How Could We Know Whether Nonhuman Primates Understand

How Could We Know Whether Nonhuman PrimatesUnderstand Others’ Internal Goals and Intentions?Solving Povinelli’s Problem

Robert W. Lurz & Carla Krachun

Published online: 10 August 2011# Springer Science+Business Media B.V. 2011

Abstract A persistent methodological problem in primate social cognition research hasbeen how to determine experimentally whether primates represent the internal goals ofother agents or just the external goals of their actions. This is an instance of DanielPovinelli’s more general challenge that no experimental protocol currently used in thefield is capable of distinguishing genuine mindreading animals from their complemen-tary behavior-reading counterparts. We argue that current methods used to test forinternal-goal attribution in primates do not solve Povinelli’s problem. To overcome theproblem, a new type of experimental approach is needed, one which is supported by analternative theoretical account of animal mindreading, called the appearance-realitymindreading (ARM) theory. We provide an outline of the ARM theory and show howit can be used to design a novel way to test for internal-goal attribution in chimpanzees.Unlike protocols currently in use, the experimental design presented here has thepower, in principle and in practice, to distinguish genuine mindreading chimpanzeesfrom those who predict others’ behavior solely on the basis of behavioral/environmental cues. Our solution to Povinelli’s problem has important consequencesfor a similar debate in developmental psychology over when preverbal infants shouldbe credited with the ability to attribute internal goals. If what we argue for here in thecase of nonhuman primates is sound, then the clearest tests for internal-goal attributionin infants will be those that test for attributions of discrepant or ‘false’ perceptions.

1 Introduction

For some 30 years now, there has been a lively debate within comparative psychologyand philosophy over whether nonhuman animals are capable of attributing mental states

Rev.Phil.Psych. (2011) 2:449–481DOI 10.1007/s13164-011-0068-x

R. W. Lurz (*)Department of Philosophy, Brooklyn College - CUNY, Brooklyn, NY, USAe-mail: [email protected]

C. KrachunDepartment of Psychology, Grenfell Campus Memorial University of Newfoundland,Corner Brook, Canadae-mail: [email protected]

to others (also known as mindreading or theory-of-mind).1 One strand of this debatehas focused on whether primates attribute the psychological state of intending orhaving goals (i.e., internal goals, see below) to other creatures when predicting andmaking sense of their behaviors. Until about a decade ago, the general opinion in thefield was that they did not. In their comprehensive volume on primate cognition,Tomasello and Call (1997) expressed the then-prevailing view. “Nonhuman primates,”they wrote, “perceive and understand others as ‘animate’ based on their ability tomove and do things spontaneously and as ‘directed’ toward external objects and eventsin the sense that they have learned certain antecedent–consequent sequences ofbehavior … [but this] does not, however, necessarily make for a theory of mind, oreven a theory of intentions” (pp. 384–386). In order to be considered as having atheory of mind or intentions, the researchers argued, primates must demonstrate anability to attribute states that mediate between the antecedent–consequent sequences ofbehavior in others. And at the time, Tomasello and Call concluded, there was simplyno unequivocal evidence that primates did in fact attribute such mediating states whenpredicting or making sense of the behavior of other creatures.

In recent years, Tomasello and Call (see Tomasello et al. 2005), as well as anumber of other prominent researchers in the field, have reversed their opinion onthis issue. They now hold that a new set of experimental data “compels” them to saythat primates (in particular, chimpanzees) understand and predict others’ behavior,not simply by understanding the sorts of behavioral/environmental cues that can beused to reliably predict others’ future actions, “but also in terms of the underlyinggoals, and possibly intentions” that, in others’ minds, mediate between suchantecedent cues and consequent actions (Call and Tomasello 2008, p. 189). Agrowing consensus among animal researchers, based on this new set of data, is thatprimates do in fact attribute psychological states of intending and having goals whenthey predict and make sense of other agents’ behavior.

Throughout this shift in opinion, Daniel Povinelli has consistently argued thatno experimental protocol currently in use is capable of distinguishing genuinemindreading animals from their complementary behavior-reading counterparts.As a result, none of the positive data produced from such protocols could ever providecompelling reasons for saying that animals engage in mindreading rather than somecomplementary form of behavior-reading (see Penn and Povinelli 2007). We agree that‘Povinelli’s problem’ has not been overcome by recent studies that Tomasello, Call andothers now take as providing compelling grounds for thinking that primates attributethe psychological states of intending and having action goals.2 While this research hassignificantly advanced the field in many ways, in every study it is unfortunatelyimpossible to definitively rule out an equally plausible complementary behavior-reading explanation. However, we do believe that there is a way to move forward onthis question by designing more sensitive tests that do have the capacity to solvePovinelli’s problem. What is needed is a new type of experimental protocol in thefield, one which is supported by an alternative theoretical account of animal

1 For convenience, nonhuman animals and nonhuman primates will henceforth be referred to as animalsand primates.2 In some writings (Hurley and Nudds 2006; Lurz 2009, 2011a, b), Povinelli’s problem is referred to as thelogical problem.

450 R.W. Lurz, C. Krachun

mindreading that we call the appearance-reality mindreading (ARM) theory. In thispaper, we outline the ARM theory of animal mindreading and show how it can beused to design a novel test for internal-goal attribution in chimpanzees. Unlikeprotocols currently in use, the experimental design presented here has the power, inprinciple and in practice, to overcome Povinelli’s problem.

Our solution to this problem has important consequences for a similar debate indevelopmental psychology over when preverbal infants should be credited with theability to attribute internal goals. A variety of experimental protocols have been usedin the field to assess goal attribution in preverbal infants (e.g., Behne et al. 2005;Csibra et al. 1999; Gergely et al. 2002; Meltzoff 1995; Repacholi and Gopnik 1997;Woodward 1998, 1999). However, a number of researchers (Gergely and Csibra2003; Perner and Doherty 2005; Povinelli 2001; Povinelli et al. 2005; Sirois andJackson 2007) have argued that these various protocols are incapable of determiningwhether infants predict/understand other agents’ behavior by interpreting certainbehavioral/environmental cues as indicating underlying goals/intentions in agents(the mindreading hypothesis) or whether they do this without interpreting these cuesas such (the complementary behavior-reading hypothesis). The issue is just Povinelli’sproblem as it applies to the developmental research. What we need to move thedevelopmental field beyond this impasse, some have argued (e.g., Perner and Doherty2005), are nonverbal tests for goal attribution in infants that can distinguish betweenthese two hypotheses. Perner and Doherty, however, despair of there ever being suchtests. For the clearest way to distinguish between the two hypotheses would be a testin which infants are expected to predict/understand an agent behaving in one way (X)if they deploy an internal-goal interpretation of the relevant behavioral/environmentalcues, but in a different way (Y) if they do not deploy a mentalistic interpretation ofthese cues. But if the cues alone (i.e., without the mentalistic interpretation) areexpected to lead infants to predict/understand the agent doing Y, then in order topredict/understand the agent doing X instead, infants would have to understand theagent’s internal goal state as somehow representing the cues differently from what theyreally are (otherwise, why would infants be expected to predict/understand behavior inthe agent that was different from what the cues alone would lead them to predict/understand?). This, however, would require our taking infants to understand internalgoals as states capable of ‘misrepresentation,’ similar to that observed in beliefs andperceptions. And this, Perner and Doherty claim, is highly implausible since “goalrepresentations do not depend on manipulable ingoing information, and goals cannotbe misrepresented” (p. 711). Thus, on such a view of the nature of internal goals, therewould appear to be little chance of solving Povinelli’s problem as it applies to theresearch on goal attribution in preverbal infants. However, if what we argue for here inthe case of nonhuman primates is sound (especially that part of our argument in whichwe show that there is a theoretically plausible account of internal goals that takes themto be perceptual states), then there is a plausible way to address Perner and Doherty’sworry, and thus a way toward solving Povinelli’s problem as it applies to thedevelopmental research in question.

We begin our examination of Povinelli’s problem in the primate research by firstclarifying the distinction between internal and external goals and then explaininghow the problem applies to current studies aimed at testing primates’ attribution ofinternal goals.

Solving Povinelli’s Problem 451

2 Representing Internal Versus External Goals

The concept of a goal (and of an intention) admits of an ambiguity. In one sense,‘goal’ represents a particular type of motivational or conative state in an agent, suchas when we say that an agent has a goal in mind or intends to do something. Goalsqua conative states are intentional states of mind; they are behavior-guiding states ofan agent directed at objects or states of affairs in the world that need not exist andmay never come to exist despite the agent’s every effort to realize them. We followTomasello et al. (2005) in labeling this psychological understanding of ‘goal’ aninternal goal. In another sense, ‘goal’ is used merely to represent the external objector state of affairs that an agent’s action is understood to be directed at or expected tobring about, such as when we say that grasping an object was the goal of the agent’sreaching.3 Following Tomasello et al. (2005), we label this non-psychological senseof ‘goal’ an external goal.

To illustrate this ambiguity, imagine that we are playing a game. Given my lowscore and past poor performance, you predict that I am going to lose. Unbeknownstto you that is exactly what I intend to do. I intend to throw the game. In such a case,however, you do not represent me as intending or as having the goal to lose thegame, for you do not think that I intend to do this or have this as my goal. So inpredicting my losing the game, you do not represent my internal goal. Nevertheless,you do represent what I intend to do, since you predict my losing the game, andlosing the game is what I intend to do. And so in a non-psychological sense of‘goal,’ you represent my external goal in predicting the outcome (losing the game) ofmy current behavior. The distinction here is the difference between merelyrepresenting what an agent intends or has a goal to do (which in many cases isjust representing the expected outcome of the agent’s action or actions, such aslosing the game) and representing an agent as intending or as having a goal to dosomething (which is always representing an agent as being in a psychological state,such as the conative state of intending to lose the game). Unfortunately, both types ofrepresentations are called ‘the agent’s goal,’ but only the latter sort counts as a typeof mental state attribution.

Attributions of external goals, or goal-directed (transitive) actions, as with manysuch teleological interpretations of events, do not in every case involve or requireinterpreting agents as having conative states of mind (though in some cases theymay). The trajectories of baseballs and the attraction of iron filings to magnets are allevents that we naturally take to be directed at certain future states of affairs, but wedo not take these events to be the result of the baseball or the iron filings havinggoals or intentions. And in the natural world, we quite often interpret behaviors ofanimals and humans in teleological terms without thereby attributing conative statesto them. We interpret a spider’s web building as having the external goal to catchinsects, and the waggle dance of honeybees as having the external goal to informnest mates of the location of nectar, for example. But such teleologicalinterpretations of these behaviors are in no way dependent upon our seeing the

3 Note that grasping an object is not a mental state of the agent, in contrast to the agent’s intending orhaving the goal to grasp the object, which are mental states.


spider or the bee as having in their mind the goal to catch insects or to inform nestmates of the location of nectar.

