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HypotHesis and tHeory articlepublished: 05 April 2011

doi: 10.3389/fnins.2011.00043

game, reusing results in less absolute gain, but increased relativeequity, as both participants get nothing; Bolton and Zwick, 1995).

Note, too, that peoples reusals in this context are not inequityaversion as proposed byFehr and Schmidt (1999), as individu-als are increasingrather than decreasing inequity. However, recentevidence indicates that people routinely reuse unequal oers in

the impunity game, making this an important set o circumstancesto evaluate more ully (Yamagishi et al., 2009).

Humans are not alone in this. Other species show very similarbehaviors. In a game reminiscent o an impunity game, two specieso primate have been shown to reuse rewards more oten whentheir partners get better rewards than when their partners receive

the same, lower-value rewards (Brosnan, 2006; details in next sec-tion). While this behavior seems economically irrational, it is avery consistent response in both these primates and humans. This

raises questions about the evolution o this behavior on a numbero levels. First, is it possible that the underlying unction o thisbehavior is similar across species, including humans? Second, even

i the behavior has been selected or similar reasons, what are themechanisms that lead to the behavior? Related to this, how do weaccount or the individual dierences seen in the behavior, andare these dierences, and their causes, consistent across species?

Although the data we have now provide pieces o the puzzle,there is as yet no satisying unctional explanation, and the very

ubiquity o the response demands an answer. One hypothesis isthat recognizing and responding to inequitable outcomes increases

IntroductIon

When making decisions about resources, humans show an intense

interest in how their outcomes compare to those o others. In thelaboratory, people will reject absolute gains in order to keep oth-ers rom receiving more (Guth et al., 1982), even i this results ingreater inequity (Yamagishi et al., 2009). This behavior seems to

aect oers, too; individuals oer more when their partner has achance to reuse than when they have no recourse (e.g., compar-ing the ultimatum to the dictator games; Camerer, 2003). Peoplesreaction to inequity is also highly context dependent. Subjects makedierent decisions i the right to control distributions must beearned (Homan et al., 1994) or i they have an opportunity to

respond in some other way (Xiao and Houser, 2005). We do notmake decisions in a vacuum, but instead seem to care very muchhow our outcomes compare to those o our social partners.

Although much o this research is done using the ultimatumgame, another important game in this vein is the impunity game. Inthis game, one individual, the proposer, is given an endowment omoney and must decide how to split the money between themselves

and another individual, the responder. The responder then has twooptions; i they accept, the distribution is given to each individual asproposed, as in the ultimatum game, but i they reject, the proposerreceives their money, but the responder receives nothing. This gameis little studied, likely because reusing results in both less absolutegain and increased relative inequity or the responder, thus it was

assumed that no rational actor would reuse (in the Ultimatum

A hypothesis of the co-evolution of cooperation and responses

to inequity

Sarah F. Brosnan*

Department of Psychology and Neuroscience Institute, Georgia State University, Atlanta, GA, USA

Recent evidence demonstrates that humans are not the only species to respond negatively

to inequitable outcomes which are to their disadvantage. Several species respond negatively

if they subsequently receive a less good reward than a social partner for completing the same

task. While these studies suggest that the negative response to inequity is not a uniquely

human behavior, they do not provide a functional explanation for the emergence of these

responses due to similar characteristics among these species. However, emerging data support

the hypothesis that an aversion to inequity is a mechanism to promote successful long-term

cooperative relationships amongst non-kin. In this paper, I discuss several converging lines of

evidence which illustrate the need to further evaluate this relationship. First, cooperation can

survive modest inequity; in explicitly cooperative interactions, individuals are willing to continue

to cooperate despite inequitable outcomes as long as the partners overall behavior is equitable.

Second, the context of inequity affects reactions to it in ways which support the idea thatjoint efforts lead to an expectation of joint payoffs. Finally, comparative studies indicate a link

between the degree and extent of cooperation between unrelated individuals in a species and

that species response to inequitable outcomes. This latter line of evidence indicates that this

behavior evolved in conjunction with cooperation and may represent an adaptation to increase

the payoffs associated with cooperative interactions. Together these data inform a testable

working hypothesis for understanding decision-making in the context of inequity and provide

a new, comparative framework for evaluating decision-making behavior.

Kywods: nqty, coopaton, non-hman pmats, aps, monkys, volton of bhavo, compaatv appoach

Edited by:

Daeyeol Lee, Yale University School of

Medicine, USA

Reviewed by:

Ming Hsu, University of California

Berkeley, USA

Masataka Watanabe, Tokyo

Metropolitan Organization for Medical

Research, Japan

*Correspondence:

Sarah F. Brosnan, Department of

Psychology and Neuroscience

Institute, Georgia State University, PO

Box 5010, Atlanta, GA 30302-5010,USA.

e-mail: [email protected]

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individuals payos rom cooperation. I this is the case in humans,it may be in other species as well, but only recently has enoughdata emerged to begin to address this hypothesis in species otherthan humans. In this paper, I discuss this emerging evidence innon-human primates, and based on three main lines o inquiryput orth the hypothesis that there has been co-evolution between

cooperation and inequity across the animal kingdom. I discuss the

implications o this hypothesis or other, related, areas o inquiry.Finally, I end with ideas or urther tests to evaluate and rene thishypothesis, both in primates and other species.

InequIty paradIgms In other specIes

Given inherent species dierences, most o the protocols or study-ing inequity in other species are simpler than those in humans, or

instance requiring no verbal instruction. In the typical paradigm,two individuals rom the same social group are paired, and theymust alternately complete a task to receive a reward (see Figure 1or details o the experimental procedure). Each can see the othersperormance and the others outcomes. In the baseline condition,rewards are the same, but in the inequity condition, one partner

receives a reward which is more preerred (e.g., based on previouspreerence tests between the rewards; see Table 1). Thus we cancompare individuals reactions to a reward when their partner getsthe same reward as they do versus a more preerred one. Additionalcontrols can examine potentially mediating actors such as the roleo dierential eort or the way in which the mere presence ohigher-value rewards (which are not given to a conspecic) may

aect reactions. Such studies have now been done in a variety oprimates, as have similar studies in other taxa (Heidary et al., 2008;Range et al., 2008).

