Rev.Phil.Psych. (2011) 2:449–481
DOI 10.1007/s13164-011-0068-x
How Could We Know Whether Nonhuman Primates
Understand 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 has
been how to determine experimentally whether primates represent the internal goals of
other agents or just the external goals of their actions. This is an instance of Daniel
Povinelli’s more general challenge that no experimental protocol currently used in the
field is capable of distinguishing genuine mindreading animals from their complementary behavior-reading counterparts. We argue that current methods used to test for
internal-goal attribution in primates do not solve Povinelli’s problem. To overcome the
problem, a new type of experimental approach is needed, one which is supported by an
alternative theoretical account of animal mindreading, called the appearance-reality
mindreading (ARM) theory. We provide an outline of the ARM theory and show how
it 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 the
power, in principle and in practice, to distinguish genuine mindreading chimpanzees
from those who predict others’ behavior solely on the basis of behavioral/
environmental cues. Our solution to Povinelli’s problem has important consequences
for a similar debate in developmental psychology over when preverbal infants should
be credited with the ability to attribute internal goals. If what we argue for here in the
case of nonhuman primates is sound, then the clearest tests for internal-goal attribution
in 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 psychology
and philosophy over whether nonhuman animals are capable of attributing mental states
R. W. Lurz (*)
Department of Philosophy, Brooklyn College - CUNY, Brooklyn, NY, USA
e-mail: [email protected]
C. Krachun
Department of Psychology, Grenfell Campus Memorial University of Newfoundland,
Corner Brook, Canada
e-mail: [email protected]
450
R.W. Lurz, C. Krachun
to others (also known as mindreading or theory-of-mind).1 One strand of this debate
has focused on whether primates attribute the psychological state of intending or
having goals (i.e., internal goals, see below) to other creatures when predicting and
making sense of their behaviors. Until about a decade ago, the general opinion in the
field 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 to
move and do things spontaneously and as ‘directed’ toward external objects and events
in the sense that they have learned certain antecedent–consequent sequences of
behavior … [but this] does not, however, necessarily make for a theory of mind, or
even a theory of intentions” (pp. 384–386). In order to be considered as having a
theory of mind or intentions, the researchers argued, primates must demonstrate an
ability to attribute states that mediate between the antecedent–consequent sequences of
behavior in others. And at the time, Tomasello and Call concluded, there was simply
no unequivocal evidence that primates did in fact attribute such mediating states when
predicting or making sense of the behavior of other creatures.
In recent years, Tomasello and Call (see Tomasello et al. 2005), as well as a
number of other prominent researchers in the field, have reversed their opinion on
this issue. They now hold that a new set of experimental data “compels” them to say
that primates (in particular, chimpanzees) understand and predict others’ behavior,
not simply by understanding the sorts of behavioral/environmental cues that can be
used to reliably predict others’ future actions, “but also in terms of the underlying
goals, and possibly intentions” that, in others’ minds, mediate between such
antecedent cues and consequent actions (Call and Tomasello 2008, p. 189). A
growing consensus among animal researchers, based on this new set of data, is that
primates do in fact attribute psychological states of intending and having goals when
they predict and make sense of other agents’ behavior.
Throughout this shift in opinion, Daniel Povinelli has consistently argued that
no experimental protocol currently in use is capable of distinguishing genuine
mindreading animals from their complementary behavior-reading counterparts.
As a result, none of the positive data produced from such protocols could ever provide
compelling reasons for saying that animals engage in mindreading rather than some
complementary 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 and
others now take as providing compelling grounds for thinking that primates attribute
the psychological states of intending and having action goals.2 While this research has
significantly advanced the field in many ways, in every study it is unfortunately
impossible to definitively rule out an equally plausible complementary behaviorreading explanation. However, we do believe that there is a way to move forward on
this question by designing more sensitive tests that do have the capacity to solve
Povinelli’s problem. What is needed is a new type of experimental protocol in the
field, 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 animals
and primates.
2
In some writings (Hurley and Nudds 2006; Lurz 2009, 2011a, b), Povinelli’s problem is referred to as the
logical problem.
Solving Povinelli’s Problem
451
mindreading that we call the appearance-reality mindreading (ARM) theory. In this
paper, we outline the ARM theory of animal mindreading and show how it can be
used to design a novel test for internal-goal attribution in chimpanzees. Unlike
protocols currently in use, the experimental design presented here has the power, in
principle and in practice, to overcome Povinelli’s problem.
