Does the chimpanzee have a theory of mind? 30 years later

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1 Review Does the chimpnzee hve theory of mind? 30 yers lter Josep Cll nd Michel Tomsello Mx Plnck Institute for Evolutionry Anthropology, Deutscher Pltz 6, D Leipzig, Germny On the 30th nniversry of Premck nd Woodruff s seminl pper sking whether chimpnzees hve theory of mind, we review recent evidence tht suggests in mny respects they do, wheres in other respects they might not. Specificlly, there is solid evidence from severl different experimentl prdigms tht chimpnzees understnd the gols nd intentions of others, s well s the perception nd knowledge of others. Nevertheless, despite severl seemingly vlid ttempts, there is currently no evidence tht chimpnzees understnd flse beliefs. Our conclusion for the moment is, thus, tht chimpnzees understnd others in terms of perception gol psychology, s opposed to full-fledged, humn-like belief desire psychology. Introduction In 1978 Premck nd Woodruff sked, Does the chimpnzee hve theory of mind? In this brief review we ttempt to nswer this question bsed on much reserch tht hs been conducted in the 30 yers since tht time, prticulrly in the lst decde or so (see Tomsello nd Cll [1] for the stte of the rt s of the mid-1990s). The nswer will not be simple yes or no, however, becuse prt of the progress tht hs been mde in recent yers is recognition tht there re mny different wys in which orgnisms might understnd the psychologicl functioning of others. A brief history Premck nd Woodruff s [2] originl study ws ctully bout chimpnzees understnding of humn gols. But soon there ws new reserch suggesting tht perhps these results were experimentl rtifcts [3,4] nd other reserch on socil lerning tht suggested chimpnzees did not hve n understnding of humn gols [5]. Negtive evidence lso ccrued during the 1990s bout chimpnzees understnding of visul perception, especilly from the well-known studies of Povinelli nd Eddy [6] in which chimpnzees begged indiscrimintely from humns fcing them nd others with buckets over their heds (see lso Ref. [7]). There ws lso one negtive study on chimpnzees understnding of flse beliefs [8]. All of these dt led Tomsello nd Cll [1] to the generl conclusion tht chimpnzees nd other nonhumn primtes do not understnd the psychologicl sttes of others. Tht is, they cn predict the ctions of others in mny situtions bsed on pst experience (nd perhps specilized cognitive dpttions), but they do not Corresponding uthor: Cll, J. (cll@ev.mpg.de). go beneth the surfce to n understnding of the gols, perceptions, knowledge nd beliefs tht guide ction (nor to n understnding of underlying physicl forces either). But not ll of the dt existing t tht time were consistent in suggesting this conclusion, nd moreover, chimpnzees hd been observed doing things tht would seem to require more thn just n understnding of surfce-level behvior, for exmple, tcticl deception [9]. The story since the lte 1990s hs been one of experimenters finding better wys to tp into wht chimpnzees know bout the psychologicl sttes of others nd so getting mny more positive results. In most cses this hs been guided by ttempts to model the experiments more closely on situtions tht chimpnzees routinely encounter in their nturl environments, for exmple, presenting them with problems not in situtions in which they must cooperte with others but, rther, in situtions in which they must compete with others (see Box 1). Skeptics still bound, s represented most prominently by Povinelli nd collegues [10,11]. They cling to the hypothesis tht chimpnzees understnd only surfce-level behvior (forming behviorl rules ), nd indeed this explntion is lmost lwys possible for ny single experiment. But there re now in mny cses multiple experimentl prdigms ll imed t single psychologicl stte ech presenting chimpnzees with highly novel problem tht mkes the positing of lerned behviorl rules difficult explntory strtegy [12]. Here, we review current experimentl evidence bout chimpnzees understnding of: (i) the gols nd intentions of others nd (ii) the perception, knowledge nd beliefs of others. Understnding gols nd intentions To compete nd cooperte effectively with others in their group, highly socil nimls, such s chimpnzees, must be ble not only to rect to wht others re doing but lso to nticipte wht they