altruism and weak spite (doing nothing led to the same outcome). If chimpanzees are not averse to disadvantageous inequity, they should make altruistic choices; if averse to disadvantageous inequity, they should be as likely to do nothing because to be spiteful as both would have the same outcomes; an absence of a preference would again suggest a lack of awareness—or indifference—towards the outcomes of another. In the final study, the competing choices were between altruism and true spite (here, unlike experiment 2, inaction would lead to a positive outcome for the other). If not averse to disadvantageous inequity, chimpanzees should either do nothing or make altruistic choices; if averse to disadvantageous inequity, they would choose spitefully; again, the absence of any preference would suggest that chimpanzees are not other-regarding or are not averse to perceived inequities.