Our solution to this problem has important consequences for a similar debate in
developmental psychology over when preverbal infants should be credited with the
ability to attribute internal goals. A variety of experimental protocols have been used
in 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 Csibra
2003; Perner and Doherty 2005; Povinelli 2001; Povinelli et al. 2005; Sirois and
Jackson 2007) have argued that these various protocols are incapable of determining
whether infants predict/understand other agents’ behavior by interpreting certain
behavioral/environmental cues as indicating underlying goals/intentions in agents
(the mindreading hypothesis) or whether they do this without interpreting these cues
as such (the complementary behavior-reading hypothesis). The issue is just Povinelli’s
problem as it applies to the developmental research. What we need to move the
developmental field beyond this impasse, some have argued (e.g., Perner and Doherty
2005), are nonverbal tests for goal attribution in infants that can distinguish between
these two hypotheses. Perner and Doherty, however, despair of there ever being such
tests. For the clearest way to distinguish between the two hypotheses would be a test
in 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/environmental
cues, but in a different way (Y) if they do not deploy a mentalistic interpretation of
these cues. But if the cues alone (i.e., without the mentalistic interpretation) are
expected to lead infants to predict/understand the agent doing Y, then in order to
predict/understand the agent doing X instead, infants would have to understand the
agent’s internal goal state as somehow representing the cues differently from what they
really are (otherwise, why would infants be expected to predict/understand behavior in
the agent that was different from what the cues alone would lead them to predict/
understand?). This, however, would require our taking infants to understand internal
goals as states capable of ‘misrepresentation,’ similar to that observed in beliefs and
perceptions. And this, Perner and Doherty claim, is highly implausible since “goal
representations do not depend on manipulable ingoing information, and goals cannot
be misrepresented” (p. 711). Thus, on such a view of the nature of internal goals, there
would appear to be little chance of solving Povinelli’s problem as it applies to the
research on goal attribution in preverbal infants. However, if what we argue for here in
the case of nonhuman primates is sound (especially that part of our argument in which
we show that there is a theoretically plausible account of internal goals that takes them
to be perceptual states), then there is a plausible way to address Perner and Doherty’s
worry, and thus a way toward solving Povinelli’s problem as it applies to the
developmental research in question.
We begin our examination of Povinelli’s problem in the primate research by first
clarifying the distinction between internal and external goals and then explaining
how the problem applies to current studies aimed at testing primates’ attribution of
internal goals.
452
R.W. Lurz, C. Krachun
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, such
as when we say that an agent has a goal in mind or intends to do something. Goals
qua conative states are intentional states of mind; they are behavior-guiding states of
an agent directed at objects or states of affairs in the world that need not exist and
may never come to exist despite the agent’s every effort to realize them. We follow
Tomasello et al. (2005) in labeling this psychological understanding of ‘goal’ an
internal goal. In another sense, ‘goal’ is used merely to represent the external object
or state of affairs that an agent’s action is understood to be directed at or expected to
bring about, such as when we say that grasping an object was the goal of the agent’s
reaching.3 Following Tomasello et al. (2005), we label this non-psychological sense
of ‘goal’ an external goal.
To illustrate this ambiguity, imagine that we are playing a game. Given my low
score and past poor performance, you predict that I am going to lose. Unbeknownst
to 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 the
game, for you do not think that I intend to do this or have this as my goal. So in
predicting 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, and
losing 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) of
my current behavior. The distinction here is the difference between merely
representing what an agent intends or has a goal to do (which in many cases is
just representing the expected outcome of the agent’s action or actions, such as
losing the game) and representing an agent as intending or as having a goal to do
something (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 of
representations are called ‘the agent’s goal,’ but only the latter sort counts as a type
of mental state attribution.
Attributions of external goals, or goal-directed (transitive) actions, as with many
such teleological interpretations of events, do not in every case involve or require
interpreting agents as having conative states of mind (though in some cases they
may). The trajectories of baseballs and the attraction of iron filings to magnets are all
events that we naturally take to be directed at certain future states of affairs, but we
do not take these events to be the result of the baseball or the iron filings having
goals or intentions. And in the natural world, we quite often interpret behaviors of
animals and humans in teleological terms without thereby attributing conative states
to them. We interpret a spider’s web building as having the external goal to catch
insects, and the waggle dance of honeybees as having the external goal to inform
nest mates of the location of nectar, for example. But such teleological
interpretations 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 or
having the goal to grasp the object, which are mental states.