will do. One wy of ccomplishing this is by observing wht others do in prticulr situtions nd deriving set of behviorl rules (or, in some cses, hving those built in). This will enble behviorl prediction when the sme or highly similr sitution rises gin. But nother wy to do it indeed more flexible wy to do it is to discern directly wht the other is trying to do, wht stte of the environment he is trying to bring bout nd wht his gol is. This enbles behviorl prediction not only in previously observed or highly similr situtions but lso in novel situtions. The methodologicl problem is tht when n ctor cts towrd gol successfully, then wht he is trying to do nd /$ see front mtter ß 2008 Elsevier Ltd. All rights reserved. doi: /j.tics

2 Box 1. Assessing visul nd uditory perspective tking in chimpnzees Hre et l. [26] hd chimpnzees compete for food with humn competitor who ws inside glss booth (see Figure I). In the min study chimpnzees preferentilly chose to pproch the food tht ws on the side of the booth with n opque brrier so tht the humn could not see them pproching. They, thus, ttempted to influence wht the humn could see, nd they did so from the very first trils. Melis et l. [27] went one step further by hving booth with opque brriers on both sides, but chimpnzees hd to choose between reching through cler tunnel, in which cse the humn competitor could see their reching rm, nd n opque tunnel, in which cse she could not. Even though they could not see the humn under ny conditions, the chimpnzees reched more through the opque tunnel thn the cler tunnel presumbly imgining the perspective of the humn. In n extension of this, Melis et l. [27] used this sme booth rrngement but with two cler tunnels leding to food. Here the experimenter lowered her hed between her knees so tht she ws unble to see the chimpnzee. In this cse ech tunnel hd smll door midwy through, one of which ws very noisy nd one of which ws silent (they hd lerned bout the doors properties erlier in different context). If the humn herd door opening, she would rise her hed nd grb the food before the chimpnzee hd chnce, but not if she could not her door opening. Chimpnzees in this cse preferentilly reched through the silent door, nd gin they did so from the very first trils. Box 2. Chimpnzees infer humn s intentions Buttelmnn et l. [28] used the Gergely et l. [29] method to test six humn-rised chimpnzees in the so-clled rtionl-imittion prdigm. The chimpnzees were shown how to operte n pprtus to produce n interesting result (e.g. lights or sounds), nd then they were given turn. The most nturl behvior for them in ll cses ws to operte it with their hnds. But this obvious behvior ws never demonstrted for them; they lwys sw humn mnipulte the pprtus in novel wy with some other body prt. The ide ws tht in some cses the physicl constrints of the sitution dictted tht the humn (referred to s E in the figure) hd to use tht unusul body prt; for exmple, he hd to turn on light with his hed becuse his hnds were occupied holding blnket or he hd to operte light with his foot becuse his hnds were occupied with hevy bucket (see Figure I). When the chimpnzees sw this forced use of the unusul body prt, they mostly discounted it nd used their hnds s they normlly would (becuse the constrints were not present for them). However, when they sw the humn use the unusul body prt when there ws no physicl constrint dictting this, they quite often copied the unusul behviorl mens themselves. If we interpret this experiment the wy it is interpreted for humn infnts, the conclusion is tht the chimpnzees understood not only wht the experimenter ws trying to do (his gol) but lso why he ws doing it in the wy he ws doing it the rtionlity behind the choice of the pln of ction towrd the gol. According to Tomsello et l. [30], n understnding of the ction pln chosen towrd gol constitutes n understnding of the intention. Figure I. Turning on the light with the foot becuse () E wnted to or (b) E hd to (becuse his hnds were occupied) in the Buttelmnn et l. [28] experiments. Figure I. Experimentl setup for the studies of Hre et l. [26] nd Melis et l. [27]. wht he ctully does re the sme, nd so it is very difficult, if not impossible, to distinguish when n observer is reding the ctor s behvior versus reding his gols. But there re situtions in which wht n ctor does does not mtch