Decisions to reuse a reward stem rom many actors, nonethe-less these results cannot be ully explained by processes other thanthe aversion to inequitable outcomes. For instance, it has been pro-

posed that reusals o rewards may be due to a rustration eect,in which subjects compare their current outcomes to those whichthey received previously, and protest i the comparison comes upwanting (Roma et al., 2006). Such individual contrast eects areseen in a wide variety o species (Tinklepaugh, 1928; Friedan et al.,

2009), however they do not explain these results. In controlledexperiments, the mere presence o the higher-value rewards didnot cause increased rejections, even when the experimenter calledthe subjects attention to the preerred reward prior to each inter-action (Brosnan et al., 2010a). O course it is likely that these twophenomena are built on similar cognitive underpinnings; in both

cases, subjects compare their current outcomes to some other re-

erent and nd the current outcome wanting. However, in the caseo the rustration eect, the reerent is individual, or ones ownprevious outcomes, whereas in the case o inequity, the reerent issocial, or ones partners outcomes. Thus the response to inequitycan be thought o as a social contrast or social rustration eect.Given the ubiquity o individual contrast eects, as well as atten-

tion to others outcomes (e.g., in the context o social learning), wemight expect inequity responses to be widespread.

Primates also show quite a bit o variation in their responses.Considering only chimpanzees, responses vary both within andbetween experiments. Thus ar, actors which have been implicatedin this variation include dominance rank, sex, and group identity.

Other actors, such as personality and individuals relationships

are also likely to play a role. However there is not yet consistencyin which actors aect behavior, probably due to both interactionsbetween these and other eects and the relatively small sample sizewhich has been tested (approximately three dozen chimpanzees,which is large or primate studies, but too small to pinpoint such

interactions). Larger studies are currently underway to investigatethis variation. It is also not yet clear how this compares to inter-individual dierences in humans, as studies with humans typicallyocus on mean responses rather than individual behavior.

hypotheses for the functIon of the response to

InequIty

It has been proposed that the inequity response unctions to increase

the success o long-term cooperative relationships amongst unre-lated individuals (hereater cooperation; Fehr and Schmidt, 1999;Brosnan, 2006). Although this was originally proposed with respectto humans, new evidence provides support or this hypothesis in an

evolutionary context. Specically, the ability to recognize situationsin which one is receiving a less good outcome than a partner, orinequity, may allow individuals to determine when their coopera-tive partners are taking more than their air share and are thus nolonger to ones benet as a partner. In other words, an aversionto inequity can be a mechanism which encourages individuals to

switch to a new partner when they nd themselves in a situationwhich is not to their advantage. This ultimately unctions to increasepayos by encouraging individuals to seek out new partners. I the

new partner is more equitable, then there is a benet and the indi-vidual will have an absolute gain, despite temporary costs associatedwith time spent searching or new partners or potential interludeswith other inequitable individuals. This mechanism would be under

strong positive selection due to the potential or large tness gains.Note that while the original inequity aversion ormulation assumedthat individuals were averse to decisions which increased relativeinequity (Fehr and Schmidt, 1999), even reactions that increaseshort-term inequity (e.g., in the Impunity game) may serve toincrease long-term equity by moving actors in to relationships

which are more benecial.

Figure 1 | A schmatc daam ndcatn th pocd fo ach tal of

an nqty tst. Two primates (P1 and P2) are tested in a pair. For each

condition (see Tabl 1, columns 1 and 2), the primates must sequentially

perform a task with the experimenter (typically a token exchange; Tabl 1,

column 3) in order to receive a food reward (Tabl 1, column 4; see full

description of each condition in column 5). An arrow indicates the order in

which the object (token or food) moves between the experimenter (E) and the

primates in each step of the trial. In all studies in my laboratory, primates are

seated side-by-side. Monkeys are separated by a mesh barrier which they can

reach through while apes are not separated. The details of the exchange task

and the foods given to each primate are determined by the test condition; for

details see Tabl 1.

Fonts n Noscnc | Decision Neuroscience April 2011 | Volume 5 | Article 43 | 2

Brosnan Cooperation and responses to inequity
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This hypothesis also ties in nicely with other hypotheses or

why individuals respond negatively to inequity. It has been arguedthat responding negatively to inequity, including when one is the

beneted party, can unction as a commitment device (Frank, 1988,2001; Yamagishi et al., 2009). Responding in this way indicates toboth ones current and potential uture partners that you are a verygood partner (i.e., you do not treat your other partners inequita-

bly), and may also send a signal which increases ones reputation(Frank, 2004). Similarly, reusing absolute gains which are relativelyunequal sends a signal to potential partners that they cannot getaway with such behavior with you, which may increase uture pay-os (Yamagishi et al., 2009). Such behavior represents a short-termcost (in the orm o lost immediate gains) or a long-term gain (in

the orm o long-term benecial relationships).Evidence is beginning to emerge to support the hypothesis that

cooperation and the response to inequity are linked in species besides

humans (see Existing Evidence in Support o the Hypothesis). Sucha link may provide evidence or how the response evolved. Thisquestion is more than academic. Understanding the evolutionary

trajectory o a behavior can help elucidate its evolutionary unc-tion. Moreover, while it is oten assumed that traits within a taxonare likely to be homologous, this does not need to be the case. Myresearch indicates that responding negatively to receiving less thanones partners is not homologous among primates, but rather is con-vergent, and the trait which best maps on to it is cooperation. Below

I summarize lines o evidence leading to this conclusion, relying ondata rom a number o dierent species in several dierent contexts.

Note, too, that this would not need to be consciously understood

by the individual; those who developed an aversion to inequitythrough whatever mechanism (e.g., an emotional reaction) would

be more likely to succeed, increasing the requency o the reactionin the population. Considering one possible scenario, in most casesin which individuals experience inequity during or immediatelyollowing an interaction with another individual (e.g., not just inproximity to another, but ater explicitly interacting with them), the

inequity is likely to be related to the others actions. Although therewould undoubtedly be a ew situations in which this was not thecase, this would be somewhat ameliorated i individuals kept tracko more than one interaction in the relationship history, as theycan do (Brosnan et al., 2006). Thus individuals who happened torespond in these situations could develop the behavior withoutan

understanding o the others intentions or motives (although notethat primates likely have the requisite understanding o intention-

ality and ability to inhibit; see The Context o the Interaction andInequity and Sel-Control). Alternatively, this mechanism couldunction similarly to that which has been proposed or attitudinalreciprocity, in which individuals base their moves on their cur-

rent eelings or their partner (Brosnan and de Waal, 2002; Schinoand Aureli, 2010). Thus there is no expectation that an inequityresponse needs to be paired with any higher-order cognition inorder to unction in this way. In act, only partner recognitionis required, and this is likely widespread throughout the animalkingdom (including invertebrates; Karavanich and Atema, 1998;

Tibbets, 2002; Steiger et al., 2008).