Regarding other creatures’ external goals, there is little serious doubt that animalscan represent and track them. Many animals form reliable expectations about whatothers are likely to do or are currently doing, and in many instances theseexpectations represent these other creatures’ external goals. RL’s cat, for instance,manifests clear signs of expecting him to feed her in the kitchen at lunchtime. WhenRL gets off the couch at noon and begins to move toward the kitchen (where thecat’s bowl is located), the cat, upon seeing RL’s behavior at this time of day, reliablyscampers ahead in clear anticipation of being fed in the kitchen. Since feeding her inthe kitchen is what RL intends to do, the cat represents RL’s external goal. On thisthere seems little to object. Nevertheless, it is rather questionable whether the catrepresents RL’s external goal to feed her by representing him as having an internalgoal (a psychological state of intending or having the goal) to feed her.

As this case is meant to illustrate, the controversy within animal social cognitionresearch is not whether animals can represent and track other creatures’ externalgoals (RL’s cat shows rather intuitively that they can) but whether they ever do so byattributing internal goals to other creatures. Thus, the distinction between theexternal goal of an agent’s action in the teleological sense and the internal goal of theagent in the psychological sense should be kept in mind when examining the studieson primate goal and intention attribution. Even if primates do show an aptitude forunderstanding other agents’ actions as directed at or tending toward objects andstates of affairs in the world, as we believe that some of the data suggest, this alonewould not show that these animals understand other agents as psychological agentsthat have goals and intentions. They may well see other agents and their actionsmuch in the way that we see spiders and their web-building behaviors, and as RL’scat sees RL’s kitchen-directed behavior at noon: in teleological but not psychologicalterms.

3 Povinelli’s Problem

The general problem for any animal mindreading hypothesis, according to DanielPovinelli (and his colleagues), is that since mental state attribution by animals mustbe based on observable features of an agent’s behavior and environment—for theminds of others are not open to direct inspection—every mindreading hypothesis hasa complementary behavior-reading hypothesis. Such a hypothesis proposes that theanimal in question uses the very same behavioral/environmental cues to predict theagent’s behavior that, on the mindreading hypothesis, the animal is taken to use as itsinferential grounds for attributing a mental state. What is more, according to thistheory, complementary behavior-reading hypotheses for primate social behaviors(especially, the great apes), are not idle, ad hoc competitors; they are hypothesesentailed by an independently credible evolutionary theory of mindreading, called thereinterpretation theory. On the reinterpretation theory, mindreading is a uniquelyhuman specialization that was “grafted into existing cognitive systems for reasoningabout social behavior that we inherited from our ancestors with the African apes.”Thus, the distinctly human ability to represent others’ mental states “did not replace


the ancestral system for representing behavioral abstractions but was integrated withsuch systems” (Vonk and Povinelli 2006, p. 375). On the reinterpretation theory,complementary behavior-reading explanations of great apes’ social behavior are allbut expected to be true.

The problem, according to Povinelli, is that no experimental protocol that hasbeen used to test a mindreading hypothesis with apes (or other animals) has beenable to control for an equally plausible complementary behavior-reading explanationof the data. In their 2006 paper, Vonk and Povinelli provide the following succinct(though somewhat technical) expression of the problem:

The general difficulty is that the design of … tests [in animal mindreadingresearch] necessarily presupposes that the subject notices, attends to, and/orrepresents, precisely those observable aspects of the other agent that are beingexperimentally manipulated. Once this is properly understood, however, itmust be conceded that the subject’s predictions about the other agent’s futurebehaviour could be made either on the basis of a single step4 from knowledgeabout the contingent relationships between the relevant invariant features of theagent and the agent’s subsequent behaviour, or on the basis of multiple stepsfrom the invariant features, to the mental states, to the predicted behavior.Without an analytical specification of what additional explanatory work theextra cognitive step is doing in the latter case, there is nothing to implicate theoperation of Sb+ms [mindreading] over Sb [behavior-reading] alone. (p. 393)

To illustrate Povinelli’s problem here, imagine that an experiment is run to test thehypothesis that an animal, A, predicts what an agent, S, will do (Saction) in a certainsetting by attributing to S a particular mental state (Sms). The hypothesis, forexample, might be that A will predict that S will eat the food in the middle of theroom and not behind the pillar because A knows (or believes) that S sees the food inthe middle of the room but not behind the pillar. Of course, A cannot know of S’smental state (Sms) directly but must infer it from some observable behavioral/environmental cue (Sb) in the setting. A might, for instance, infer that S sees the foodin the middle of the room but not behind the pillar because S has an unobstructedline of gaze to the food in the middle of the room but not behind the pillar (the pillarbeing an opaque barrier intersecting S’s line of gaze to the food). And so, amindreading hypothesis, when fully articulated, will need to make two fundamentalassumptions about A. It will need to assume that (1) on the basis of representing theobservable cue Sb, A infers that S has a particular type of mental state (Sms),

5 andthat (2) on the basis of representing Sms, A predicts a particular type of action by S(Saction).

On the complementary behavior-reading hypothesis, however, A is taken topredict Saction on the basis of Sb without representing Sms. A, for example, might betaken to predict that S will eat the food in the middle of the room but not behind the

4 Vonk and Povinelli overstate the problem here a bit. It should not be assumed that a behavior-readinganimal’s prediction of an agent’s behavior must involve only a single inferential step—a step from theobserved behavioral/environmental cue to the predicted behavior. There is no reason why the behavior-reading animal could not engage in several inferential steps. The important point is that whatever furthersteps the animal may use, they will not involve the attribution of mental states.5 In the above quotation, Vonk and Povinelli represent assumption (1) here with the notation ‘Sb+ms’.


pillar because it knows (or believes) that S has a direct line of gaze to the food in themiddle of the room but not behind the pillar—without A interpreting the formerspatial relation between S’s eyes and the food as evidence of S seeing the food, orthe latter obstructed spatial relation between S’s eyes and the food as evidence of Snot seeing it. Thus, on the complementary behavior-reading hypothesis, A is taken toreason in accordance with what might be called a ‘behavioral rule’ (in this case, therule that Sb-type cues lead to Saction-type behaviors) rather than, as the mindreadinghypothesis assumes, according to a ‘mindreading rule’ (the rule that Sb-type cueslead to Sms-type mental states, which in turn lead to Saction-type behaviors).6

If it is asked how A might come to know or follow such a behavioral rule, fourtypes of answers are now routinely offered in the field. First, A may have simplyobserved Sb-type cues followed by Saction-type behaviors in the past. Thus, A mayhave come to know/follow the behavioral rule by means of associative learning—learning that may well involve associating environmental cues with representationsof “behavioral abstractions” in others (Vonk and Povinelli 2006, p. 375), as opposedto associating environmental cues with representations of mere “surface behaviors”in others (Call and Tomasello 2008, p. 189).

Second, according to the teleological theory of action comprehension, A mightcategorize these past Sb–Saction sequences in terms of their efficiency within theparameters of the context of action (see Gergely and Csibra 2003). Thus A may havelearned through past observations that S (or S-type agents) typically do Saction-typebehaviors under Sb-type observable conditions by the most efficient means availablein the context (according to A’s standards of efficiency). On this way ofunderstanding the behavioral rule, A may be able to predict S doing Saction underSb conditions by the most efficient means available in the context (again, accordingto A’s standards), even if the “surface behaviors” exhibited by S’s new efficientmeans of action have never been observed by A to follow Sb-type conditions in thepast.

Third, A may come to follow the behavioral rule by engaging in an embodiedsimulation-based process (Gallese 2007). On the embodied simulation account, Amay predict Saction-type behavior in others under Sb-type observable conditions byusing its own past behaviors or current behavioral dispositions under observed Sb-type conditions as a model. A, for instance, may expect that agent S is likely to doSaction-type behavior under S-b type situations, since A itself has done or currentlyfinds itself prepared to do (via the stimulation of its mirror-neuronal system) Saction-type behavior under Sb-type observable situations.

6 As should be clear from the above discussion, not all instances of behavior-reading fit into the categoryof complementary behavior-reading. In many instances, animals learn (or innately know how) to anticipateother agents’ behavior (e.g., fleeing, feeding, mating) on the basis of certain behavioral/environmentalcues (e.g., forward-facing torso, sound of a bell, smell of sex pheromones in conspecifics, etc.) that are notin any plausible way indicative of kinds of mental states in the other agent. Lurz (2009, 2011b) calls suchnon-mindreading methods of behavior prediction minimal behavior-reading (for evidence of this inchimpanzees see Povinelli and Eddy 1996; Vonk and Subiaul 2009). The categories of mindreading andcomplementary behavior-reading are, therefore, not exhaustive; an animal can be a minimal behavior-reader and fail to be in either of these categories. Thus, it bears reiterating that it is the realistic possibilitythat chimpanzees (and perhaps other primates) are complementary behavior-readers—not minimalbehavior-readers—that creates Povinelli’s problem for chimpanzee (primate) theory-of-mind research.


And fourth, it is quite possible that Saction-type behaviors are stereotypicalbehaviors under Sb-type conditions for A’s species and, thus, A may be credited withbeing innately equipped (‘hardwired’) to expect Saction-type behaviors from othersunder Sb-type conditions (see Povinelli 2001).

What the four behavior-reading strategies here have in common is that they are allreality-based methods for representing other creatures’ external goals (i.e., methodsthat take the behavioral/environmental cues animals use to predict other agents’behavior as facts that the animal itself currently perceives or believes obtain orobtained). An important point in mentioning these various possible explanations forhow an animal might come to know or follow a behavioral rule is to avoid the falsedichotomy that animals predict others’ behaviors either by mindreading or bylearning to associate certain antecedent behavioral/environmental cues with certainconsequent surface behaviors in others (i.e., movements of others’ limbs in spaceindependent of contextual matters). Thus, to overcome Povinelli’s problem, oneneeds to show how an animal could predict another agent’s action (Saction) by itstaking a behavioral/environmental cue (Sb) as evidence of an underlying mental state(Sms) but could not make this prediction by following a behavioral rule (e.g., Sb-typecues lead to Saction-type behaviors) according to one of the four behavior-readingmethods outlined above. The question is whether any of the experimental protocolsthat Call, Tomasello, and others take as providing compelling data that primatesattribute internal goals overcomes Povinelli’s problem.

4 Compelling Data of Internal-Goal Attribution by Primates?

It is beyond the scope of this paper to examine every empirical study on primateinternal-goal attribution. But neither is it necessary to do so for the purpose of thispaper. The studies that we have selected are representative of the sorts ofexperimental approaches currently used in the field to test for primates’ attributionof internal goals. Demonstrating that these kinds of experimental approaches cannotovercome Povinelli’s problem is enough to motivate pursuing an alternativeexperimental strategy.