his gol, specificlly, in unsuccessful ttempts nd ccidents. The best evidence tht n observer understnds gols, therefore, is when she rects specificlly to the ctor s gol nd not to his overt behvior when he is trying unsuccessfully or hving n ccident. Tble 1 lists ten seprte studies suggesting tht chimpnzees understnd others gols. At lest six of these re modeled closely on studies with humn infnts (see Box 2), which re routinely used s evidence of gol understnding. These ten studies involve severl different situtions tht chimpnzees must understnd nd severl different response mesures s well. Thus, in the first seven studies in Tble 1 (briefly described there, points 1 7), chimpnzees rect not towrd the ctul behvior of the humn but towrd his gol, s they ttempt to get rewrd or nticipte his behvior. For exmple, they rect differently to very similr behviors when the humn is refrining from giving food becuse he is unwilling versus unble to do so (Tble 1, points 1 nd 3), when he is doing something on purpose versus by ccident (Tble 1, point 2) or else when he is n innocent bystnder (Tble 1, point 7). They lso discern tht nother s gol is to rech n out-ofrech object nd then they either help him to rech it (Tble 1, point 4) or grb it first if it indictes the loction of food (Tble 1, point 5). And they nticipte humn s 188

3 Tble 1. Studies on chimpnzees nd humn infnts understnding of gols nd intentions Studies References Chimpnzees Getting/finding food 1. Leve erlier nd beg more intensely from n E who is unwilling s opposed to unble to deliver food [31] [32] (behvior similr in the two cses) 2. Select the box cted on intentionlly versus ccidentlly (behvior similr in the two cses) [33] [33] 3. Leve erlier when E is plying with s opposed to trying to open box with food (behvior identicl in the two cses) Recting to prtner s ctions 4. Give the object tht the E is trying to rech [34,35] [34] 5. Tke the food tht competitor is trying to rech [36] 6. Anticipte where E is going bsed on potentil gols vilble 7. When food is stolen retlite ginst thief, not ginst innocent receiver of stolen food [37] Imittion 8. Produce trget ction bsed on observing filed ttempt [38,39] [40] 9. Copy intentionl ctions more often thn ccidentl ctions [38] [41] 10. Selectively copy freely chosen cts but not those forced by circumstnces [28] [29] (D. Buttelmnn et l., personl communiction). Gps in the tble indicte no informtion vilble. Infnts impending ctions when he stnds nd turns in certin direction bsed on wht gol he is probbly pursuing (Tble 1, point 6). Importntly, in two of these studies (Tble 1, points 3 nd 6) the behvior t the time the chimpnzee must rect is identicl in experimentl nd control conditions, with the only difference being in the immeditely preceding context (e.g. given tht the humn hs been opening boxes, the current mbiguous behvior is probbly n ttempt to open new box, wheres without this preceding context the humn s gol is unknown). One could of course ttempt to explin ny one of these results in terms of behviorl rules tht chimpnzees re either born with or lern, nd this might be plusible in prticulr cse. But this explntory strtegy is not plusible cross ll of the seven studies becuse of the diversity of situtions nd rections required. Moreover, to explin the studies in which the humn s behvior is identicl in experimentl nd control conditions t the time of rection, one would need to lso posit something like contextul rules becuse there is no differentil behvior to red in the two situtions. In three further studies in Tble 1 (points 8 10) explntions in terms of behviorl or contextul rules re not possible. This is becuse these ll use n imittion prdigm in which the chimpnzee subject ctully cts out in her own behvior wht she understnds the other to be ttempting to do, nd this does not lwys correspond to the overt ctions (note tht ll three of these studies use humn-rised chimpnzees, who re very probbly the only ones cpble of copying ctions in specific wys [13]). In these three studies (ll modeled on similr studies with humn infnts), chimpnzees imitted wht the humn ws trying to do (not wht he did) (Tble 1, point 8), his purposeful rther thn his ccidentl ctions (Tble 1, point 9), nd they even selectively imitted ctions bsed on n understnding of why the ctor chose this prticulr ction (Tble 1, point 10), which might