Tabl 1 | Dscpton of xpmntal condtons (fo a smmay of th pocd, s F 1 and assocatd capton).

Abbvaton Condton nam exchan Food Dscpton

ETLV Equity test, low value Both exchange Both low value Both subject and partner exchanged for low value reward.

ETHV Equity test, high value Both exchange Both high value Both subject and partner exchanged for high value reward.

FC Food control Both exchange Both see high value before Prior to exchange, high value reward is held in front of

exchange, receive low exchanger and then is placed back in container. After

value following exchange successful completion of exchange, exchanger

receives low value reward.

IT Inequity test Both exchange Subject low value Partner exchanges for high value reward and subject

Partner high value exchanges for low value reward.

GR Gift reward No exchange Subject low value Partner is given a high value reward for free (e.g.,

Partner high value without exchange) and then subject is given a low

value reward.

DT Delay test Both exchange, Both high value Partner exchanges for a high value reward and

subject waits 10 s subject exchanges and must wait 10 s before

after exchange receiving high value reward.

before receiving food

DETLV Differential exchange Subject exchanges Both low value Partner is given a low value reward for free (e.g.,

test, low value Partner does not without exchange) and subject must exchange for a

exchange low value reward.

DETHV Differential exchange Subject exchanges Both high value Partner is given a high value reward for free (e.g.,

test, high value Partner does not without exchange) and subject must exchange for a

exchange high value reward.

Not all conditions are used in all experiments, but all are provided to give an overview of the types of questions which have been asked. The most critical tests are

the ETLV, which collects baseline data on responses to lower-value rewards, the IT, which collects data on responses to lower-value rewards when ones partner

receives higher-value ones, and the FC, which is a control to determine reactions to lower-value rewards are present, but no primates receive one. Reprinted with

permission from Brosnan et al. (2010a).


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intrinsically less motivated to cooperate when rewards are easilymonopolizable than when rewards are distributed such that nei-ther individual can dominate them (de Waal and Davis, 2002),indicating that they preer to avoid situations which may resultin inequity. Although the same studies have not been run withchimpanzees, these apes are more likely to work to obtain a joint

reward when paired with a partner who tolerantly shares ood than

one who does not (Melis et al., 2006a) and, when given the choice,will choose a more tolerant rather than less tolerant partner withwhom to work (Melis et al., 2006b). Together, these studies indi-cate a close connection between cooperation and inequity, suchthat cooperation can survive modest or short-term inequity, but

greater or more pervasive inequity leads to the cessation o thecooperative interaction. Such a response is indeed advantageous,as it allows individuals to identiy partners with whom coopera-tion is not paying o and alter their choices in the uture withoutoreiting a benecial partnership due to one instance o cheating,misunderstanding, or coincidence.

the context of the InteractIon

In humans, much has been done to investigate the role o the experi-ments context on how subjects respond to distributional inequity

(although using monetary payos rather than ood, as in non-human species). One area relevant to cooperation is intentionality.Humans clearly distinguish between acts which were determinedby another human being and those which were determined ran-domly, without human intervention (e.g., by a computer). Humansrespond behaviorally, less oten reusing unair outcomes when they

are determined by computer (Blount, 1995) and also show dier-ential brain activation between human-initiated and computer-initiated distributional inequity (Knoch et al., 2006). It is typicallyassumed that the explanation or this is that subjects are sensitiveto the intention behind the action; intentional actions must be

responded to, while those which were the result o chance werebad luck that is not due to ones social partners and so do not

require a response. This sensitivity to intentionality makes sense inlight o cooperation. I your partner gets more than you do becauseo chance, then there is no reason to go nd a new partner; thisoutcome provides no inormation at all about their value as a socialpartner. I, on the other hand, you were working together with apartner who then took a greater share o the benets reaped, that

is a clue about the partners value, and a sign to nd a new partner.In other words, joint eorts should lead to joint outcomes (vanWolkenten et al., 2007).

In primates, there are ew studies comparing intentional andaccidental inequity, no doubt in part due to the diculty o exper-

imentally distinguishing chance occurrence rom intentionality innon-verbal species. However, those which do exist indicate behav-ior similar to that seen in humans. Primates are sensitive to theintentionality o a human experimenter, responding more stronglywhen they drop a reward intentionally than when it appears to bean accident (Call et al., 2004). Within their own species, chimpan-zees react more strongly to punish (e.g., take away access to ood)

a partner who previously stole that ood rom them than when itis simply an inequitable distribution (Jensen et al., 2007). In thislatter study, chimpanzees had the option to pull down a tablewhich held ood their partner could access (thus taking away access

exIstIng evIdence In support of the hypothesIs

I cooperation and responding to inequity are linked, a number opredictions emerge. Three among these have already accrued someevidence. First, inequitable outcomes should aect cooperation.Second, negative responses to inequity should be evident in thecontext o cooperation. Third, species which typically cooperate

should show a greater tendency to reject inequitable outcomes than

those which do not. Below I explore the evidence which addressesthese predictions, ocusing on behavior which is disadvantageousto the individual.

the Interplay of cooperatIon and InequIty

I cooperation and inequity are related, cooperation should beaected by inequitable outcomes (Brosnan, 2006; de Waal and

Suchak, 2010). In act, this question is o vital importance as it israre that interactions result in complete equity, at least in the shortterm (Aghion et al., 1999). Thus any species which both successullycooperates and responds to inequity must have some capacity ortaking into account context or an ability to extrapolate over thelonger term in order to maintain cooperative interactions.