4.1 The Accidental Versus Intentional Protocol

One of the earliest studies of goal/intention attribution in primates was by Call andTomasello (1998).7 In their study, orangutans and chimpanzees were first trained tochoose a baited box from among three that were presented. The baited box wasalways distinguished from the others by a marker (a block) placed on top. The apes’choice of the marked box was rewarded by having the experimenter remove the foodfrom underneath the box and give it to the animal. The apes were then trained on adelayed version of the task in which the experimenter removed the marker beforeallowing the animal to make a choice. In the testing stage, the apes were initiallypresented with the three boxes without a marker. They then watched while theexperimenter intentionally put the marker on one box and then (before or after this)

7 See Povinelli et al. (1998) for a similar study that produced negative results.


accidentally knocked the marker onto another box. In those cases where the markerwas intentionally put onto the box, the experimenter either gently placed the markeron top of the box or held the marker 10 cm directly over the top of the box andcarefully dropped it onto the box; in both cases, the experimenter looked at the boxand marker while acting. In those cases where the marker ended up on a boxaccidentally, the experimenter either looked away from the boxes and, by moving theapparatus on which the boxes were set, caused another marker (which waspositioned above the boxes) to fall onto a box; or she simply knocked the positionedmarker off its perch with her arm and onto a box while pretending to inspect somepart of the apparatus. Whether the marker was placed accidentally or intentionally,the experimenter left it on the box for 3 s before removing it. For the unintendedmarker only, the experimenter made a facial expression of disapproval and in someinstances said “Oops!” as she removed it.

Pooling the data across subjects, it was discovered that the apes as a group tendedto choose the intentionally marked boxes over the accidentally marked ones (at leastin the early trials). There was an unexpected difference in the animals’ choicebetween the two kinds of intentionally marked boxes, however. Most of the apestended to choose the intentionally marked box on which the marker had beendropped by the experimenter, choosing the box on which the marker had beenintentionally placed at roughly the same rate as the accidentally marked box.However, the language-trained orangutan, Chantek, chose the box on which theexperimenter had intentionally placed the marker on every trial, choosing the box onwhich the marker had been intentionally dropped at roughly the same rate as theaccidentally marked box. Call and Tomasello speculated that this difference was dueto a difference in the apes’ rearing histories. Food sharing among great apes is notvery common, but when it does occur, the donor usually just drops the food on theground in front of the recipient, rather than handing it to him as a human might. As aresult of this species-typical type of behavior in food-sharing contexts, Call andTomasello suggested that the unenculturated (not reared by humans) apes in theirstudy “may have regarded dropping as more intentional than placing” (p. 202).Chantek, however, is an enculturated orangutan. Call and Tomasello speculated thatChantek’s “early exposure to human interaction and enculturation” may haveinclined him toward seeing the experimenter’s act of placing the marker as moreintentional than her act of dropping it. This seems plausible, for Chantek’s trainersand caregivers, especially when he was a juvenile in the language-training program,would often gently place food into his hand or a feeding tray, rather than simply dropit onto the floor next to him. And during his language training period, Chantek cameto use many signs to request food, whereupon the trainer would (in many cases)carefully place the food into Chantek’s hands (see Miles 1990). And so it is quitepossible that Chantek understood the arrangement of Call and Tomasello’sexperiment as one of a food-sharing kind and, as a result of his enculturation, wasdisposed to interpret the gentle placement of the marker by the experimenter assimilar to the intentional placement of food into his hand or feeding tray in pastfood-sharing encounters with humans.

Call and Tomasello interpreted the results of their study as showing that their apes“understood something about the experimenter’s intentions” (p. 192). The questionis whether a behavior-reading hypothesis complementary to Call and Tomasello’s


intentional-state attribution hypothesis can provide an equally plausible account. Theanswer, we believe, is yes. First, let’s consider the question of why the apes as awhole tended to choose the intentionally marked boxes over the accidentally markedones. The complementary behavior-reading answer is simply that the apes as a wholepreferred the former type of boxes to the latter because the former were more like thebaited boxes in the training phase vis-à-vis the experimenter’s behavior toward them.That is, the former boxes were such that the experimenter had direct eye gaze withthe marker on them and removed the marker without disapproving facial expressionsor saying “Oops!”

The second question to be answered is why did the apes choose the particulartype of intentionally marked box that they did in the test trials? Call and Tomasello,recall, speculated that the unenculturated apes chose the intentionally droppedmarker boxes because they saw the experimenter’s action of dropping the marker asbeing like those actions of intentionally dropping food in a typical ape-ape food-sharing context; and that Chantek chose the intentionally placed marker boxesbecause he saw the experimenter’s action of carefully placing the marker as beinglike those actions of intentionally placing food in typical human-ape food-sharingencounters. But, of course, the animals would not have seen the experimenter’sactions in such intentional terms if their respective past food-sharing encountersnever led to them (or others) receiving the food in the location where it was droppedor gently placed by the donor. If the food was quickly eaten by the donor (ormiraculously disappeared) as soon as it hit the floor or lay in the recipient’s hand, forexample, the apes would not have taken such actions of dropping or placing food asfood-sharing at all, let alone as expressing the donor’s intention to share food. Andso, if Call and Tomasello’s explanation here is correct, the apes are likely to knowfrom previous experience that dropping or gently placing food in a food-sharingcontext generally leads to the food remaining in the area that it was dropped orplaced long enough for the recipient to retrieve it. But now we are in position to givea complementary behavior-reading answer to our second question. The unencultu-rated apes tended to choose the box on which the marker was intentionally droppedbecause they saw the experimenter’s action as being similar to the act of droppingfood by a conspecific and expected a similar type of outcome: food in that location.And Chantek chose the box on which the marker was intentionally placed becausehe saw the experimenter’s action as being similar to the careful placement of food byhumans and expected a similar type of outcome, too: food in that location. We do notsee that the complementary behavior-reading explanations here are any lessplausible, or any more speculative, than Call and Tomasello’s own intentional-stateattribution explanation.

4.2 The Unwilling Versus Unable Protocol

In 2004, Call and colleagues ran another intentional-state attribution study withchimpanzees. In that study, the subject chimpanzee was fed grapes by anexperimenter through a hole in a Plexiglas wall. Immediately following 2 to 6motivational trials in which the chimpanzee received a grape, a test trial was run inwhich the experimenter suddenly stopped feeding the chimpanzee either becausehe was unable (but still willing) to do so or because he was unwilling (but still


able) to do so. In the various unable test trials, the experimenter’s willingness tocontinue feeding the chimpanzee was suggested to the animal by the fact that hehad been, up to that point, feeding the chimpanzee quite regularly, and that hissudden cessation of feeding was due to his being physically unable to do so (e.g.,because a barrier, obstacle, or clumsy behavior prevented him from passing a grapeto the chimpanzee, or because his hands were currently occupied with anothertask). In the various unwilling test trials, the experimenter’s sudden unwillingnessto feed the chimpanzee was suggested to the animal by the fact that he wasphysically able to deliver a grape (i.e., there were no barriers, obstacles, or clumsybehavior preventing delivery of the grape, nor were his hands otherwise occupied)yet he did not.

Chimpanzees responded differently to these two types of test trials. In the unabletrials, chimpanzees tended to wait and remained quiet (i.e., they engaged in little tono begging behaviors). Such “patient” behavior on the chimpanzees’ part wassuggestive of their understanding that the experimenter would eventually come toremove the physical constraint or cease the clumsy or distracted behavior and returnto feeding them. By contrast, in the unwilling test trials, chimpanzees tended to begmore often (as if trying to cause the experimenter to continue feeding) and ended upleaving the testing room soon after begging (as if understanding that there was nopoint to sticking around and begging if the experimenter was so bent on notdelivering the food). The researchers concluded:

The current study provides suggestive evidence that chimpanzees spontane-ously (i.e., without training) are sensitive to others’ intentions. Observing thebehavior of a human not giving them food, chimpanzees demonstrated in theirspontaneous behavior that they recognized a difference between cases in whichhe was not giving food because he was unwilling to or because, for variousreasons, he was unable. (p. 496).

Thus, Call et al. (2004) take their chimpanzees to be following something like thefollowing mindreading rules:

Unable-yet-willing: If the experimenter has been feeding me regularly andsuddenly stops because he is physically unable to continue (e.g., because of abarrier, obstacles, clumsy behavior, preoccupied hands), then he is still willing(still has the intention) to feed me; in which case, the thing to do is to just waitand be patient.Unwilling-yet-able: If the experimenter has been feeding me regularly andsuddenly stops though not because he is physically unable (e.g., there are nobarriers, obstacles, etc. preventing him from feeding me), then he is no longerwilling (i.e., no longer has the intention) to feed me; in which case, the thing todo is to beg to see if this might cause him to start feeding me again, and if thisdoesn’t initially work, to just give up.

Of course, one might wonder how the chimpanzees came to know what to do ineach of the two kinds of test situations, since they could not have learned thesedifferent response strategies from repeated exposure to the test trials (in whichchimpanzees’ were not differentially rewarded). It is plausible to suppose that thechimpanzees might have learned these different strategies from past encounters with


other feeding agents (e.g., other humans or conspecifics).8 On the basis of such pastencounters, the chimpanzees could have learned that the best strategy (i.e., the onemost likely to lead to receiving food) to adopt toward an agent who appears to bewilling but temporarily unable to feed you is to sit tight and wait; whereas, the beststrategy to adopt toward an agent who appears to be unwilling though quite able tofeed you is to beg, and if that doesn’t work, to just give up.

Unfortunately, Call et al.’s (2004) study does not overcome Povinelli’s problem.To know what the experimenter’s intention is in any given test trial, chimpanzeesmust infer it from observed behavioral/environmental cues. As noted above, the cuesthat the chimpanzees presumably used to infer that the experimenter remainedwilling to feed them in the unable trials were that he had been feeding them regularlybut had suddenly stopped because he was physically unable. And the cues that thechimpanzees presumably used to infer that the experimenter was no longer willing tofeed them in the unwilling test trials were that he had suddenly stopped feeding themeven though he remained physically able to do so. But once these observablegrounds for attributing the different intentions to the experimenter are made explicit,a complementary behavior-reading explanation of the chimpanzees’ performancequickly suggests itself. It seems just as plausible that instead of using the abovemindreading rules, chimpanzees simply used the following behavioral rules:

Stops-feeding-because-unable: If the experimenter has been feeding meregularly and suddenly stops because he is physically unable to continue (e.g.,because of a barrier, obstacle, clumsy behavior, preoccupied hands, etc.), thenthe thing to do is to just wait and be patient.Stops-feeding-though-able: If the experimenter has been feeding me regularlyand suddenly stops though not because he is physically unable (e.g., there are nobarriers, obstacles, etc. preventing him from feeding me), then the thing to do isto beg to see if this might cause him to start feeding me again, and if this doesn’tinitially work, to just give up.