be construed s understnding his intention, in the sense of the ction pln he chose for pursuing his gol. Behviorl nd contextul rules only help subject to predict wht n ctor will do; they provide no guidnce whtsoever for socil lerning. We believe tht there is only one resonble conclusion to be drwn from the ten studies reviewed here: chimpnzees, like humns, understnd the ctions of others not just in terms of surfce behviors but lso in terms of the underlying gols, nd possibly intentions, involved. Behviorl or contextul rules might be concocted to explin the results of ny one of the seven studies in which the chimpnzees rect to or predict the behvior of others, but this requires mny different d hoc behviorl nd contextul rules for which there is bsolutely no positive evidence. Indeed consistent use of this explntory strtegy would lso deny humn children n understnding of gols nd intentions becuse most of the chimpnzee studies re modeled on child studies. Moreover, the three imittion studies would not seem to be menble to behviorl-rules explntions t ll. Understnding perception nd knowledge To understnd how nother works s gol-directed gent, n observer must understnd not only his gols but lso his perceptions becuse wht he sees nd knows helps to determine wht he does. Here, we exmine wht chimpnzees understnd bout wht n ctor sees not just wht he is oriented to, but wht he registers from the environment in wys tht ffect his ctions nd bout wht tht ctor knows in the sense of informtion he hs previously registered tht still ffects his current ctions (e.g. his knowledge of where food is even though he cnnot see it now). Understnding flse beliefs is the specil cse in which n observer predicts or explins the behvior of n ctor bsed on judgment of wht tht ctor believes to be the cse, not wht relly is the cse s the observer knows it (e.g. the ctor believes the food is in one plce when the observer knows tht it is relly in nother). Agin in the current cse, the min lterntive hypothesis tht we must consider is tht chimpnzees either re born with or lern certin behviorl or contextul rules tht determine how they respond to others surfce behviors (orienting behviors in this cse) without ny understnding of their perception or knowledge. And nlogous to the cse with gols, the best evidence tht n individul understnds nother s perception is when 189

4 Tble 2. Studies on chimpnzees nd humn infnts understnding of perception nd knowledge Studies References Chimpnzees Infnts Gze following 1. Follow gze to distnt loctions behind self [42 44] [45] 2. Follow gze on the bsis of both fce nd eye direction [46] [46,47] 3. Check bck with gzer if nothing relevnt t the trget loction [48,49] [50] 4. Stop looking fter few trils if nothing relevnt t the trget loction [51] 5. Ignore distrcting objects on the wy to the trget loction [52] [53] 6. Move to the side of opque brriers to view the trget loction [42,49,52] [54] 7. Understnd tht gze stops t n opque brrier - unless it hs window in it [55] [56] Gesturl communiction 8. Use visul gestures mostly when conspecifics or E re oriented to them [6,14,57,58] [6] 9. Position oneself to gesture in front of others [59,60] [61] 10. Both fce nd eye orienttion of recipient determine gesture production [62] [6] Food competition 11. Pick the food tht the E is not looking t [26] 12. Pick the food tht dominnt individul or E cnnot see becuse of brrier [26,63,64] [65] 13. Visully concel pproch to food (using brrier) [26,27] 14. Auditorilly concel pproch to food (choosing silent door) [27] 15. Tke food tht dominnt individul did not see being hidden [15] [66] 16. Understnd tht if competitor picks first, he will hve chosen the food he sw (not food he did not see) being hidden (J. Kminski et l., personl communiction). Gps in the tble indicte no informtion vilble. the two individuls perceptions differ (e.g. they see different sides of brrier), nd the observer cts on the bsis not just of wht she herself sees or knows but lso on the bsis of wht the other sees or knows. Tble 2 lists 16 different studies relevnt to the question of whether chimpnzees understnd wht others see nd know. The first set of studies (Tble 2, points 1 7) involve gze-following behviors, which re, dmittedly, not such powerful evidence of n understnding of perception. The key fct here is tht chimpnzees follow the gze direction of