Capuchin monkeys are excellent subjects in which to investigatethis interaction as they are known to cooperate in many contexts,and to understand the contingencies o cooperation (Brosnan,2010). Although in most cases cooperation experiments utilizesituations in which both individuals receive identical rewards,the ew which do not are telling. In a study designed explicitly toaddress this question, monkeys could cooperate on a mutual task

in which joint eorts resulted in rewards to both, but sometimesthe rewards were unequal (Brosnan et al., 2006). Unlike in manycooperation experiments, the monkeys were not separated and thushad to decide between themselves which individual would pullwhich bar and, hence, receive which reward. Since the monkeysdetermined which one worked or which reward, rather than the

experimenter, we could determine whether the presence o inequityaected cooperation, and whether they were able to work around it.In act, the distribution o rewards (equal or unequal) did not aectcooperation, but the partners behavior infuenced it greatly. I oneindividual consistently dominated the better rewards, cooperationdropped to almost a third o the rate seen in partnerships in which

both monkeys alternated receiving the better rewards.Intriguingly, this cessation o cooperation happened in all con-

ditions, including equitable ones in which no reward dierencewas possible. Thus, the monkeys were reacting to their partnersbehavior rather than the distribution o rewards. This is an impor-tant point; clearly the monkeys are willing to tolerate inequity on

a trial-by-trial basis as long as the overalloutcomes are approxi-

mately the same. In other words, cooperation only breaks downwhen aced with long-term inequity, not the occasional unequaloutcome. This explains how cooperation can be maintained intasks such as group hunting or mating coalitions, which may resultin unequal outcomes in the short term. Individuals will continue

to cooperate despite the occasional lesser reward i their overallrewards remain equitable.

Other studies support this conclusion. Capuchin monkeys willhelp partners obtain rewards as long as the partner then shares someo the spoils, but, again, cooperation breaks down i no sharingoccurs (de Waal and Berger, 2000). Moreover, capuchins appear



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the response is likely not a homology among primates. Althoughany such analysis without inormation on all species is necessarilylimited, we are at the point where it is useul to begin consideringthe phylogenetic data. Inequity seems to aect cooperative behavior,and the species in which the response to inequity was rst docu-mented, capuchins and chimpanzees, are known to cooperate, both

in the laboratory (de Waal and Berger, 2000; Melis et al., 2006b)

and in the eld (Creel and Creel, 1995; Fragaszy et al., 2004; Mitani,2006; Langergraber et al., 2007), lending credence to this hypothesis.However, while the cooperation hypothesis is compelling, it is notthe only possibility. For instance, inequity could be a homologyamong primates (or a more specialized group within the primate

taxon), in which case investigations in to unction will necessarilyrequire other taxa. Second, this behavior could be an emergentbehavior that arises when a species develops sucient cognitiveabilities to remember and compare outcomes between themselvesand others. Third, it could also emerge as a by-product o socialliving. I individuals were already predisposed to pay attention to

their partners outcomes, as is required, or instance, in some ormso social learning, they may then begin comparing their outcomes in

other situations. Notice an important subtlety in these latter cases;the ability to gain benets rom social comparison may requirecertain cognitive skills, such as individual recognition, but thatis independent o whether cognition is a unctional explanation

or the behavior. We can address these hypotheses by comparingprimates which vary on these dimensions. Data exist or severalspecies o great apes and new world monkeys, providing a morene-grained analysis.

For these comparisons, it is important to keep the tasks anddependent variables as similar as possible across studies. Given that

it is clear that a task is required to elicit inequity (see The Contexto the Interaction), in all o the studies done in my laboratory weutilized an exchange-based task (Figure 1). The dependent variables

were whether the primate completed the task and whether or notthey accepted the reward, combined as a participation measure, andthe latency to complete the interaction. Since the latter measure hasnot been seen to dier between conditions o equity and inequity, I

ocus here only on whether the subject reused to participate (e.g.,reused to complete the task or accept the reward).

First considering the great apes, the response has been well doc-umented in both humans and chimpanzees, as discussed above,although there is variation which needs to be urther explored toully elucidate how context aects the reaction (Brosnan et al., 2005,

2010a; Bruer et al., 2006, 2009). Considering chimpanzees sisterspecies, bonobos, only one study has investigated bonobos in acomparable paradigm to chimpanzees. The bonobos reused twice

as oten in the inequity (approximately 20%) as compared to theequity (approximately 10%) condition, although this dierence wasnot signicant (possibly due to the small sample size; Bruer et al.,

2009). This result rules out none o the potential hypotheses, butthe act that bonobos also cooperate, both in the laboratory (Hareet al., 2007) and the eld (e.g., in social relationships; Parrish, 1996;Hohmann and Fruth, 2000; Fruth and Hohmann, 2002), providessupport or the cooperation hypothesis.

Orangutans have also been well studied (Bruer et al., 2006,

2009; Brosnan et al., in review). These apes do not respond neg-atively to inequity, even in a study which directly replicated the

rom the partner). I the partner got the ood by taking away accessrom the subject, subjects were ar more likely to pull down thetable than i the partner was simply granted access to the ood bythe experimenter.

Primates are also quite sensitive to how rewards are received.Specically, primates are most sensitive to inequity in the context o

completing a task. Most inequity experiments require the subjects

to complete a task (typically an exchange o a token) to receivetheir rewards (see Figure 1). However in other studies, subjects arehanded ood rewards alternately, but or ree, i.e., with no taskrequired to obtain the ood (e.g., panels 2 and 4 in Figure 1). Thusar, no reactions to inequity have been ound in any study which

did not involve a task (Bruer et al., 2006; Dubreuil et al., 2006;Roma et al., 2006; Fontenot et al., 2007). This includes one study(Dindo and de Waal, 2006) which used the same capuchin subjectswhich both previously and subsequently responded to inequity inparadigms which involved tasks. Several additional studies haveexplicitly compared the presence versus absence o a task, three

using a within-subjects design and one a between-subjects design.In a within-subjects design, chimpanzees showed no reaction to

inequity in the absence o a task (e.g., when rewards were handedout or ree), while responding when they had to complete thetask or their rewards (Brosnan et al., 2010a). In the other threespecies, all reused more oten when no task was used despite sub-

jects not responding dierentially between the equity and inequity

conditions (Neiworth et al., 2009; Brosnan et al., in review; Talbotet al., in press).

There are several possibilities as to why the primates onlyrespond to inequity when a task is involved. First, despite claimsthat the presence or absence o a task should be irrelevant (Romaet al., 2006), animals are known to treat rewards which are earned

dierently than those which are received or ree (Carder andBerkowitz, 1970). Moreover, since captive animals routinely receive

ood rom keepers in situations which are not equitable, it is possiblethat they have grown accustomed to inequity in non-task situations,and so do not respond in these situations. Finally, it may also be thatthe presence o a task mimics a joint activity, despite the sequential

nature o the interaction, thus priming the individuals to expectmore equitable outcomes (Brosnan et al., 2010a).