Again, one might wonder how chimpanzees could come to know such behavioralrules, given that they did not learn them over the test trials. To this question, the sameanswer that was given above can be given here. Through their past encounters with otherfeeding agents, the chimpanzees could have learned that the best strategy to adopttoward an agent who has been feeding them but suddenly stops because he is physicallyunable to continue is to sit tight and wait; whereas, the best strategy to adopt toward anagent who has been feeding them but suddenly stops though not because he is physicallyunable to deliver the food is to beg, and if that doesn’t work, to give up. Thus, both themindreading explanation and the complementary behavior-reading explanations areforced to provide the same type of explanation regarding how the chimpanzees came toknow the rules that they followed in the test trials.

Note also that both the mindreading and the complementary behavior-readingexplanations credit chimpanzees with possessing the abstract behavioral concepts of‘being physically able to deliver food’ and ‘being physically unable to deliver food,’

8 All the chimpanzees tested were captive born with a mean age of 15.4 years. Thus, by the time they weretested, they would have had plenty of opportunity to learn such response strategies as a result of theirinteractions with human caretakers who fed them.


concepts that apply to a perceptually heterogeneous class of behaviors andenvironmental circumstances. Only the mindreading hypothesis, however, takeschimpanzees to use these abstract behavioral concepts as their basis for ascribing themental state of willing (or intending) to the experimenter. Yet nothing is apparentlygained in explaining the chimpanzees’ performance by making this additionalassumption. The chimpanzees’ performance is just as well explained in terms of theirfollowing the behavioral rules above. Thus, Call and colleagues’ study does not solvePovinelli’s problem.9

4.3 The Imitative Learning Protocol

In the past 20 years, there has been a lot of interest regarding imitative learning inanimals, with some imitation studies being used to test for intentional-stateattribution in primates. In one such study, Buttelmann et al. (2007) adapted animitation protocol developed by Gergely et al. (2002) for testing human children. InButtelmann et al.’s study, enculturated chimpanzees observed while an experimenterdemonstrated how to illuminate a light mounted onto a box by pressing on the lightwith his forehead. Half the chimpanzees observed the experimenter do so while hishands were occupied with holding a blanket around his shoulders; the other halfobserved the experimenter do so while his hands were free. In both cases, afterobserving the demonstration, chimpanzees were allowed to manipulate the apparatuson their own. Chimpanzees were more likely to turn on the light as the experimenterhad (with his head) if the experimenter’s hands were free during the demonstrationthan if they were occupied. When the experimenter’s hands were occupied,chimpanzees were more likely to use their hands or mouth to illuminate the light.10

The researchers interpreted the results as demonstrating that chimpanzees “possessthe ability to understand others’ intentions as rational choices of action to achieve goals”(p. 38). Thus, chimpanzees are taken to reason according to the mindreading rule thatagents generally intend to perform actions that they (the agents) know or believe to bemost efficient in the environmental setting. In the hands-free demonstration, accordingto this rule, chimpanzees are taken to reason that since (1a) the experimenter’s hands

9 A similar complementary behavior-reading explanation can be given for Phillip et al.’s (2009) study withcapuchin monkeys. In one of their experiments, however, the researchers used a spoon, rather than ahuman hand, to deliver food to the animal subject during motivational trials. In contrast to their differentialresponses in human-hand experiments, monkeys in the spoon experiment did not discriminate between aspoon that was ‘unable yet willing’ and one that was ‘unwilling yet able’ to deliver food. In each case, themonkeys behaved impatiently and quickly left the testing room. The researchers argue that the monkeys’differential responses to the spoon and the human hand provided evidence of goal-attribution when itcomes to humans but not to inanimate objects. However, such results are in no way inconsistent with whatone would expect from a complementary behavior-reading account of the monkeys’ behavior. For thecomplementary behavior-reading hypothesis does not say that monkeys learn to follow behavioral rulesthat apply to any moving object (even spoons). Rather, the hypothesis holds, just as does the mindreadinghypothesis that the researchers endorse, that monkeys follow rules that they learned from (and take toapply specifically to) human or other animal agents. Thus, the complementary behavior-readinghypothesis is no more expected to predict a differential response pattern from monkeys in the differentspoon trials than is the mindreading hypothesis endorsed by Phillip and colleagues.10 Similar results were found with two other apparatuses that the demonstrator also illuminated using anunusual body part, although no significant differences were found with apparatuses that produced sounds(Buttelmann et al. 2007).


are unoccupied and yet he employs his head, (2a) he must have intended to turn on thelight in this unusual manner for some (unknown) advantage in efficiency (otherwise,he would have used his hands). Thus (3a) desiring to imitate what they understood tobe the experimenter’s intention in using this unusual means of operation, thechimpanzees turn on the apparatus with their head. And in the hands-occupieddemonstration, according to the mindreading rule, chimpanzees are taken to reasonthat since (1b) the experimenter’s hands are occupied and could not be used and heinstead employs his head, (2b) the experimenter must have intended to turn on thelight in this unusual manner for the reason that it was the most efficient meansavailable. Thus (3b) desiring to imitate what they understood to be the experimenter’sintention in using his head, chimpanzees turn on the apparatus by the most efficientmeans available, which for them is their hands or mouth.

The question is whether chimpanzees could have imitated as they did not becausethey desired to copy what they understood to be the experimenter’s intention in using hishead, but rather because they desired to copy what they understood to be the mostefficient means to turn on the light. This interpretation, which takes chimpanzees tofollow the behavioral rule that agents generally perform actions that are the mostefficient in the environmental setting, is consistent with interpretations of similarperformance by infants in analogous imitation studies (see Gergely et al. 2002). In thehands-free demonstration, according to this rule, chimpanzees are taken to reason thatsince (1c) the experimenter’s hands are unoccupied and yet he employs his head, (2c)there must be some (unknown) advantage in efficiency in using this unusual means ofoperation (otherwise, the experimenter would have used his hands). Thus (3c) desiringto turn on the light in what they understand to be the most efficient way, chimpanzeesturn on the light with their head. In the hands-occupied demonstration, according tothe behavioral rule, chimpanzees are taken to reason that since (1d) the experimenter’shands are occupied and could not be used and he instead employs his head, (2d) theexperimenter turned on the light by the most efficient means available in the situation.Thus (3d) desiring to turn on the light by what they understand to be the most efficientmeans to them, chimpanzees turn on the light with their hands or mouth.

The above complementary behavior-reading explanation is no less plausible or morespeculative than Buttelmann et al.’s intentional-state explanation. Both hypotheses restupon the common assumption that chimpanzees judge agents’ actions within a contextas being more or less usual and efficient. The significant difference between them is thatthe intentional-state hypothesis credits chimpanzees with inferring the experimenter’sunderlying intention in performing the unusual mode of operation, whereas thecomplementary behavior-reading hypothesis credits chimpanzees with inferring themost efficient way to operate the apparatus in the circumstance.

4.4 The Violation-of-Expectancy, Looking-Time Protocol

In 2004, Claudia Uller used an innovative looking-time methodology (inspired byGergely et al. 1995) to test for goal attribution in chimpanzees.11 In the study, fourinfant chimpanzees were familiarized (habituated) to one of two video displays inwhich a block moved toward and finally made contact with a ball, at which point the

11 See Rochat et al. (2008) for a similar violation-of-expectancy, looking-time study with macaques.


block stopped moving. In the experimental video, the block is shown making aparabolic jump over a barrier before making contact with the ball on the other side.In the control video, the barrier is off to the side and the block is shown making thesame parabolic jump in the air before making contact with the ball. The experimentalvideo, of course, was designed to make it look as if the block had the goal to makecontact with the ball by the most direct path possible (in the circumstance, jumpingover the barrier); whereas, the control video was designed to make it look as if theblock either had no goal at all or the goal of making contact with the ball by jumpingin the air first (an indirect path). After the habituation phase, the chimpanzees werethen shown two videos in which the barrier was removed. In one of the videos (oldaction test), the block performed the same movements as before, a parabolic jumpand contact with the ball; in the other (new action test), the block moved in a straightline toward the ball. Uller hypothesized that if the chimpanzees that were habituatedto the experimental video interpreted the block as having the goal of contacting theball in the most direct way possible, then they should find the old action testsurprising and look longer at it than at the new action test; and if the chimpanzeesthat were habituated to the control video interpreted the block as having no goal orthe goal of contacting the ball by jumping, then they should find the new action testsurprising and look longer at it than at the old action test. And these are precisely theresults Uller got.

Although this may be one of the better studies of internal-goal attribution inchimpanzees—for it tests the animal’s ability to predict an agent’s behavior ratherthan perform some task (e.g., discrimination or imitation) that is only distantlyrelated to the agent’s action—we do not believe it shows that chimpanzees attributethe mental state of having goals or intentions to agents, as opposed to attributinggoal-directed movement (see Gergely and Csibra 2003 for a similar interpretation ofthe infant data). To see this, let us use an analogy. Water, as we know, flows from ahigher level of elevation to a lower one by means of the most direct path—it ‘seeks’the path of least resistance, as we say. Thus, when there is an obstacle in its path, weexpect water to flow around the obstacle, and when the obstacle is removed, weexpect the water to flow in a straight path. Does this understanding of hydraulicsrequire or involve our interpreting water as having (in its mind!) the goal or intentionto reach the lowest level of elevation by means of the most direct path? Surely not.But it does involve our seeing the flow of water in teleological terms, as movingtoward an end state (the lowest level of elevation) in a predictable and efficient way(direct path). And so, it is quite possible that this is how the chimpanzees in Uller’sstudy understand the movement of the block in the experimental video, inteleological but not mental-state terms. They observe the block following the mostdirect path possible toward the ball (as we would see the movement of water arounda barrier toward a lower level of elevation). When the barrier is removed, thechimpanzees should (and do) expect the block’s movements to once again follow themost direct path to the ball—in this case, moving along a straight line (again, just aswe would expect the water to flow down a straight path after the barrier wasremoved). This does not involve the chimpanzees seeing the block as having a goal;it merely involves the chimpanzees seeing the block’s movement as directed towarda certain end state (contact with ball) and following the most direct route to it. Thechimpanzees that watch the control video, on the other hand, observe the block


following an indirect path toward the ball, and so they should (and do) expect theblock to follow the same indirect path in the test video. Again, there is no mentalstate attribution here, just attribution of goal-directed movement.

4.5 The Neurological Protocol

In the mid-nineties, researchers at the University of Parma quite unexpectedlydiscovered that a set of neurons in the F5 area of the macaque monkey’s premotorcortex not only fired when the monkey performed a goal-directed action, such aspicking up a grape, but also fired when it observed another monkey or humanperforming the same goal-directed action. F5 neurons were subsequently called“mirror-neurons,” and researchers soon began to speculate that they were used bymonkeys (as well as other primates) to interpret other agents’ actions in goal-directedways (Gallese et al. 1996; Rizzolatti et al. 1996).