others behind themselves, round brriers nd pst distrctors; they check bck with the gzer if nothing interesting is to be found in the indicted direction, nd they eventully stop following if n individul lwys gzes t nothing interesting. These re ll chrcteristics of gze following shred with humn infnts, nd even though humn infnts use the eyes, s opposed to the hed, more often thn chimpnzees do in following gze direction, chimpnzees do use the eyes to some extent s well (Tble 2, point 5). The second set of studies involves gesturl communiction. The generl finding is tht chimpnzees tke into ccount the visul orienttion of the recipient when gesturing (Tble 2, point 8), even moving themselves in front of the recipient to gesture when necessry (Tble 2, point 9). Of prticulr importnce, Kminski et l. [14] found chimpnzees much more sensitive to the recipient thn did Povinelli nd Eddy [6] in similr prdigm when the humn ws ctully in position to deliver food (i.e. fcing them bodily). It is true tht chimpnzees re not s sensitive to the eyes s re humn infnts, but still they understnd when someone is or is not in position to receive their communictive ct (Tble 2, point 10) gin in mnner very similr to humn infnts. By fr the most powerful evidence comes from the third set of studies, which use food-competition prdigms (Tble 2, points 11 16, lso Box 1). Ech of these studies is whole experimentl prdigm with severl control conditions, tht cnnot be fully described here. But the bsic ide is tht when competing with others for food, chimpnzees tke into ccount wht their competitor cn nd cnnot see, wht he cn nd cnnot her nd even wht he does nd does not know. In some cses they even ttempt to influence wht the other cn nd cnnot see nd her by ctively conceling their own pproch to food either visully or uditorilly (Tble 2, points 13 nd 14). Chimpnzees in these studies lso know wht others know in the sense tht they keep trck of wht nother hs just seen moment before (Tble 2, points 15 nd 16), just s humn infnts. Agin, we believe tht there is only one resonble conclusion to be drwn from the totlity of the studies reviewed here: chimpnzees, like humns, understnd tht others see, her nd know things. We hve mny different methodologies involving severl different experimentl prdigms nd response mesures ll leding to the sme conclusion. Agin, behviorl rules might be concocted to explin the results of ech of the vrious studies individully, but gin this will require creting vriety of post hoc explntions on the bsis of no direct evidence of the requisite pst experiences. And gin, if one were to use the behviorl rules critique rigorously nd firly cross the bord, one would hve to conclude tht humn infnts nd young children lso hve no understnding of the perception or knowledge of others becuse mny of the studies correspond rther closely to studies conducted with infnts. No understnding of flse belief? Despite ll of this positive evidence for chimpnzees understnding the gols, intentions, perceptions nd knowledge of others, there is currently no experimentl evidence tht they understnd flse beliefs by, for exmple, predicting wht nother will do bsed on wht tht other knows (when the subject knows something else to be the cse). First re the negtive findings of Cll nd Tomsello [8], 190

5 who hd positive results with five-yer-old children in the sme nonverbl prdigm (which correlted with stndrd verbl flse-belief prdigm). Second, there re now supporting negtive results from competitive version of tht tsk in which subjects were required to infer from competitor s ctions where she believed the hidden food to be, which then led the subject to the ctul loction (C. Krchun et l., personl communiction). In ddition, Hre et l. [15] presented two versions of the bsic chimpnzee competition tsk. In one version the dominnt chimpnzee either witnessed (informed condition) or not (uninformed condition) the loction where the food ws plced. In nother version, the dominnt lwys witnessed the initil loction of the rewrd but then the rewrd ws moved to second loction. In some trils the dominnt witnessed this movement (informed condition), wheres in others she did not (misinformed condition), which creted flse belief in the dominnt bout the rewrd s loction. If subjects understood tht in the misinformed condition their competitor hd flse belief, they should hve