O course, i cooperation is linked with inequity, one wouldexpect other experimental or social contexts to be important as well.Reactions to inequity are aected by group membership (Brosnanet al., 2005), sex (Brosnan et al., 2010a), rank (Bruer et al., 2006;

Brosnan et al., 2010a), and experimental design (Brosnan et al.,2010a) and, as discussed above, tolerance between individualsinfuences cooperative outcomes (Melis et al., 2006a). Similarly,

individuals personalities or relationships may play a role. This hasnot been investigated in great detail in humans, possibly due to thetendency to test subjects in completely anonymous situations, with

strangers, in part in an eort to rule out these actors as potentialcauses. Non-human primate studies oer a wonderul opportunityto investigate these actors in longitudinal studies.

a phylogenetIc approach

Not all species o primates respond to inequitable outcomes. Arecent surge o studies have provided inormation on a ar widerrange o primate species than were initially tested, indicating that



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cooperation. In the case o cooperative breeders, the interdepend-ency between males and emales (the partners in these experiments)may explain these results. I males and emales rely on each otheror their reproductive tness, negative responses to minor inequitieswould end up being more costly than benecial. Thus, individualsmay do best to tolerate minor inequities and respond only when

the inequitable outcomes become egregious.

Taken together, the current phylogenetic results most stronglysupport the hypothesis that inequity and cooperation are inter-linked, providing evidence in avor o the cooperation hypoth-esis or the evolution o the inequity response. Cognition may benecessary, but is not sucient, as great apes with brain-to-body

ratios on par with chimpanzees and capuchins, and comparableperormance in cognitive tasks, do not respond negatively to ineq-uity. The response is not a by-product o group-living, as severalprimates which are gregarious do not respond, including the highlysocial Callitrichids. However, species which routinely cooperatewith non-kin in several dierent contexts respond negatively to

inequity, while those who do not cooperate to this degree ail to doso. Moreover, interdependent species, which may not benet rom

such a response, ail to respond to inequity.

ImplIcatIons of the hypothesIs

In short, there is emerging comparative evidence that links coopera-tion and inequity. It is interesting to consider urther implicationso the hypothesized relationship between the negative response toinequity and cooperation.

InequIty and Interdependence

One intriguing aspect o the phylogenetic data is the possible role

o interdependence in the response. Extreme interdependenceoccurs among cooperatively breeding species, in which the (unre-lated) adults genetic tness is directly tied to the tness o others.

This promotes cooperation (Roberts, 2005) and has been arguedto explain the unusually high levels o prosocial behavior amongmated pairs in cooperative breeding species (Clutton-Brock, 2002;Hrdy, 2009; Jaeggi et al., 2010; Van Schaik and Burkart, 2010). In

other words, the parents are so dependent upon one another ortheir tness that it is in their best interests to help each other inmost circumstances, regardless o the cost. While there is currentlyonly a single study investigating inequity responses in coopera-tive breeders, the lack o evidence or an inequity response amongmated partners may indicate that interdependence plays a role

in inequity. Once the relationship is established, continuing theinteraction may be worthwhile even i their partner is getting abetter deal. The cost o abandoning ones breeding partner to nd

a new one due to a small act o inequity would be strongly selectedagainst, and only extreme inequity may be sucient to change thiscostbenet calculation.

Humans are worth considering in this respect. Humans alsoappear to be a cooperatively breeding species (Hrdy, 2009), yet werespond to distributional inequity. However, unlike Callitrichids,humans routinely orm cooperative relationships with individu-als outside o the pair bond, and virtually all experimental testso inequity involve strangers, and oten occur in anonymous set-

tings. Thus, humans may be subject to two selective orces. First,we are likely selected to be sensitive to inequity in interactions with

methodology which ound a response in chimpanzees (Brosnanet al., in review). These results seem to rule out the possibilitythat cognitive dierences are related to inequity, as orangutans areequally skilul in cognitive (Russon, 1998; Shumaker et al., 2001)and exchange (Flemming et al., in revision) tasks as other apes.Moreover, they support the cooperation hypothesis, as orangutans,

while cooperative in experimental studies (Chalmeau et al., 1997;

Duour et al., 2008), are not known to cooperate in the wild to agreat degree, possibly due to their more solitary social organization(van Schaik and van Hoo, 1996). These results also support thehypothesis that inequity is a by-product o sociality, as orangutansare less gregarious than other great apes, although they do orm

relationships and interact much more requently in some contextsthan initially recognized (Edwards and Snowdon, 1980; Singletonand van Schaik, 2002; van Schaik et al., 2009). The orangutan resultsalso indicate that any homology among the apes must be morerecent than the orangutan split rom the Arican apes.

Amongst the new world monkeys, brown capuchins (Cebusapella) have been documented to respond negatively to inequityin all but one o our studies which employed a task (Brosnan and

de Waal, 2003; van Wolkenten et al., 2007; Fletcher, 2008; Silberberget al., 2009). Taken with the data on their responses to inequity inthe context o cooperation (see The Interplay o Cooperation andInequity above), it is clear that in many contexts, these monkeys aresensitive to receiving less than their partner. To explicitly address

the homology hypothesis, we replicated the test on squirrel mon-keys (Saimirispp.), a species which shares a phylogenetic amily,Cebidae, with capuchins. Squirrel monkeys have a smaller brainand neocortex volume per body size than do capuchins (Rillingand Insel, 1999) and cooperate in only limited situations (Boinski,1987). They are also a highly gregarious, group-living species, and

are even sympatric with capuchins in some areas. However, in ourstudy, squirrel monkeys did not respond negatively to inequity,

completing the interaction whether or not their partner receiveda greater reward (Talbot et al., in press). They were sensitive to theexperimental paradigm; subjects were more likely to reuse to par-ticipate i the rewards violated their expectations than in the control

condition. These results indicate that the inequity response is nothomologous within the Family Cebidae, and the distribution acrossnew world monkeys and great apes suggests that neither socialitynor cognition are sucient eature to explain it. Given these data,the one eature in common to all is requent cooperation amongnon-kin, indicating the possibility o either a convergence or the

secondary loss o the trait in non-cooperative species.The new world monkeys also introduce an interesting caveat in

the orm o the Callitrichids. Callitrichids are cooperative breeders,

meaning that the parents and, sometimes, adult ospring, worktogether to rear young. Given this intensive level o cooperation(Cronin et al., 2005), even when conronted with unequal outcomes