However, some researchers in recent years have gone further and argued that themirror-neuronal system found in the inferior parietal lobe (IPL) of the macaque brainis used by the monkey to attribute intentional states to agents. Fogassi et al. (2005)discovered that some units of neurons in the macaque’s IPL discharged selectivelydepending on the type of goal-directed action it executed. Some neurons (the “grasp-and-eat” neurons) discharged more strongly when the monkey grasped and ate agrape than when it grasped an object and placed it into a container (receiving a grapefrom the experimenter as a reward); while others (the “grasp-and-place” neurons)discharged more intensely when the monkey grasped an object and placed it into acontainer (again, receiving a grape as a reward) than when it grasped and ate a grape.Controls were added to make sure that the two types of actions—grasp-and-eat andgrasp-and-place—were otherwise kinematically the same. When the monkey wasthen allowed to observe an experimenter perform one of these two types of graspingactions, it was discovered that its grasp-and-eat neurons were more likely to firewhen it observed the experimenter grasping a grape (without the container on table)than when it observed the experimenter grasping an object (or grape) with thecontainer on the table; whereas, its grasp-and-place neurons were more likely to firewhen it observed the experimenter grasping an object (or grape) with a container onthe table than when it observed the experimenter grasping the grape (without thecontainer). The researchers interpreted these findings as showing that the IPL mirror-neuron system in the macaque brain “allows the monkey to predict the goal of theobserved action [i.e., eating or placing] and, thus, to ‘read’ the intention of the actingindividual” (p. 666).

But we do not see how these findings show that the macaque understands that theexperimenter has a particular internal goal or intention when he grasps a grape orobject, as opposed to it merely being able to anticipate the end state (eating orplacing) of the experimenter’s initial grasping act by way of comparing the initial actto its own past grasping behaviors in similar contexts. For one thing, the researcherssimply speculate that the monkey predicts the end state (eating or placing) of theexperimenter’s grasping act, but there is absolutely no evidence of this in theexperiment. Although its grasp-and-eat neurons fire when it observes theexperimenter grasping a grape, for example, this may indicate nothing more thanthat the monkey is recollecting its own act of grasping and eating grapes under


similar conditions. However, despite the absence of any empirical support for thisspeculation, it is certainly possible that the monkey uses its mirror neurons to makesuch a prediction. But even on this assumption, there is no more reason to think thatthe monkey predicts the end state of the experimenter’s grasping act by attributing anintentional state to the experimenter than to think that it predicts the end state bycomparing the experimenter’s grasping act to its own past grasping acts in similarcontexts. There is no more reason, that is, to assume the intentional-state attributionhypothesis which takes the monkey to think something like,

“When I grasp a grape in such a context, I intend to eat it; so theexperimenter’s grasping the grape in such a context indicates that he has thesame type of intention and will likely eat the grape,”

rather than the complementary behavior-reading hypothesis that takes the monkey tothink something like,

“When I grasp a grape in such a context, I typically eat it; so theexperimenter’s grasping the grape in such a context indicates that he willlikely eat it.”

For example, in observing the experimenter grasping a grape in the absence of thecontainer, the monkey may recall its own grasping of grapes in the same context,which led to it consuming the grape that it had grasped. It is not implausible tosuppose that this association with its own past grasping-and-eating behavior in thecontext of an absent container would selectively increase the firing among its grasp-and-eat neurons, perhaps allowing it to “predict the goal [eating grape] of theobserved action.” Likewise, in observing the experimenter grasping an object (or agrape) in the presence of the container, the monkey may recall its own grasping ofobjects in the same context, which led to its placing the object into the container.Again, this association with its own past grasping-and-placing behavior may haveselectively increased the firing of its grasp-and-place neurons, allowing the monkeyto anticipate the placing of the object into the container by the experimenter.12

So in the end, we do not find any of the different methodological approachescurrently used to study goal attribution in animals capable of overcoming Povinelli’sproblem. In each case, a behavior-reading hypothesis complementary to theintentional-state hypothesis under consideration provides an equally plausibleaccount of the data.

5 Relevance to Children’s Ability to Attribute Internal Goals, and a Questionof Parsimony and Convergence

Call and Tomasello (2008) have responded to the above complementary behavior-reading strategy to understanding the data on internal-goal attribution in primateswith the following parity-of-reasoning argument:

12 The researchers did observe that the monkey’s grasp-and-eat neurons also fired when it observed theexperimenter grasping a grape in the context of a container, but they did not fire as strongly as its grasp-and-place neurons (p. 664).


Indeed consistent use of this explanatory strategy would also deny humanchildren an understanding of goals and intentions [qua mental states] becausemost of the chimpanzee [and monkey] studies are modeled on child studies.(p. 189).

Such a charge against the complementary behavior-reading strategy would be acompelling reason, we believe, to abandon it as a way to understand the primate dataif there were not a similar controversy in developmental psychology over how tointerpret the infant studies on goal attribution. But there is a similar controversy. Asnoted above, a number of researchers have argued that the data from the preverbalinfant studies are just as well accounted for by infants understanding somethingabout others’ external goals as by their understanding something about others’internal goals. Povinelli’s problem is just as much a problem for these infant studiesas it is for analogous primate studies. Since parity-of-reasoning arguments arecompelling only insofar as their analogue base is more certain than their target, wedo not find Call and Tomasello’s parity-of-reasoning argument convincing.

On the other hand, it may be possible to avoid Povinelli’s problem with regard tothe research on preverbal infants, just as we are suggesting it is for research onprimates. If internal-goal attribution in primates is anything like what we suggestbelow, then it follows from our account that internal-goal attribution in preverbalinfants will be most unambiguously manifested in their ability to attribute discrepantor ‘false’ perceptions. The one study to date on false perception attribution in infantsby Song and Baillargeon (2008), unfortunately, does not overcome Povinelli’sproblem. In that study, infants at around 14.5 months showed signs of expecting ahuman agent to reach for a container that deceptively appeared to contain a preferredtoy (a doll), but they did so only when the agent was absent during the baiting of thecontainers (false-perception trials). When the human agent was present during thebaiting of the containers (true-perception trials), infants showed signs of expectingthe agent to reach for the container that actually contained the doll, even though itwas not the container that deceptively appeared to contain the doll. However, assome have suggested (Hogrefe et al. 1986; Wellman 2011), infants could haveperformed as they did simply because they expected the absent (‘ignorant’) agent tobehave incorrectly (e.g., to choose the non-preferred container). Alternatively, infantscould have themselves been deceived by the misleading appearance in false-perception trials but not in true-perception trials. In true-perception trials, it ispossible that infants simply failed to register the deceptive appearance of thecontainer as a result of their focusing their attention on the human agent and his/herunobstructed line of gaze to the baiting process; whereas in false-perception trials,without the distraction of the human agent, infants may have noticed and beenfooled by the deceptive appearance of the container, resulting in their wronglythinking that the doll was in that container. Neither of these alternative explanationscredits the infants with understanding anything about perceptual states in others.

As will be shown below, our protocol is designed to control for such alternativeexplanations. Thus, on our account, consistent use of the complementary behavior-reading strategy to understand the infant data on internal-goal attribution does notentail an outright denial that infants understand internal goals and intentions; atmost, it entails reserving our strongest assertion of internal-goal attribution in infants


until the time that children begin to attribute discrepant or false perceptions.What we are recommending, then, is that if researchers want to demonstrateunambiguously that infants attribute internal goals, they may wish to adopt anexperimental protocol along the same lines as the one we are proposing herefor chimpanzees.

Call and Tomasello (2008) also charge that consistent use of the complementarybehavior-reading strategy is particularly unparsimonious compared to the mind-reading strategy that they favor. As our own analysis of the five studies aboveillustrate, no one behavioral rule accounts for all the data. In some of the studies, thebehavioral rule invoked to explain animal performance is hypothesized to be basedon associative learning, while in other studies it is hypothesized to be based onembodied simulation or (non-mentalistic) teleological reasoning. Of course, since thecomplementary behavior-reading explanations in each case were modeled on theircompeting mindreading explanations, the same point can be made for Call andTomasello’s mindreading strategy. There is no one mindreading rule that accounts forall the data, either. As we saw above, in some studies, the mindreading rule invokedto explain animal performance is hypothesized to be based on some type ofassociative learning, while in other studies it is hypothesized to be based on a type ofmental simulation or a psychologically enriched version of teleological reasoning.So both strategies (behavior-reading and mindreading) are required to use differentrules of inference and prediction in order to explain the results of the differentstudies. Nevertheless, it may be said, as Call and Tomasello (2008) suggest, that atleast the data from the various studies can be given a unified explanation under thegeneral mindreading hypothesis that primates understand something about theinternal goals underlying others’ actions. But, of course, the complementarybehavior-reading strategy can offer a similar unifying hypothesis: primatesunderstand something about the external goals of others’ actions.

Alternatively, one might argue that the point raised by Call and Tomasello is thatthe alternative behavior-reading explanations discussed above appeal to verydifferent kinds of cognitive mechanisms (e.g., associative learning, embodiedsimulation, non-mentalistic teleological reasoning); whereas Call and Tomasellocan explain the data in terms of the very same kind of cognitive mechanism:mindreading. The problem with this argument is that mindreading is no more anunderlying cognitive mechanism than is complementary behavior-reading. Likecomplementary behavior-reading, mindreading is a general ability or competency forpredicting other agents’ behaviors when provided with certain kinds of behavioral/environmental cues. Animals may manifest this ability through the use of differentcognitive mechanisms or processes, such as associative learning, mental simulation,or mentalistic teleological reasoning. Thus, what the data from the above studiesindicate, if one interprets them as showing that primates have the general ability topredict other agents’ behavior by means of attributing internal goals, is that thisability is realized by different cognitive mechanisms in different situations and bydifferent kinds of primates. Similarly, what the data indicate if one interprets them asshowing that primates have the general ability to predict other agents’ behavior byattributing external goals, is again that this ability in primates is realized by differentcognitive mechanisms in different situations and by different kinds of primates. Theonly difference between these two views is that the various cognitive mechanisms


understood to be employed in the manifestation of mindreading in primates alloperate on representations of mental states as well as on representations of thevarious behavioral/environmental cues used to apply these mental states to otheragents; whereas, the mechanisms understood to be employed in the manifestation ofcomplementary behavior-reading in primates operate only on the latter kinds ofrepresentations. The difference is not that the studies show that the ability tomindread in primates can be understood to be realized by a single underlyingcognitive mechanism while the ability to engage in complementary behavior-readingmust be understood to be realized by an assortment of very different cognitivemechanisms.