Solving Povinelli’s Problem
453
spider or the bee as having in their mind the goal to catch insects or to inform nest
mates of the location of nectar.
Regarding other creatures’ external goals, there is little serious doubt that animals
can represent and track them. Many animals form reliable expectations about what
others are likely to do or are currently doing, and in many instances these
expectations 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. When
RL gets off the couch at noon and begins to move toward the kitchen (where the
cat’s bowl is located), the cat, upon seeing RL’s behavior at this time of day, reliably
scampers ahead in clear anticipation of being fed in the kitchen. Since feeding her in
the kitchen is what RL intends to do, the cat represents RL’s external goal. On this
there seems little to object. Nevertheless, it is rather questionable whether the cat
represents RL’s external goal to feed her by representing him as having an internal
goal (a psychological state of intending or having the goal) to feed her.
As this case is meant to illustrate, the controversy within animal social cognition
research is not whether animals can represent and track other creatures’ external
goals (RL’s cat shows rather intuitively that they can) but whether they ever do so by
attributing internal goals to other creatures. Thus, the distinction between the
external goal of an agent’s action in the teleological sense and the internal goal of the
agent in the psychological sense should be kept in mind when examining the studies
on primate goal and intention attribution. Even if primates do show an aptitude for
understanding other agents’ actions as directed at or tending toward objects and
states of affairs in the world, as we believe that some of the data suggest, this alone
would not show that these animals understand other agents as psychological agents
that have goals and intentions. They may well see other agents and their actions
much in the way that we see spiders and their web-building behaviors, and as RL’s
cat sees RL’s kitchen-directed behavior at noon: in teleological but not psychological
terms.
3 Povinelli’s Problem
The general problem for any animal mindreading hypothesis, according to Daniel
Povinelli (and his colleagues), is that since mental state attribution by animals must
be based on observable features of an agent’s behavior and environment—for the
minds of others are not open to direct inspection—every mindreading hypothesis has
a complementary behavior-reading hypothesis. Such a hypothesis proposes that the
animal in question uses the very same behavioral/environmental cues to predict the
agent’s behavior that, on the mindreading hypothesis, the animal is taken to use as its
inferential grounds for attributing a mental state. What is more, according to this
theory, complementary behavior-reading hypotheses for primate social behaviors
(especially, the great apes), are not idle, ad hoc competitors; they are hypotheses
entailed by an independently credible evolutionary theory of mindreading, called the
reinterpretation theory. On the reinterpretation theory, mindreading is a uniquely
human specialization that was “grafted into existing cognitive systems for reasoning
about 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
454
R.W. Lurz, C. Krachun
the ancestral system for representing behavioral abstractions but was integrated with
such systems” (Vonk and Povinelli 2006, p. 375). On the reinterpretation theory,
complementary behavior-reading explanations of great apes’ social behavior are all
but expected to be true.
The problem, according to Povinelli, is that no experimental protocol that has
been used to test a mindreading hypothesis with apes (or other animals) has been
able to control for an equally plausible complementary behavior-reading explanation
of 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 mindreading
research] necessarily presupposes that the subject notices, attends to, and/or
represents, precisely those observable aspects of the other agent that are being
experimentally manipulated. Once this is properly understood, however, it
must be conceded that the subject’s predictions about the other agent’s future
behaviour could be made either on the basis of a single step4 from knowledge
about the contingent relationships between the relevant invariant features of the
agent and the agent’s subsequent behaviour, or on the basis of multiple steps
from the invariant features, to the mental states, to the predicted behavior.
Without an analytical specification of what additional explanatory work the
extra cognitive step is doing in the latter case, there is nothing to implicate the
operation 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 the
hypothesis that an animal, A, predicts what an agent, S, will do (Saction) in a certain
setting by attributing to S a particular mental state (Sms). The hypothesis, for
example, might be that A will predict that S will eat the food in the middle of the
room and not behind the pillar because A knows (or believes) that S sees the food in
the middle of the room but not behind the pillar. Of course, A cannot know of S’s
mental 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 food
in the middle of the room but not behind the pillar because S has an unobstructed
line of gaze to the food in the middle of the room but not behind the pillar (the pillar
being an opaque barrier intersecting S’s line of gaze to the food). And so, a
mindreading hypothesis, when fully articulated, will need to make two fundamental
assumptions about A. It will need to assume that (1) on the basis of representing the
observable cue Sb, A infers that S has a particular type of mental state (Sms),5 and
that (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 to
predict Saction on the basis of Sb without representing Sms. A, for example, might be
taken 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-reading
animal’s prediction of an agent’s behavior must involve only a single inferential step—a step from the
observed behavioral/environmental cue to the predicted behavior. There is no reason why the behaviorreading animal could not engage in several inferential steps. The important point is that whatever further
steps 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’.