(Cronin and Snowdon, 2008), one might expect them to be particu-larly sensitive to inequity. However, among tamarins, there is little,i any, evidence that they reuse to participate when outcomes areunequal (Neiworth et al., 2009). Given the variation seen amongspecies, this result needs to be validated with additional studies ocooperative breeders. Nonetheless, this nding may serve to vali-

date the main tenet o the cooperation hypothesis, which is thatresponding to inequity serves to increase the long-term gains rom



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InequIty and punIshment

While the main purpose o the response to inequity seems to berecognizing when it is time to nd a new partner with whom tocooperate, in some situations switching to a new partner maynot be an option (e.g., due to low availability o other partners, ahigh cost to switching, or the diculty in establishing sucient

trust or cooperation to emerge at the same level). In these cases,

the recognition o a partners low quality that derives rom theinequity response may secondarily be used to identiy situations inwhich it is worth an attempt to alter the current partners behavior,including punishment (Jensen, 2010; Raihani et al., 2010). Thisprovides an alternate option through which individuals can try

to increase their benets or cooperating; i they cannot leave andnd a new partner, they may be able to try changing the behav-ior o the current one. Even the occasional act o punishmentmay be sucient to alter the partners behavior or the better(Bergmller and Taborsky, 2005).Thus punishment may be morelikely in those species which have limited options or nding a

new partner or or which such an action is extremely costly, onepractical implication o which is that it may be easier to identiy

punishment in species which are known to react behaviorallyto inequity, but in situations in which individuals have limitedoptions or nding new partners.

InequIty and related behavIors

Inequity is related to several behaviors, either because these behav-iors may contribute to the mechanisms which allow individuals toreuse inequitable outcomes, or because the ability has implicationsor the behavior. Below I consider two o these.

InequIty and self-control

Reusing a present and available reward seems to require quite a loto sel-control. Primates are known to be good at this (Beran, 2002;

Evans and Westergaard, 2006; Duour et al., 2007), and even usebehavioral distraction strategies to assist in reraining rom reach-ing or oods (Evans and Beran, 2007a), both o which indicate thatprimates have the requisite abilities or the reusals seen in inequity

responses. One intriguing possibility is that dierences in speciesability to delay gratication may be related to their tendency toreuse rewards in the context o inequity. Given than there is speciesvariation in the ability to delay gratication, or instance with apesoutperorming monkeys (Evans and Beran, 2007b), it will be interest-ing to see whether this prediction is supported. It may also be that

increasing selection or negative reactions to inequity simultaneouslyselected or increased sel-control ability. Ecological orces are knownto shape sel-control ability in other contexts (Stevens et al., 2005). On

the fip side, the cognitive challenge o sel-control may serve to limitwhich species are able to respond to inequity by reusing rewards.

InequIty and prosocIal behavIor

All o the studies discussed above ocus on how individuals respondwhen they receive less than a partner. However, it is equally interest-ing to investigate responses when they receive more than a partner.

In particular, whether species will bring rewards to their partnershas been o interest to investigate the evolution o human socialbehavior. Presumably the mechanisms which allow individuals tocompare their outcomes to those o others and recognize when

individuals other than our partner, as is likely captured in studieso inequity and cooperation in the laboratory (e.g., with non-pairbonded individuals, who are oten both strangers and anonymous).On the other hand, we may be selected to ignore inequity within

close relationships, such as the pair bond (Clark and Grote, 2003),as do other cooperatively breeding species. In act, it is likely thathumans are not the only species which will show dierent behavior

depending upon the relationship involved, an area which needsurther investigation.

InequIty and relatIonshIps

Related to this, it is possible that interactions may dier within thesame species depending upon the relationships among the indi-viduals in question. For instance, in species which orm mated

pairs but nonetheless regularly interact and cooperate with otherindividuals, one might expect no response to inequity among matedpairs (due to interdependence), but a present response to inequityamong non-mated pair adults. For instance, this might describethe interactions we would expect in humans, as discussed above.Humans show reduced sensitivity to inequity in close relationships

as compared to more distant ones, which are typically more con-tingent (Clark and Grote, 2003). As another example, individualswithin a cooperative breeding group may have dierential invest-ment in the group, or dierential cost to nding a new partner.Less invested individuals, or those who can more easily nd newpartners, should be less tolerant o inequity. Similar patterns mayapply in other species which cooperate across many dierent types

o relationships.

InequIty and lIfe hIstory

Another prediction which emerges is that responses to inequity

may dier across lie history stages. Some species have dierentrelationships with others o their age class depending upon where

they are in development. For instance, polygynous species maygo through a stage in an adult single-sex group (e.g., a bachelorgroup) prior to becoming the alpha male o a mixed-sex group.In some cases, individuals may even have dierent relationships

with dierent individuals during the same lie history stage, and soshow several dierent behaviors at any given time (but with dier-ent partners, e.g., humans; see above sections). Since relationshipsand the requency o cooperation dier in these dierent stages odevelopment, reactions to inequity may vary as well.

An intriguing corollary o this prediction is the possibility o

particularly enhanced responses to inequity among individuals whoare in the process o orming pair bonds. Given that individuals arein the process o orming a bond which is both critical to repro-

ductive tness and costly to break, individuals may be best servedto be hyper vigilant or signs o inequity. In this way they may beable to predict uture behavior rom current actions and limit the

subsequent costs o either inequity or the requirement to nd anew mate. This may also provide a likely opportunity to investigatedeception, as there would be strong selection in avor o behavingmore equitably in the mating market than once the pair bond wasormed. Finally, individuals may be particularly likely to tolerateinequity once there is a reproductive investment in the relationship.

Thus inequity may be a mechanism individuals can use to assesspartner potential early in a relationship.



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that many species pay close attention to their partners behavioraloutcomes. Any species that socially learns receives inormation roma conspecic which changes their behavior, even i they are notconsciously aware o it (as they do not need to be consciously awareo inequity or it to provide a benet). Social learning is widespreadamong animals (Zentall et al., 1988; Heyes et al., 1996), and some

species are quite nuanced, with individuals paying attention to

relevant eatures o the social partner when determining whetherto copy their actions (Swaney et al., 2001; Biro et al., 2003; Perry,2009; Hopper, 2010; Horner et al., 2010).

a potentIal evolutIonary pathway

O course, this is a correlative relationship; we do not know whichcame rst, the response to inequity or the tendency to cooperate.