There is one further objection to our project that needs to be considered.According to this objection, one must look at the body of evidence for internal-goalattribution (and perhaps other types of mental state attribution) in primates ratherthan just at individual studies, as we have done above. For in both claims abouthuman theory of mind and primate theory of mind, it is the convergence of a body ofevidence—both experimental and observational—that has led some researchers toconclude that primates (in particular, chimpanzees) understand something aboutothers’ mental states such as internal goals.

Many researchers would certainly agree that it is important to consider the body ofevidence in favor of primate mindreading, and that the body of evidence may indeedcome to converge on the conclusion that primates understand something about mentalstates in others. However, a number of prominent researchers who have examined thisbody of evidence (Heyes 1994, 1998; Macphail 1998; Penn et al. 2008; Penn andPovinelli 2007; Shettleworth 1998, 2010) have argued that there is no compellingreason to think that it does converge on the hypothesis that primates are mindreadersrather than on the alternative hypothesis that primates are complementary behavior-readers. To be genuinely confident that the body of evidence converges on the formerrather than the latter hypothesis, it is argued, would require that one “show not merelythat [the theory of mind hypothesis] can be applied to diverse phenomena but that foreach of a range of phenomena it provides a better explanation than alternative,nonmentalistic hypotheses” [emphasis added] (Heyes 1998, p. 111). Yet criticalexamination of the most compelling theory-of-mind studies (both experimental andobservational) in primates has failed to show that the data are better explained by amindreading hypothesis than by an alternative nonmentalistic hypothesis. What areneeded to move the primate theory-of-mind debate forward, the researchers argue, areexperimental protocols that could provide such data. Until then, they hold, there issimply no more reason to think that the body of evidence converges on the hypothesisthat primates are mindreaders than on the hypothesis that they are complementarybehavior-readers. Of course, some researchers may be presently convinced that thebody of evidence does converge on the former rather than the latter hypothesis, but anumber of researchers, as noted, are not. And it is to these skeptical researchers thatwe are appealing in providing an experimental protocol to test for internal-goalattribution in chimpanzees that overcomes Povinelli’s problem.

Thus, we do not find Call and Tomasello’s parity-of-reasoning or parsimonyarguments, or the argument from the convergence of the body of evidence, strongenough to overcome the motivation and importance to solve Povinelli’s problemdirectly by designing more discriminating tests.


6 The Appearance-Reality Mindreading Theory

In this section and the next, we sketch an experimental protocol and its theoreticalrationale that show how researchers can acquire data that would provide compellingevidence that primates—chimpanzees specifically—represent others’ internal goals. Theexperimental protocol that will be outlined below is based on an appearance-realitymindreading (ARM) theory, earlier versions of which can be found in Humphrey (1976)and Gallup (1982). More recent applications and defenses of the theory can be found inKrachun (2008), Krachun et al. (2010), and Lurz (2011a, b). According to the ARMtheory, mental state attribution, such as attributions of internal goals, evolved in theanimal kingdom for the purpose of predicting the behavior of other agents(conspecifics, predators, or prey) in environmental settings in which the animal’scompeting behavior-reading counterparts could not. In many environmental settings,the way distal objects (e.g., food, partially occluded objects, other animals) perceptuallyappear (e.g., look, sound, smell) to an agent is a better predictor of how the agent islikely to act toward the objects than the way the objects objectively are. Behavior-reading animals can appeal only to the latter sorts of reality-based, mind-independentfacts, such as facts about agents’ past behavior or their current line of gaze to objects inthe environment. Mindreading animals, in contrast, can appeal to the subjective waysenvironmental objects perceptually appear to agents to predict their behavior.

In illusory settings where distal objects appear differently from what they reallyare, the ability to predict another agent’s behavior on the basis of how objects look orsound to the agent (contrary to the way the object might really be) has the potentialto contribute to the animal’s overall level of fitness. Understanding that a dominantconspecific, for instance, fails to see the camouflaged insect or distant piece of fruitin its line of gaze as an insect/piece of fruit (seeing it instead as perhaps a leaf/darkspot on the forest floor) would enable a subordinate mindreading animal to predictthat the dominant is unlikely to try to eat the insect/fruit. Such understanding couldprovide the subordinate animal with the valuable opportunity to eat the insect/fruitlater in private, free of any agonistic encounter with the dominant. Were a behavior-reading subordinate placed in the very same situation as this mindreading one itwould not be capable of availing itself of this opportunity, however. For such asubordinate would understand only the reality-based fact that the dominant is gazingdirectly at an edible insect/piece of fruit. Knowing this fact about the dominant andthe environment would not enable the subordinate to predict that the dominant isunlikely to try to eat the insect/fruit. If anything, such a fact would likely lead thesubordinate to wrongly predict that the dominant is likely to try to eat the insect/fruit(since quite probably that is how dominants typically behave when they are lookingdirectly at edible insects/fruit). This, in turn, would likely discourage the subordinatefrom attempting to eat the insect/fruit now or to make plans to eat it later. Althoughsituations like this are not likely to occur frequently in primate natural habitats, theymay well occur often enough, and provide enough of a fitness benefit to themindreader, to exert a selection pressure in favor of mindreading over behavior-reading. Obviously, more detailed ecological observations of chimpanzee (and otherprimate) behavior in illusory environmental settings are needed. However, there is noa priori reason to think that such situations would not provide enough selectionpressure to favor mindreading over behavior reading.


Thus, the ARM theory hypothesizes that internal-goal attribution in chimpanzees (andperhaps other primates), in so far as it exists, may have evolved as a result of chimpanzeescoming to introspect their own ability to distinguish the way environmental objectsperceptually appear to them from the way they know (believe) them to really be, andusing this introspective knowledge of perceptual appearances in illusory (as well as non-illusory) settings to predict other agents’ behavior. There is currently no direct behavioralevidence supporting the ARM theory, other than the fact that chimpanzees, of all speciesof nonhuman animal that have been studied, have shown the strongest (though equivocal)evidence for mindreading and have also shown themselves capable of discriminatingperceptual appearance from reality (Krachun et al. 2009a). The ARM theory predicts theco-occurrence of these two cognitive abilities in chimpanzees; the complementarybehavior-reading theory does not. What is needed, and what we aim to provide in thefinal section, is an empirically more direct way to test the ARM theory—a way thatovercomes Povinelli’s problem.

Before turning to the experimental protocol designed to test the ARM theory, we wishto make two further points that will help in clarifying the relevance of the experimentalprotocol in relation to the question of whether chimpanzees attribute internal goals. Thefirst has to do with a particular type of simple internal goal. In many animals, the simplestforms of internal goals are action-guiding perceptual states. Certain types of perceptualstates in animals—perceptions of predators, mates, food, species-specific warning andmating calls, types of colors, odors, and sounds, for example—feed rather directly intothe production of predictable types of behaviors in the animals. Amale stickleback, to useone well-known example, will quite predictably attack another male upon seeing its redunderbelly. Perceptions of red bellies in sticklebacks are, according to Millikan (2004),“perceptions on the one hand and directives on the other” (pp. 158–159). Millikan aptlycalls such simple forms of perception that act as direct guides to behavior “pushmi-pullyu” mental representations. Pushmi-pullyu perceptual states are, thus, simple formsof internal goals. And so if animals, such as chimpanzees, demonstrate that they canpredict the external goals of other creatures by attributing such simple behavior-guidingperceptual states to them, then this, we believe, counts as compelling evidence thatthese animals can attribute a type of internal goal to others.13

13 Those familiar with research on perceptual-state attribution in primates might be inclined to object here,pointing out that chimpanzees have already demonstrated that they can attribute such internal goals toother agents. A number of researchers, for example, interpret studies by Hare et al. (2000, 2001) asproviding some of the strongest evidence that chimpanzees can predict dominant conspecifics’ feedingbehaviors on the basis of whether they think the dominant sees the food in the environment or saw wherethe food was last hidden. However, Heyes (1994, 1998), Lurz (2009), and Povinelli and Vonk (2006) havepersuasively argued that these studies, as well as other perceptual-state attribution studies in primates andother animals, are unable to determine whether chimpanzees predict other agents’ behavior on the basis ofwhat they think the agent sees or cannot see, or simply on the basis of what they think the agent has ordoes not have an unobstructed line of gaze to (either currently or in the recent past). Having a line of gazeto a distal object is not a psychological state, and it is not the psychological state of seeing the object (e.g.,blind people can and often do have unobstructed lines of gaze to objects in their environment; they justdon’t see the objects). However, line of gaze is one of the principal observable cues on which an animalmight reasonably infer that another subject sees an object. The main objective of this paper is to show howresearchers can overcome this methodological problem by running a type of test in which an animal canpredict an agent’s external goal by attributing a perceptual state (internal goal) to the agent but not byrepresenting facts about what the agent has (or lacks) a line of gaze to.


It is also important to mention that some researchers and philosophers have beenquite skeptical about the very idea of animals attributing mental states, even simpleinternal goals. In many cases, the skepticism is grounded on an assumption thatattributing mental states necessarily involves attributing states that the attributorunderstands as being hidden inside the other agent’s body, as being the cause of theagent’s outward behavior, and as having representational (i.e., truth-evaluable)contents. But as these researchers and philosophers argue, animals do not appearcapable of representing hidden internal causes of external events or of grasping thenormative ideas of truth and veridicality that are needed to understand the idea ofmental representations (for such arguments, see Bermúdez 2009; Carruthers 1998;Davidson 2001; Penn et al. 2008).

We do not find this line of skepticism compelling, and one of us has arguedagainst it at length elsewhere (Lurz 2011a, b). Here we simply wish to expressour rejection of this line of skepticism by noting that we do not think it at allnecessary that animals, in representing others’ internal goals, must represent themas spatially internal, causally efficacious, representational states.14 If an animal, A,successfully predicts what another creature, B, intends to do (e.g., eat a particulargrape among several) by understanding how some aspect of the environmentperceptually appears to B (e.g., by understanding that that particular grape lookslarge to B), then A’s attribution of this perceptual state to B is an attribution of aninternal goal, a genuine psychological state. However, to do this A need notrepresent B’s perceptual state as something that is spatially inside B’s body, or assomething that literally causes B to eat the grape, or even as something that has atruth-evaluable representational content. A need only represent B’s perceptual stateas a triadic relation (e.g., the relation <object> looks <property> to <subject>)holding between a distal object (a grape), a property (large), and a subject (B).15 Aneed not understand that this triadic state of affairs is something that is realized(partially or entirely) by events occurring inside B’s body or brain (though, it likelyis). Nor does A need to understand that this triadic state of affairs is the cause (orpartial cause) of B’s eating the large grape (though, it may be). A may simplyknow, from past occurrences, that B (or others of B’s type) will typically eat thegrape that looks large to him (or them). And neither does A need to understand thatthe representational content of B’s perceptual state—the content that thatparticular grape is large—is something that is made true or false by the actualstate of affairs obtaining in B’s environment (e.g., by the fact that there is/isn’t a

14 This is not to say that animals do not have spatially internal, causally efficacious, representational statesby virtue of which they think, reason, perceive, and attribute mental states to others. What we are claimingis that, in their ability to attribute mental states (even discrepant or ‘false’ mental states), animals do notneed to be understood to represent mental states in such ways. Representing mental states in such termsmay be the way that we (philosophers, scientific researchers, and perhaps most people) represent them, butit should not be assumed that it is the way that animals represent them.15 An interesting feature of the triadic-relation model of perceptual-state attributions is that some animals—most notably apes (see Tomasello and Call 1997)—have been shown to understand other types of triadicrelations, both in the social domain (e.g., understanding the relation of their being between individuals x and yin dominance rank) and in the domain of tool use (e.g., understanding the relation between a demonstrator, atool, and the object the tool is used to manipulate).


large grape in front of B).16 A need have no grasp of the notions of truth or falsity,veridicality or unveridicality, or representation in order to represent that a particulargrape looks large to B.