Solving Povinelli’s Problem
455
pillar because it knows (or believes) that S has a direct line of gaze to the food in the
middle of the room but not behind the pillar—without A interpreting the former
spatial relation between S’s eyes and the food as evidence of S seeing the food, or
the latter obstructed spatial relation between S’s eyes and the food as evidence of S
not seeing it. Thus, on the complementary behavior-reading hypothesis, A is taken to
reason in accordance with what might be called a ‘behavioral rule’ (in this case, the
rule that Sb-type cues lead to Saction-type behaviors) rather than, as the mindreading
hypothesis assumes, according to a ‘mindreading rule’ (the rule that Sb-type cues
lead 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, four
types of answers are now routinely offered in the field. First, A may have simply
observed Sb-type cues followed by Saction-type behaviors in the past. Thus, A may
have come to know/follow the behavioral rule by means of associative learning—
learning that may well involve associating environmental cues with representations
of “behavioral abstractions” in others (Vonk and Povinelli 2006, p. 375), as opposed
to 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 might
categorize these past Sb–Saction sequences in terms of their efficiency within the
parameters of the context of action (see Gergely and Csibra 2003). Thus A may have
learned through past observations that S (or S-type agents) typically do Saction-type
behaviors under Sb-type observable conditions by the most efficient means available
in the context (according to A’s standards of efficiency). On this way of
understanding the behavioral rule, A may be able to predict S doing Saction under
Sb conditions by the most efficient means available in the context (again, according
to A’s standards), even if the “surface behaviors” exhibited by S’s new efficient
means of action have never been observed by A to follow Sb-type conditions in the
past.
Third, A may come to follow the behavioral rule by engaging in an embodied
simulation-based process (Gallese 2007). On the embodied simulation account, A
may predict Saction-type behavior in others under Sb-type observable conditions by
using its own past behaviors or current behavioral dispositions under observed Sbtype conditions as a model. A, for instance, may expect that agent S is likely to do
Saction-type behavior under S-b type situations, since A itself has done or currently
finds itself prepared to do (via the stimulation of its mirror-neuronal system) Sactiontype behavior under Sb-type observable situations.
6
As should be clear from the above discussion, not all instances of behavior-reading fit into the category
of complementary behavior-reading. In many instances, animals learn (or innately know how) to anticipate
other agents’ behavior (e.g., fleeing, feeding, mating) on the basis of certain behavioral/environmental
cues (e.g., forward-facing torso, sound of a bell, smell of sex pheromones in conspecifics, etc.) that are not
in any plausible way indicative of kinds of mental states in the other agent. Lurz (2009, 2011b) calls such
non-mindreading methods of behavior prediction minimal behavior-reading (for evidence of this in
chimpanzees see Povinelli and Eddy 1996; Vonk and Subiaul 2009). The categories of mindreading and
complementary behavior-reading are, therefore, not exhaustive; an animal can be a minimal behaviorreader and fail to be in either of these categories. Thus, it bears reiterating that it is the realistic possibility
that chimpanzees (and perhaps other primates) are complementary behavior-readers—not minimal
behavior-readers—that creates Povinelli’s problem for chimpanzee (primate) theory-of-mind research.
456
R.W. Lurz, C. Krachun
And fourth, it is quite possible that Saction-type behaviors are stereotypical
behaviors under Sb-type conditions for A’s species and, thus, A may be credited with
being innately equipped (‘hardwired’) to expect Saction-type behaviors from others
under Sb-type conditions (see Povinelli 2001).
