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Animal behaviour

Fair refusal by capuchinmonkeys

B rosnan and de Waal1 report that

capuchin monkeys show evidence of asense of fairness or inequity aversion

because they rejected a less preferred rewardwhen they saw a partner monkey receive apreferred reward for the same task. How-ever, this does not show that monkeys areaverse to inequity, only that they reject alesser reward when better rewards are avail-able. There are risks inherent in seekinganthropomorphic explanations for non-human behaviour.

In the inequality test, the monkeysrefused to exchange a token for a cucumberslice (non-preferred reward) on 43% of trialswhen they saw a partner monkey receive apreferred grape reward for the same effort.However, in the food control condition, in

which the partner was not present, thesesame monkeys were just as likely to refuse thecucumber slice when they saw a grape placedwhere the partner normally sat (49%refusals). There can be nothing inequitousabout receiving a non-preferred reward if nobody is receiving anything better. In thefood-control condition, the monkeys arerefusing the non-preferred reward simply because they can see that a better reward ispotentially available. This is therefore themost parsimonious explanation for theirrefusal to accept the non-preferred rewardwhen they see another monkey receive a

better one.Brosnan and de Waal 1 reject this reward-availability explanation for two reasons.First, in a third condition (the effort controlcondition), where monkeys saw their part-ner receive a grape without having toexchange a token, the monkeys were morelikely to refuse the cucumber slice than inthe food-control condition. On its own, thecomparison of the effort-control and food-control conditions is in the directionrequired by a fairness account. But fairnesscannot account for the equally large differ-ence between the effort-control and inequal-

ity-test conditions.The basis of Brosnan and de Waals sec-ond reason for rejecting the reward-avail-ability explanation is in their Fig. 2, whichseems to show an increasing trend of non-exchange for the two conditions in whichanother monkey was present (inequality testand effort control) and a decreasing trend of rejections in the food-control conditionwhere no other monkey was present. TheirFig. 2 shows mean rejections for the first 10and last 15 trials (not, as stated in the paper,the first 15 and last 10 trials; Brosnan andde Waal,personal communication) averagedacross two sessions.

When the cumulative rate of rejections is

represented across all trials of both sessionsfor Brosnan and de Waals monkeys, we findthat there is no overall increase in rejectionrate in the inequality-test and effort-controlconditions,and that the rate does not declineacross sessions in the food-control condition(results not shown).

Although explanations of animal behav-iour in anthropomorphic terms are notori-ously prone to imprecision 2, if fairness orinequity aversion mean anything in thiscontext, they surely imply that individualsreject rewards more often when they seeanother receive a better reward than whenthe better reward is simply in view with noone else there to consume it.The very similarlevels and patterns of cucumber rejection inthe inequality-test and food-control condi-tions therefore contradict an account basedon fairness or inequality.Clive D. L. WynneDepartment of Psychology, University of Florida,Box 112250, Gainesville, Florida 32611, USA

e-mail: wynne@ufl.edu 1. Brosnan,S. F. & de Waal, F. B.M. Nature 425, 297299

(2003).2 . Wynne, C.D.L . Do Animals Think? (Princeton Univ.Press, in

the press).

Brosnan and de Waal reply We haveshown 1 that animals compare their ownrewards with those of others, and accept orreject rewards according to their relativevalue. Our aim was not to demonstratethat capuchin monkeys make a humanresponse to inequality, but rather to eluci-date evolutionary precursors to inequity aversion. We use this term as in ref. 2

people resist inequitable outcomes; thatis, they are willing to give up some materi-al pay-off to move in the direction of moreequitable outcomes and specifically focus on disadvantageous inequity aver-sion 2. The monkeys in our experimentcould not change the reward division, andhence could not actively avoid inequality,but we wanted to determine whether they would at least recognize inequality if sub- jected to it. We found that the capuchinsreacted negatively, refusing to complete theinteraction.

It is unlikely that inequity aversion

appeared de novo in humans. It almostcertainly evolved because individuals whoresponded to inequality disadvantageous tothemselves increased their relative fitnesscompared with those who did not. Werecognize several potential evolutionary precursors to disadvantageous inequity aversion (S.F. B., H. C. Schiff and F. B. M. deW., manuscript in preparation). First is theability to recognize that rewards and effortsdiffer between individuals, which is alsorequired for social learning, a skill present incapuchins 3. Second is the propensity to reactif another individual receives a better rewardfor a specific task. Third is sacrifice to alteranother individuals outcome.

Our study mainly concerned the secondability, showing that capuchin monkeysreact negatively when another individualgets a better reward for the same or less efforton a specific task.This finding suggests thatprecursors to inequity aversion are presentin animals from which our lineage split mil-lions of years ago. Although capuchins may be reacting somewhat differently from adulthumans, we have still learned somethingabout the behaviours possible evolutionary trajectory.

Regarding the cross-cultural study, thelowest mean offer by a proposer in the ulti-matum game was 26% of the total, whereasthe lowest modal offer was 15%,both by theMachiguenga of Peru 4. Such relatively highoffers would not seem to be consistent withcompletely selfish individuals who lack any conception of fairness 5.

As stated earlier 1,although the mere pres-ence of a higher-value reward affects thecapuchins, their reaction is not the same as

when a conspecific receives the higher-valuereward. To ignore the differences betweenthe inequality test and the food-control testis unwarranted. Our Fig. 1 does not permitany conclusions about the effect of the food-control test and was not used for this pur-pose; it is the data in our Fig. 2 that inspiredour claim.

The frequency of refusals across trialsincreases when a partner receives the rewardand decreases when a reward is merely visible.The conservative statistic we chose did notallow significance (P 0.05)1, but we havesince subjected these data to a comparison of

the slopes of the linear regressions across tri-als for each test 6. This re-analysis shows thatrefusals in the food-control test decreaseacross time, whereas those in the inequality test and effort-control condition increase(F 2,69 28.71,P 0.001).Our subjects there-fore discriminate between a situation inwhich higher-value food is being consumedby a conspecific and one in which such food ismerely visible, intensifying their rejectionsunder only the former condition.Sarah F. Brosnan, Frans B. M. de WaalYerkes National Primate Research Center,Emory University, Atlanta, Georgia 30329, USA

1. Brosnan,S. F. & de Waal, F. B.M. Nature 425, 297299 (2003).2. Fehr,E. & Schmidt,K. M. Q.J. Econ.114, 817868 (1999).3. Brosnan,S. F. & de Waal, F. B.M. J. Comp.Psychol.(in the press).4. Henrich,J. et al. Am.Econ. Rev.91, 7378 (2001).5. Sigmund,K., Fehr, E.& Nowak, M.A. Sci.Am.8387 (2002).6. Zar,J .H. Biostatistical Analysis 3rd edn (Prentice Hall, Saddle

River,NJ, 1996).

CorrigendumWhen the American sea sturgeon swam east

Arne Ludwig, Lutz Debus, Dietmar Lieckfeldt, IsaacWirgin, Norbert Benecke, Ingo Jenneckens,Patrick Williot, John R. Waldman,Christian PitraNature 419, 447448 (2002).The institutional address of A. L. should have been listedas the Institute of Freshwater Ecology and InlandFisheries and Institute for Zoo and Wildlife Research.

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