It is important to note, however, that even though A may not represent B as beingin a representational (truth-evaluable) state, this does not mean that A cannotattribute a discrepant or ‘false’ perception (as it is frequently called). In representingthat a grape looks large to B, A is not required to believe that the grape is large, oreven to see the grape as being large at the time of attribution. A, for example, mayhave reason to believe that the grape is not actually large (e.g., A may have seen thesmall grape placed behind a magnifying lens), and A may currently not see the grapeas being large (e.g., A may be currently looking at B and not at the grape). Yet at thesame time, A may have reason to represent that the grape looks large to B (e.g., Amay observe B staring directly at the grape behind the distorting lens but know thatB did not witness the grape being placed behind the lens). Therefore, A may be in aposition to attribute a perceptual state to B even though A is not currently undergoingthat type of perceptual state itself, and even though A is not currently having the type ofbelief (that the grape is large) that B is likely undergoing in having that type ofperceptual state. Thus, A can attribute a discrepant (‘false’) perceptual state withoutrepresenting B as being in a representational (i.e., truth-evaluable) mental state.

7 Experimental Protocol for Internal-Goal Attribution in Chimpanzees

Krachun et al. (2009a) recently ran an appearance-reality discrimination test withchimpanzees using magnifying and minimizing lenses. The researchers first confirmedthat chimpanzees would naturally choose (by pointing to) the larger of two grapespresented to them. Then, in the initial demo trial of the experiment, chimpanzees werepresented with a small grape displayed behind a magnifying lens (making it appearlarge) and a large grape displayed behind a minimizing lens (making it appear small).The animals consistently pointed to the magnifying lens over the minimizing lenswhen requesting a grape, demonstrating that they were fooled by the distorted size ofthe grapes behind the lenses. In subsequent test trials, chimpanzees were allowed towitness the change in the apparent size of the grapes as the experimenter placed thembehind the lenses. More than half the chimpanzees chose the minimizing lens (i.e., theapparently smaller but truly larger grape) in these trials. The results suggest, as theresearchers maintain, that some chimpanzees are capable of distinguishing between theapparent size of an object (a grape) and its real size.17 The question is whetherchimpanzees can use this understanding of visual appearances to predict the behaviorof others, as understood on the ARM theory of animal mindreading.

16 On representational theories of perception, the truth-evaluable contents of perceptual states of the form‘x looks F to S’ are often identified with propositions (e.g., the proposition that x is F; see Armstrong1968; Thau 2002) or, on some accounts, with non-propositional, truth-evaluable entities such as positionedscenarios (e.g., the set of different ways of filling out the space before S’s body that are consistent with thefact that x looks F to S; see Peacocke 1992).17 A follow-up test was also run to rule out the possibility that chimpanzees were solving the task bysimply keeping track of which container they saw the large grape being placed into. Five chimpanzees(more than a third of the original sample) also passed this test.


As a way to test for this possibility, we propose the following experimentalprotocol modeled on Krachun et al.’s (2009b) competitive paradigm study withchimpanzees and Southgate et al.’s (2007) anticipatory-looking paradigm study withyoung children. The objective here is to test whether chimpanzees are capable ofanticipating (as evidenced by their looking behavior) the actions of a humancompetitor by understanding how an illusory stimulus (grape) looks to the human.The general setup is similar to Krachun et al.’s competitive paradigm (2009b). Achimpanzee and a human competitor are in adjoining rooms divided by a clearPlexiglas wall (see Fig. 1). The wall contains two small holes, spaced a meter apart,through which the chimpanzee can stick a finger to indicate its choice of container.Flush against the wall, on the competitor’s side, is a table with a sliding platform ontop. Two transparent (and non-distorting) glass containers with opaque lids areplaced a meter apart on the platform, in alignment with the finger holes. On the sideof the table nearest the competitor is an opaque backdrop containing two curtainedwindows, also a meter apart and aligned with the containers. The windows allow thecompetitor to stick an arm through the backdrop to indicate his choice of container.The backdrop prevents the chimpanzee from seeing the competitor’s torso and armsduring testing while allowing a clear view of his face and direction of gaze. Anexperimenter (not shown in Fig. 1) is also present in the room with the competitor,seated at the table at a 90° angle to both contestants. The experimenter’s role is toplace the grapes inside the containers and to slide the containers within reach of thecontestants.

Fig. 1 General setup for experimental design


Pretest Trials The study begins with a pretest stage, which has three purposes: (1) tofamiliarize chimpanzees with the general testing procedure; (2) to allow chimpan-zees to learn that the competitor will always prefer a large grape over a small onewhen choosing first; and (3) to make chimpanzees aware of the fact that thecompetitor will not always be successful in obtaining the large grape. (As we willsee below, this last point is important for ensuring that the chimpanzee is motivatedto attend to the competitor’s actions in the subsequent test trials.)

In the pretest stage, non-distorting glass containers are used, and the chimpanzeeand human competitor take turns choosing between these containers. The procedureis as follows: With the sliding platform in the middle of the table and the competitorabsent from the room, the experimenter opens the lid on each container and places alarge grape inside one container and a small grape inside the other while thechimpanzee observes. (The placement of the large and small grape is alternated fromtrial to trial in a pseudo-random order). The experimenter then closes the lids toprevent the competitor from peering directly into the containers from above whenlooking at the grapes. After the containers are baited, the competitor enters the roomand sits at the table. The chimpanzee observes the competitor enter and can clearlysee his head over the top of the backdrop. The competitor then conspicuously looksat each grape through the sides of the glass containers before staring straightahead.18 At this point, one of two distinctive bells is rung. One type of bell(competitor bell) indicates that the experimenter is now about to slide the platformover to the competitor (competitor-first trials); the other type of bell (chimpanzeebell) indicates that the experimenter is about to slide the platform over to thechimpanzee (chimpanzee-first trials). (Competitor-first and chimpanzee-first trialsare alternated in a pseudo-random order). Besides indicating which contestant willchoose first on a given trial, the bell also serves to cue coders when they shouldbegin coding for anticipatory looking in the chimpanzee. After the bell is rung, theplatform is slid before the appropriate contestant, and the contestant is allowed tomake a selection of one of the grapes in the containers.

Because our dependent measure is anticipatory looking, it is crucial thatchimpanzees be motivated to attend to the competitor’s actions in the upcoming testtrials. We create this motivation at the pretest stage in two ways: First, on competitor-first trials, the competitor will actually obtain the larger, preferred grape only half thetime (success trials). The other half of the time (failure trials), the competitor will beunsuccessful in obtaining either grape, so that both containers will remain baited when itis the chimpanzee’s turn to choose. Second, once the competitor has made his choice—and either succeeded or failed to obtain the large grape—the experimenter will placeopaque coverings completely over the containers before sliding them towards thechimpanzee. Thus, when it is choosing second, the chimpanzee can only know which isthe best container to choose by observing the competitor’s choice and noting whether hewas successful in obtaining the large grape.

The competitor-first trials thus proceed as follows: On half the trails (successtrials), the competitor extends an arm through the curtained window in front of thecontainer with the large grape, slides his hand under the opaque lid, extracts the large

18 This is important to note since in the test trials, it will be critical that the competitor cannot see thegrapes directly but only by looking at them through the sides of the glass containers.


grape (allowing the lid to fall back into place), retracts his arm back through thewindow with grape in hand, and conspicuously eats the grape—all in full view of theobserving chimpanzee. On the other half of trials (failure trials), the competitorextends an arm through the curtained window in front of the container with the largegrape but fails to retrieve the grape. On these trials, the competitor stops short ofreaching the large grape in the container and behaves as if his arm were stuck in thewindow. After a few short thrusts of his arm (in an apparent attempt to free it), thecompetitor retracts his arm through the window, leaving the large grape in thecontainer. (The order of success trials and failure trials are alternated in a pseudo-random order.) After the competitor has acted in one of these two ways toward thecontainer with the large grape, the experimenter places opaque coverings completelyover each container and moves the platform toward the chimpanzee. Once theplatform is in place before the chimpanzee, the chimpanzee is allowed to choose oneof the two containers (by pointing to it through a finger hole). Since the containersare covered, the chimpanzee cannot make its choice by seeing the grape or grapesinside the containers. Rather, to make its preferred choice (which we assume will bethe remaining small grape in competitor-first success trials and the large grape incompetitor-first failure trials) the chimpanzee must attend to the selection process ofthe competitor. Thus, once again, the purpose of using the opaque coverings in thecompetitor-first trials is to force the chimpanzee to attend to the competitor’s selectionbehavior in order to make a successful selection when it is allowed to choose. As notedabove, making sure that the chimpanzee is attentive to the competitor’s selectionbehavior in competitor-first trials will be important for securing anticipatory-lookingresponses from chimpanzees during the testing phase of the experiment.

On chimpanzee-first trials, the chimpanzee bell is rung immediately after thecontainers are baited, the platform is slid before the chimpanzee, and the chimpanzeeis allowed to indicate the grape of choice (presumably the large grape) by pointing toit through the aligned finger hole. The experimenter then removes the selected grapefrom its container and gives it to the chimpanzee to eat. The platform is then slid intoplace before the competitor, and the competitor is allowed to choose. The competitorsuccessfully retrieves and eats the remaining gape. The opaque coverings are notused in the chimpanzee-first trials.

This procedure is repeated a number of times, and it is expected that after playingthe game for a while, the chimpanzee will come to anticipate the following sequenceof events in competitor-first trials: (1) The competitor bell rings, (2) the platform isslid in front of the competitor, and (3) the competitor extends an arm through thecurtained window in front of the container with the large grape (and in some but notall trials succeeds in retrieving the large grape). Researchers can determine whetherthe chimpanzee at this stage has such an expectation by recording which window(and perhaps also which container) it looks to first upon hearing the competitor bellring, as well as which window/container it looks to most often in the intervalbetween the bell ringing and the competitor extending an arm through a window.(Video recordings of which window/container the chimpanzee looks at can be madeby having small recording cameras mounted above each window).

