UMD BIOL 608W - Tolerance for inequity may increase with social closeness in chimpanzees

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Tolerance for inequity may increase with social closeness in chimpanzeesIntroductionMaterial and methodsStudy subjectsExperimental testing procedureTesting paradigmsStatisticsResultsDiscussionREFERENCESdoi: 10.1098/rspb.2004.2947, 253-258272 2005 Proc. R. Soc. B Sarah F. Brosnan, Hillary C. Schiff and Frans B. M. de Waal chimpanzeesTolerance for inequity may increase with social closeness in Referenceshttp://rspb.royalsocietypublishing.org/content/272/1560/253#related-urls Article cited in:Email alerting service hereright-hand corner of the article or click Receive free email alerts when new articles cite this article - sign up in the box at the top http://rspb.royalsocietypublishing.org/subscriptions go to: Proc. R. Soc. BTo subscribe to This journal is © 2005 The Royal Society on March 18, 2010rspb.royalsocietypublishing.orgDownloaded fromProc. R. Soc. B (2005) 272, 253–258doi:10.1098/rspb.2004.2947Published online 04 February 2005Tolerance for inequity may increase with socialcloseness in chimpanzeesSarah F. Brosnan, Hillary C. Schiff and Frans B. M. de WaalYerkes National Primate Research Center, Emory University, 954 North Gatewood Drive, Atlanta, GA 30329, USAEconomic decision-making depends on our social environment. Humans tend to respond differently toinequity in close relationships, yet we know little about the potential for such variation in other species. Weexamine responses to inequity in several groups of chimpanzees (Pan troglodytes) in a paradigm similar to thatused previously in capuchin monkeys (Cebus apella). We demonstrate that, like capuchin monkeys, chim-panzees show a response to inequity of rewards that is based upon the partner receiving the reward ratherthan the presence of the reward alone. However, we also found a great amount of variation between groupstested, indicating that chimpanzees, like people, respond to inequity in a variable manner, which we specu-late could be caused by such variables as group size, the social closeness of the group (as reflected in length oftime that the group has been together) and group-specific traditions.Keywords: inequity; value; relationship quality; within-species variation; chimpanzee; Pantrogolodytes1. INTRODUCTIONInequity aversion (IA) appears to have played a key rolein the evolution of cooperation in humans (Fehr &Fischbacher 2003), yet little is known about how other spe-cies respond to unequal pay-offs. Not all species are expec-ted to show an adverse response to inequity, but such aresponse is expected in species with high levels ofcooperation (Dugatkin 1997). Furthermore, species thatare socially tolerant may have expectations about what theyshould receive (de Waal 1996), again predisposing themfor IA.As laid out by Fehr & Schmidt (1999), there are twokinds of IA: disadvantageous IA, or disliking if anotherindividual receives more than yourself, and advantageousIA, or disliking if you receive more than another individual(i.e. overcompensation; Walster [Hatfield] et al. 1978).Although in humans disadvantageous IA may manifestitself as the willingness to sacrifice potential gain to blockanother individual from receiving a superior reward (Wal-ster [Hatfield] et al. 1978; Fehr & Schmidt 1999), suchcomplex behaviour probably evolved over a series of sim-pler, intermediate steps which, at each point, increased theindividual’s relative fitness (Brosnan & de Waal 2004a). Afirst step is the recognition that other individuals obtainrewards that are different from one’s own, which is alsoimportant for other behaviours, such as social learning. Atthe second level, the maligned individual feels stronglyenough to react to the discrepancy, presumably leading tothe abandonment of the current inequitable relationship.At the third level, the individual will sacrifice their owngains to take away from those of a lucky individual, restor-ing equity. Advantageous IA, by contrast, consists ofresponses when either the self or an observed third party isovercompensated, which presumably developed after theevolution of disadvantageous IA. We focus on the secondprecursor of disadvantageous IA (i.e. whether chimpanzeesreact if they receive pay-offs different from those of a con-specific partner) and on one element of advantageous IA(i.e. whether ‘lucky’ individuals show any response to over-compensation).Chimpanzees (Pan troglodytes) are one species fromwhich we might expect to see such variation. First, chim-panzees are capable of modifying their responses as thesocial situation changes (e.g. food calls; Brosnan & de Waal2003a). Furthermore, chimpanzees are the species mostclosely related to humans and commonly thought to eluci-date the behaviour of our early ancestors (Boehm 1999;Tomasello 1999), and thus are ideal subjects for elucidat-ing the evolution of the more complex stages of IA in theprimate lineage.For several reasons, we chose to test subjects from twodifferent social groups of chimpanzees as well as twopair-housed groups. First, we know that chimpanzees’behaviour varies among different groups based upon suchcharacteristics as their housing situation (Aureli & de Waal1997; Baker et al. 2000) and their social group (Whitenet al. 1999), but, owing to a scarcity of captive chimpanzeegroups, it is rare that scientists are able to use individualsfrom more than one group. Second, the human inequityaverse response is quite variable, and social relationshipsaffect responses to inequity. Individuals in positive rela-tionships are more oriented towards equity and averse togetting more than their partner than those in negative rela-tionships (Loewenstein et al. 1989), and those in close rela-tionships follow communal orientation whereas those inmore distant relationships follow contingent rules such asequity or equality (Walster [Hatfield] et al. 1978; Clark &Grote 2003; de Waal & Brosnan 2004). Thus, more variedresponses to IA may also be expected in animal societiesthat have strong social relationships and vary their behav-iour between situations.Chimpanzees were tested for their reactions to inequi-table situations in which the reward level varied, as well asthe amount of effort required to obtain it. Based on what isknown about the inequity response in capuchins (CebusAuthor for correspondence ([email protected]).Received 15 June 2004Accepted 20 September 2004253#2005 The Royal Society on March 18, 2010rspb.royalsocietypublishing.orgDownloaded fromapella; Brosnan & de Waal 2003a), we predicted that


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