Although it is challenging to test experimentally, some evidenceprovides at least a hint o which direction evolution may havetaken. It seems, at this stage, more likely that cooperative behav-ior evolved rst, ollowed by selection or inequity. Many speciescooperate occasionally, and such interactions may lead to smallrewards (or small losses) without the risk o a major cost. It is

only when cooperation becomes common that some mechanismor avoiding excessive losses becomes essential. Thus, individu-als who came under selection or more extensive cooperationbecause o the benets such behavior accrued would similarlycome under strong selection to limit their cooperative interac-tions to partners who shared the resulting rewards. Those whomanaged to do so, by changing partners when outcomes devi-

ated substantially, would have gained ar more than those whoindiscriminately cooperated with all potential social partners withwhom there was a net benet.

It is worth considering a pathway through which the response toinequity might have evolved. Cooperation occurs when the benetsto both individuals exceed their costs (Bshary and Bergmuller,

2008; Brosnan et al., 2010c). From a purely costbenet perspec-tive, it should not matter i As benet exceeds Bs; it should stillbe to Bs benet to participate in the interaction. But what i Bcould reap a ar greater benet by cooperating with C? By recog-nizing (note, again, that this need not imply cognitive calcula-tion or understanding) only whether the cost exceeds the benet,

B may miss this opportunity. Thus in cooperative interactionsthere should be pressure to maximize outcomes by evaluatingwhether there are other, more lucrative options in the environ-ment. Responding to inequity would be a mechanism to do this;situations in which ones payos deviate substantially rom onessocial partners could be a reliable signal to evaluate other options.

In this scenario inequity aversion would serve as the mechanism

to evaluate when there is the possibility to increase ones benetrom cooperative interactions, and as such unction to maximizethe outcomes rom cooperation.

Note that this will be selected only i the additional gainsrom a new partner exceed the cost o switching partners (e.g.,

search costs) and the risk o a worse outcome. The latter occurseither i outcomes with C the new partner are worse thanthose with B or i the initial dierence in outcomes between Aand B was minimal. Moreover, i the individual accrues indirectbenets rom staying (i.e., the partner is related, or is assistingwith ospring care), responses to inequity may not be selected.

they receive less would work equally well to identiy situations inwhich they receive more. However, the selective pressures are quite

dierent, thus the behaviors may not maniest equally.Nonetheless, recent studies indicate that they co-occur and may

even interact. Although studies show that chimpanzees are unlikelyto bring ood rewards to conspecics (Silk et al., 2005; Jensen et al.,2006; Vonk et al., 2008), they do help other individuals (Warneken

and Tomasello, 2006; Warneken et al., 2007). Moreover, in oodsituations they also notice when they receive more than a partner,

reusing preerred rewards more oten when their partners receiveless preerred ones than when they also receive the preerred ruit(Brosnan et al., 2010a). This indicates that while individuals maynot work to improve their partners wellbeing, they do notice thedisparity.

Capuchins are quite prosocial, possibly moreso than chimpan-

zees, and bring rewards to conspecic partners in several situa-tions (de Waal et al., 2008; Lakshminarayanan and Santos, 2008). Arecent study specically investigated how prosocial behavior inter-acted with inequity, and ound that capuchins were prosocial (e.g.,brought ood rewards to partners) even in situations in which their

partners received more, or even when the puller received nothing.However, when inequity became greater, prosocial behavior ceased(Brosnan et al., 2010b). Thus, emerging evidence indicates thatthese two preerences interact to shape behavior. Further researchis needed that investigates these two behaviors in additional species,including those which do not respond to inequity, to determine thedegree o overlap, and the limits o prosociality.

thoughts on the evolutIon of InequIty

why the response makes sense In lIght of natural selectIon

Why should we care i another individual gets more than we

do? This is particularly true i our outcome is a net gain, ori the response actually increases inequity, a common result in

these experiments (see above and Brosnan and de Waal, 2003;Brosnan et al., 2005; Yamagishi et al., 2009). In these cases, itseems particularly surprising that any rational individual wouldchoose an outcome which makes them less well o in the short

term. On the other hand, a negative reaction to inequity is notparticularly surprising. First o all, despite a ocus on costben-et analysis in behavioral studies, the entire concept o naturalselection is based on relative gains, not absolute ones. Thus, thereis every reason to expect natural selection to avor behaviorswhich increase relative gains, even at the expense o absolute

outcomes, or which avor long-term benets over short-termcosts (e.g. Frank, 1988). Note, o course, that this does not meanthat the individual must understandthis comparison; the beauty

o natural selection is that any behavior which increases relativetness, however inadvertently, will be selected, regardless o theanimals comprehension o either their behavior, their relative

outcomes, or their benets (see Hypotheses or the Function othe Response to Inequity).

Second, many o the mechanisms or the behavior seem to be inplace. As discussed above (see Inequity Paradigms in Other Species),contrast eects occur in a variety o species, and it does not seem tobe a great cognitive leap rom comparing ones current outcomes to

ones own previous outcomes, to comparing ones current outcomesto ones partners previous outcomes. Moreover, we already know



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point to a promising new avenue or investigating sensitivity toinequity, particularly in species which do not respond to distribu-tional inequity. Such results may also help to distinguish betweensensitivity to inequity and reactivity to inequity, the latter being theimportant criteria or the current hypothesis. These situations mayalso clariy the range o situations in which responses to inequity

may be relevant.

the dIfference between notIcIng InequIty and respondIng to

InequIty

One critical area o study is whether or not individuals notice ineq-uity. Current studies measure whether individuals reactto ineq-uity, however it is possible to recognize it in situations in whichindividuals do not respond. For instance, do cooperative breeders

ail to notice that their partner receives more, or have they beenselected to not respond because o the high costs o nding a newpartner? Are the individual dierences in responses due to someindividuals ailing to notice inequity, or conditions in some situ-ations (e.g., some relationships) not avoring the response? Thishas important implications or whether behavior will change as the

degree o inequity increases. In act we do know that despite littleevidence o prosocial behavior in the orm o active giving (Silket al., 2005; Jensen et al., 2006; Vonk et al., 2008), chimpanzees doalter their behavior when they receive more than a partner (Brosnanet al., 2010a), indicating that they notice, despite not changing theirbehavior in any way that alters the outcomes. Future studies which

can continue to tease apart this dierence will be critical or ullyunderstanding the evolution o the response.