Test Trials The test trials are exactly like the pretest trials except that on some ofthese test trials (size-distorting trials), the non-distorting glass containers are replaced


with identical looking size-distorting glass containers with opaque lids. One of thesenew containers magnifies the size of objects placed inside, and the other minimizestheir size. Since the competitor is absent when the grapes are placed inside thecontainers (as in the pretest stage), he does not observe the change in apparent sizeof the grapes as they are placed inside the containers, as the chimpanzee does. Afterthe grapes are placed, the competitor enters the room, sits at the table, looks at eachgrape through the sides of the containers, stares straight ahead, and the trial proceedsas in the pretest. Thus on competitor-first test trials, the competitor bell is rung, theplatform is slid in front of the competitor, the competitor succeeds or fails to retrievea grape, the platform is slid in front of the chimpanzee, and the chimpanzee isallowed to select a container. And on chimpanzee-first test trials, the containers arebaited, the chimpanzee bell is rung, the platform is slid before the chimpanzee, andthe chimpanzee is allowed to select a container.

Predictions The question is whether, on competitor-first size-distorting test trials, thechimpanzee, upon hearing the ringing of the competitor bell, anticipates (a) thecompetitor reaching an arm through the curtained window before the magnifyingcontainer (which contains the small grape) or (b) the competitor reaching an armthrough the curtained window in front of the minimizing container (which containsthe large grape). If the chimpanzee expects (a), then it should look first to (or most oftenat) the window in front of the magnifying container upon hearing the competitorbell; if the chimpanzee expects (b) then, it should look first to (or most often at)the window in front of the minimizing container upon hearing the competitor bell.

If chimpanzees predict others’ actions by attributing states of perceptual-appearing,as understood on the ARM theory, then chimpanzees participating in the aboveexperiment should come to understand, as a result of their experience during the pretesttrials, that their competitor is disposed to reach for the grape that looks to him to be thelarge grape. If chimpanzees also understand (as in Krachun et al. 2009a) that in thesize-distorting test trials, the grape inside the magnifying container looks large but istruly small, and if they can use this knowledge to understand that the small grapelooks large to the naive competitor, then they would be expected to anticipate situation(a), above, in the competitor-first size-distorting trials.19 That is, they should anticipatethe competitor’s arm reaching through the curtained window in front of themagnifying container. If chimpanzees do in fact predict the competitor’s behavior bysuch perceptual-appearing attribution, then, on Millikan’s pushmi-pullyu theory ofaction-guiding perceptual states, we can interpret the chimpanzees’ perceptual-appearing attribution here as a type of internal-goal attribution (i.e., as the attributionof having the goal to reach for/eat the large-looking grape).

The question is whether a complementary behavior-reading hypothesis couldreasonably give the same prediction. The answer, we submit, is no. For what theperceptual-appearing/internal-goal attribution hypothesis above predicts (in accor-dance with the ARM theory) is not just that the chimpanzees will anticipate

19 The results to examine will be those from the first few competitor-first size-distorting trials.Chimpanzees’ responses in these trials will give the clearest indication of whether they show thepredicted anticipation of the competitor’s reaching. Positive results from later trials may reveal only thatchimpanzees have learned to anticipate such selection behavior from the competitor through theirexperience in previous trials.


occurrence (a) on competitor-first size-distorting trials, but that they will (c) provideevidence of their ability to discriminate appearance from reality in chimpanzee-firstsize-distorting trials by selecting the minimizing container (large grape) over themagnifying container (small grape). In other words, the chimpanzees should choosethe minimizing container when choosing first but should expect the competitor tochoose the magnifying container when he is choosing first (we call this the min-magresponse pattern).

In contrast, a complementary behavior-reading hypothesis would predict either oneof two patterns of responding: 1) Chimpanzees will anticipate (a) in competitor-first sizedistorting trials but will not perform as (c) states. That is, they will expect theircompetitor to choose the magnified grape when he chooses first, but they will alsochoose the magnified grape themselves when choosing first (a mag-mag responsepattern). 2) Chimpanzees will perform as (c) states but anticipate (b) rather than (a) oncompetitor-first size-distorting trials. That is, they will choose the minimized grapethemselves when they choose first, but they will also expect their competitor to choosethe minimized grape when he chooses first (a min-min response pattern).

This is so because all complementary behavior-reading hypotheses take animals’predictions of agents’ behavior to be reality-based. As a result, a complementarybehavior-reading hypothesis in this case would predict that chimpanzees wouldcome to understand, based on their experiences in the pretest stage, that the competitoris disposed to reach for the grape that is in fact the largest. With this reality-basedunderstanding of the competitor’s behavioral disposition, the complementary behavior-reading hypothesis would predict that chimpanzees, in the competitor-first size-distorting trials, should expect the competitor to reach for the container that, accordingto the chimpanzees’ understanding of what is real, has the truly larger grape in it (themin-min pattern, assuming that the chimpanzee distinguishes appearance from realityin such case). Thus, the only way that chimpanzees on this hypothesis would beexpected to anticipate situation (a) in competitor-first size-distorting trials, in which thecompetitor reaches an arm through the curtained window in front of the magnifyingcontainer, would be if chimpanzees (mistakenly) understood the grape in themagnifying container to be a large grape. But that would mean that chimpanzeeswere not capable of distinguishing appearance from reality in this case. As a result,they would mistakenly take the grape in the magnifying container to be the largergrape, and would therefore be expected to select the magnifying container in thosesize-distorting trials when they themselves were choosing first (the mag-mag pattern).

In short, on the complementary behavior-reading hypothesis, if chimpanzeesunderstand the difference between appearance and reality, they should show the min-min response pattern in our proposed protocol, which is not what the perceptual-appearing/internal-goal attribution hypothesis predicts. But if chimpanzees do notunderstand the difference between appearance and reality, they should show the mag-magresponse pattern, which is also not what the perceptual-appearing/internal-goal attributionhypothesis predicts. Either way, a complementary behavior-reading hypothesis cannotprovide the same predictions as the perceptual-appearing/internal-goal hypothesis. Whatthis means is that this approach has the power, in principle and in practice, to overcomePovinelli’s problem.

An added virtue of the protocol just described is that its design can be varied in anumber of ways while retaining the underlying logic that enabled it to overcome


Povinelli’s problem. This flexibility in its design will be of value to researchers whomight wish to run the study, since adjustments to the procedure may need to be made(e.g., in light of results from initial pilot tests). In the original protocol describedabove, chimpanzees’ ability to predict the competitor’s selection behavior ismeasured through anticipatory looking. In alternative protocols, other anticipatoryresponses could be used instead of (or in addition to) anticipatory looking. Forexample, researchers might use an experimental setup that requires chimpanzees tobe ‘in the right place at the right time’ in competitor-first trials; otherwise, they losetheir opportunity to receive any reward. This could be achieved with a few smallchanges to the original experimental setup and procedure, as described below.

First, rather than having an experimenter slide the containers within reach of thecompetitor so that he can make a choice, the containers would be mounted onto aspring-loaded, swivel mechanism that allowed the competitor himself to pull thecontainers within reach. Further, the mechanism would be designed such that whenthe competitor pulled a container within reach, the other container wouldautomatically move within reach of the chimpanzee, allowing the chimpanzee tochoose that container by sticking a finger through the hole in front of the container.Importantly, however, the container would only stay within reach of the chimpanzeefor a limited amount of time; once the competitor finished retrieving his chosengrape and released the container, both containers would swivel back to their originalpositions in the center of the platform. Of course, the setup would have to make itimpossible for the chimpanzee to quickly shift from one position to another in frontof the Plexiglas window. This could be achieved by erecting a clear Plexiglas dividerbetween the finger holes that chimpanzees needed to move around in order to reachone hole or the other.

Thus, in order to take advantage of their time-limited opportunity to choose theremaining grape in competitor-first trials, chimpanzees would have to move intoposition in front of the appropriate finger hole before the competitor made hisselection. Their anticipatory moving behavior would indicate which container theyexpected the competitor to choose (and note that in this protocol, the competitorwould always be successful in retrieving the chosen grape). As in the originalprotocol, chimpanzees’ predictions of the competitor’s choice of container in size-distorting versus non-distorting test trials would provide insights into theirunderstanding of the competitor’s subjective perception/internal goals. If chimpan-zees are mindreaders, according to the ARM theory, then they are expected to selectthe grape that really is large (i.e., the grape in the minimizing container) onchimpanzee-first size-distorting trials, but to predict that the naive competitor willselect the grape that merely looks large (i.e., the grape in the magnifying container)on competitor-first size-distorting trials—a min-mag response pattern. If, on theother hand, chimpanzees are complementary behavior-readers, then on size-distorting trials they are expected to produce either a min-min response pattern(showing the capacity to make the appropriate appearance-reality discrimination) ora mag-mag response pattern (showing the inability to make the appropriateappearance-reality discrimination). Either way, a complementary behavior-readinghypothesis cannot provide the same predictions as the perceptual-appearing/internal-goal hypothesis under consideration. Thus, this version of the experimental protocolalso has the power, in principle and in practice, to overcome Povinelli’s problem.


8 Conclusion

A persistent methodological problem in primate social cognition research has been howto determine experimentally whether primates represent the internal goals of otheragents or just the external goals of their actions. This is an instance of Daniel Povinelli’smore general challenge that no experimental protocol currently in use is capable ofdistinguishing genuine mindreading animals from their complementary behavior-reading counterparts. Current experimental approaches to test for internal-goalattribution in primates do not solve Povinelli’s problem, nor do these approaches solvethe problem when accompanied by parity-of-reasoning and parsimony arguments. Toovercome Povinelli’s problem as it pertains to testing internal-goal attribution inprimates, a new type of experimental approach is needed. The two versions of the size-distorting protocol presented here have the power to discriminate between chimpanzeesthat represent others’ internal goals and those that can (at most) represent the externalgoals of others’ actions. Both versions of the protocol are designed to utilize certaincompetencies of chimpanzees (e.g., the ability to distinguish appearance from realityand to make tactical choices in competitive situations) that these animals have exercisedseparately in different experimental settings. The question is whether chimpanzees willexercise these competencies together in a single competitive setting for the purpose ofpredicting a competitor’s action in terms of how a target object visually appears to thecompetitor. If chimpanzees can do this, then researchers will have finally found whatthey have been looking for: a genuinely compelling reason for saying that chimpanzeesunderstand something about the internal goals of other agents. Povinelli’s problem has asolution, the rest is now up to researchers and, of course, chimpanzees.

Acknowledgements We wish to thank the anonymous reviewers for their insightful and helpfulcomments on an earlier draft of this paper, and to thank Kristy Lee for drawing Fig. 1.

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