performed nerly perfectly in this condition becuse they would hve been ble to predict tht she should go to the wrong loction. However, if they simply understood their competitor to be ignornt, they could mke no such specific prediction bout where she would go, nd so should not perform s well. Results showed tht lthough chimpnzees distinguished between the informed nd uninformed conditions (thus showing n understnding of their competitor s knowledge versus ignornce), they did not distinguish between the uninformed nd the misinformed conditions (thus providing no evidence for understnding tht their competitor hd flse belief). Recently, J. Kminski et l. (personl communiction) found very similr pttern of results in totlly different experimentl prdigm. They presented both chimpnzees nd six-yer-old children with bck-nd-forth conspecific competition gme with both knowledge ignornce nd flse-belief versions. The bsic ide ws tht food ws plced in the opque buckets such tht in some cses the loction of the food ws known to both competitors (who wtched the other witnessing the hiding s well), in other cses only the subject witnessed the hiding of the food (the competitor ws thus ignornt) nd in still other cses both prticipnts wtched the hiding of the food but then only the subject sw it being moved to new loction (the competitor thus hd flse belief). The overll finding ws tht children pssed both the knowledge ignornce nd the flse-belief versions of the tsk, wheres chimpnzees treted both versions s involving knowledge-ignornce only nd not flse beliefs. Together with the results of Hre et l. [15], the conclusion is cler: chimpnzees understnd knowledge-ignornce, but not flse belief. This seeming lck of understnding is ll the more striking given mounting evidence tht even one- nd two-yer-old humn children understnd something in the direction of flse beliefs [16 19]. Conclusions It is time for humns to quit thinking tht their nerest primte reltives only red nd rect to overt behvior. Obviously, chimpnzees socil understnding begins with the observtion of others behvior, s it does for humns, but it does not end there. Even if chimpnzees do not understnd flse beliefs, they clerly do not just perceive the surfce behvior of others nd lern mindless behviorl rules s result. All of the evidence reviewed here suggests tht chimpnzees understnd both the gols nd intentions of others s well s the perception nd knowledge of others. Moreover, they understnd how these psychologicl sttes work together to produce intentionl ction; tht is, they understnd others in terms of reltively coherent perception gol psychology in which the other cts in certin wy becuse she perceives the world in certin wy nd hs certin gols of how she wnts the world to be. There is much less evidence overll, but it is possible tht other non-humn primte species lso hve similr understnding [20,21],ndsdo,perhps,somebirdspeciess well [22 25]. In brod construl of the phrse theory of mind, then, the nswer to Premck nd Woodruff s pregnnt question of 30 yers go is definite yes, chimpnzees do hve theory of mind. But chimpnzees probbly do not understnd others in terms of fully humn-like belief desire psychology in which they pprecite tht others hve mentl representtions of the world tht drive their ctions even when those do not correspond to relity. And so in more nrrow definition of theory of mind s n understnding of flse beliefs, the nswer to Premck nd Woodruff s question might be no, they do not. Why chimpnzees do not seem to understnd flse beliefs in prticulr or if there might be some situtions in which they do understnd flse beliefs re topics of ongoing reserch. References 1 Tomsello, M. nd Cll, J. (1997) Primte Cognition, Oxford University Press 2 Premck, D. nd Woodruff, G. (1978) Does the chimpnzee hve theory of mind? Behv. Brin Sci. 1, Svge-Rumbugh, E.S. et l. (1978) Srh s problems in comprehension. Behv. Brin Sci. 1, Povinelli, D.J. et l. (1998) Young nd juvenile chimpnzees (Pn troglodytes) rections to intentionl versus ccidentl nd indvertent ctions. Behv. Processes 42, Tomsello, M. (1996) Do pes pe? In Socil Lerning In Animls: The Roots Of Culture (Glef, B.G. nd Heyes, C., eds), pp , Acdemic Press 6 Povinelli, D.J. nd Eddy, T.J. (1996) Wht young chimpnzees know bout seeing. Monogr. Soc. Res. 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