understandIng the varIatIon

Another pressing line o research is to better understand the

variations in the response among individuals o the same species.Although a variety o actors are known to cause variation in the

response (see Inequity Paradigms in Other Species), no actor hasyet emerged which explains the variation consistently. Thus, addi-tional research to clariy the role o individual actors, such as sex,age, reproductive status, and personality, as well as social actors,

such as the relationships between individuals and group behavior,are required. This is a logistical challenge, as such comparisonsrequire a large sample sizes o individuals who are tested in multiplecombinations, which can be dicult to obtain, particularly amonglarger species such as primates. Nonetheless, research in this areais ongoing and the extra level o control possible in non-human

research may yield additional insights to and predictions abouthuman behavior (Brosnan et al., 2009).

testIng the evolutIonary model

Finally, it is important to test the proposed evolutionary modelo inequity responses. O course, testing possible evolutionarypathways is by necessity a roundabout approach, but there aresome interesting possibilities. First, the experimental economicsapproach is ideal or designing games which allow or direct com-parisons across multiple species or multiple conditions. Among

non-humans in particular, it is possible to test the same individu-als repeatedly, engaging them in multiple dierent types o games(with dierent parameters and payos) and with multiple partnersover extended periods o time. This has been done successully

Thus, the theory predicts that responding to inequity may bemore benecial or some types o cooperation than others. Forinstance, this response may be more likely to be selected in situ-ations with potentially high costs to doing relatively less well

than ones partners, or instance in winner-take-all situationssuch as mating coalitions.

what Is needed next?data from other specIes and relatIonshIps

While this paper is almost exclusively about non-human primates,it will be important to test this theory in other taxa (Drea andFrank, 2003; Beko and Pierce, 2010). There are several taxa, orinstance canids, cetaceans, and birds, in which some species showa greater tendency to cooperate than others. There are also non-

primate cooperative breeders, allowing all three aspects o thishypothesis to be tested more broadly. In particular we need dataon not just species which routinely cooperate, but also on closelyrelated species which do not do so in order to urther evaluate thelink between inequity and cooperation. Finally, it would be useulto test combinations o individuals which are not oten addressed

(e.g., cooperative breeders which are not part o the same matedpair) in order to address some o the additional implications o thetheory, as outlined above.

data from other contexts

We also need data rom a broader variety o situations. Forinstance, primates respond to dierences in outcomes, but do notseem to respond to dierences in the eort required to achievethose outcomes (Fontenot et al., 2007; van Wolkenten et al., 2007).

However, responses to inequity may occur in situations other thandierential ood distributions; or instance, several studies haveound links between equity and play behavior in non-human spe-cies (Beko, 2001, 2004; Dugatkin and Beko, 2003; van Leeuwen

et al., 2010). Despite the act that ood rewards are particularlyrelevant, making them an easy way to study behavior in experimen-tal situations, some species may respond dierently to ood than

non-ood situations (Warneken et al., 2007). Thus it is importantto collect data rom a broader, and more species-representative,array o situations.

Related to this, measures o inequity other than the rate oreward reusal or the rate o reusal to participate in the task willhelp clariy the range o situations in which responses to inequity

occur. These measures are useul, as they are easy to quantiy and themethodology can be used across a wide variety o species. However,giving up a desirably ood reward requires quite a cost on the parto the individual, potentially making this less likely to occur than

other behaviors. Thus more nuanced behavioral (such as changesin aect) and physiological (such as skin conductance or heart

rate) measures may help to uncover situations in which individu-als, responses to inequity are not so explicit. This is particularlyimportant in species or which there is no evidence o such explicitresponses to inequity.

Finally, there are other situations in which inequity may occur.A recent study nds evidence that in their play behavior, goril-

las are sensitive to their immediate social status, and work tomaintain social inequities that increase their status with respectto another individual (van Leeuwen et al., 2010). These results



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individuals who solicit other, potentially more equitable partners.Nonetheless, short-term inequity does not appear to disrupt coopera-tive relationships. Given that many interactions do not typically resultin complete equality, this fexibility may clear the way or coopera-tion to persist despite modest inequality, while long-term inequitycan be used as a cue that a particular cooperative relationship is

no longer benecial. Moreover, there is a correlation between the

degree to which individuals o a species cooperate with non-kinand the presence o a behavioral response to inequitable outcomes.Although these data are correlational, rather than causal, they provideimportant evidence regarding the unction o the inequity responsein decision-making, as well as emphasizing the power o phylogenetic

methods in illuminating evolutionary unction. Despite the dicul-ties inherent in collecting such data, it is possible to design behavioralstudies which can be run across multiple species. These phylogeneticcomparisons can be extremely useul in investigating the evolutiono decision-making and other behaviors.

acknowledgments

I thank Ralph Bergmller or stimulating discussion which led to

the ideas regarding punishment and interactions between individu-als at dierent lie stages. I also thank Ralph Bergmller as well as

Lydia Hopper, Lucie Salwiczek, and Frans de Waal or very helpulcomments on an earlier drat o this manuscript. Funding wasprovided to the author by a National Science Foundation Humanand Social Dynamics Grant (SES 0729244) and an NSF CAREERAward (SES 0847351).

using a coordination game (Brosnan et al., 2011) and this approachcan be extended to games (e.g., trust, prisoners dilemma) or payostructures which address the inequity hypothesis.

Alternatively, many o the parameters o this model could

be tested individually. For instance, we know that when given achoice, chimpanzees actively choose partners who share ood overthose who do not (Melis et al., 2006a,b). Paradigms such as this

could be expanded to investigate how increased partner choiceaects reactions to inequity. Additional experimental manipula-tions would also be useul. For instance, the model predicts thatindividuals would be more likely to choose a new partner as ineq-

uity rises, which could be tested by manipulating outcomes (e.g.,altering payos) and documenting any eects on partner choice.Although each o these may test only a component o the model,taken together they provide validation (or not) or key eatureswhich are implicated.

conclusIon

Despite the act that both humans and other species make deci-sions which indicate an aversion to inequitable outcomes which are

disadvantageous, evidence regarding the unction o the behaviorhas been limited. New evidence discussed in this review develops

the hypothesis that recognizing inequity assists individuals in nd-ing new partners with whom they will achieve a greater benet,increasing the payos rom cooperation over the long term. In jointsituations, individuals cease cooperating with partners who consist-ently dominate better rewards, giving a selective advantage to those



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This article was submitted to Frontiers

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SA, which permits use, distribution and

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