What the four behavior-reading strategies here have in common is that they are all
reality-based methods for representing other creatures’ external goals (i.e., methods
that take the behavioral/environmental cues animals use to predict other agents’
behavior as facts that the animal itself currently perceives or believes obtain or
obtained). An important point in mentioning these various possible explanations for
how an animal might come to know or follow a behavioral rule is to avoid the false
dichotomy that animals predict others’ behaviors either by mindreading or by
learning to associate certain antecedent behavioral/environmental cues with certain
consequent surface behaviors in others (i.e., movements of others’ limbs in space
independent of contextual matters). Thus, to overcome Povinelli’s problem, one
needs to show how an animal could predict another agent’s action (Saction) by its
taking 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-type
cues lead to Saction-type behaviors) according to one of the four behavior-reading
methods outlined above. The question is whether any of the experimental protocols
that Call, Tomasello, and others take as providing compelling data that primates
attribute 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 primate
internal-goal attribution. But neither is it necessary to do so for the purpose of this
paper. The studies that we have selected are representative of the sorts of
experimental approaches currently used in the field to test for primates’ attribution
of internal goals. Demonstrating that these kinds of experimental approaches cannot
overcome Povinelli’s problem is enough to motivate pursuing an alternative
experimental strategy.
4.1 The Accidental Versus Intentional Protocol
One of the earliest studies of goal/intention attribution in primates was by Call and
Tomasello (1998).7 In their study, orangutans and chimpanzees were first trained to
choose a baited box from among three that were presented. The baited box was
always 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 food
from underneath the box and give it to the animal. The apes were then trained on a
delayed version of the task in which the experimenter removed the marker before
allowing the animal to make a choice. In the testing stage, the apes were initially
presented with the three boxes without a marker. They then watched while the
experimenter 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.
Solving Povinelli’s Problem
457
accidentally knocked the marker onto another box. In those cases where the marker
was intentionally put onto the box, the experimenter either gently placed the marker
on top of the box or held the marker 10 cm directly over the top of the box and
carefully dropped it onto the box; in both cases, the experimenter looked at the box
and marker while acting. In those cases where the marker ended up on a box
accidentally, the experimenter either looked away from the boxes and, by moving the
apparatus on which the boxes were set, caused another marker (which was
positioned above the boxes) to fall onto a box; or she simply knocked the positioned
marker off its perch with her arm and onto a box while pretending to inspect some
part 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 unintended
marker only, the experimenter made a facial expression of disapproval and in some
instances said “Oops!” as she removed it.
Pooling the data across subjects, it was discovered that the apes as a group tended
to choose the intentionally marked boxes over the accidentally marked ones (at least
in the early trials). There was an unexpected difference in the animals’ choice
between the two kinds of intentionally marked boxes, however. Most of the apes
tended to choose the intentionally marked box on which the marker had been
dropped by the experimenter, choosing the box on which the marker had been
intentionally placed at roughly the same rate as the accidentally marked box.
However, the language-trained orangutan, Chantek, chose the box on which the
experimenter had intentionally placed the marker on every trial, choosing the box on
which the marker had been intentionally dropped at roughly the same rate as the
accidentally marked box. Call and Tomasello speculated that this difference was due
to a difference in the apes’ rearing histories. Food sharing among great apes is not
very common, but when it does occur, the donor usually just drops the food on the
ground in front of the recipient, rather than handing it to him as a human might. As a
result of this species-typical type of behavior in food-sharing contexts, Call and
Tomasello suggested that the unenculturated (not reared by humans) apes in their
study “may have regarded dropping as more intentional than placing” (p. 202).
Chantek, however, is an enculturated orangutan. Call and Tomasello speculated that
Chantek’s “early exposure to human interaction and enculturation” may have
inclined him toward seeing the experimenter’s act of placing the marker as more
intentional than her act of dropping it. This seems plausible, for Chantek’s trainers
and 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 drop
it onto the floor next to him. And during his language training period, Chantek came
to 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 quite
possible that Chantek understood the arrangement of Call and Tomasello’s
experiment as one of a food-sharing kind and, as a result of his enculturation, was
disposed to interpret the gentle placement of the marker by the experimenter as
similar to the intentional placement of food into his hand or feeding tray in past
food-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 question
is whether a behavior-reading hypothesis complementary to Call and Tomasello’s
458
R.W. Lurz, C. Krachun
intentional-state attribution hypothesis can provide an equally plausible account. The
answer, we believe, is yes. First, let’s consider the question of why the apes as a
whole tended to choose the intentionally marked boxes over the accidentally marked
ones. The complementary behavior-reading answer is simply that the apes as a whole
preferred the former type of boxes to the latter because the former were more like the
baited 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 with
the marker on them and removed the marker without disapproving facial expressions
or saying “Oops!”
The second question to be answered is why did the apes choose the particular
type of intentionally marked box that they did in the test trials? Call and Tomasello,
recall, speculated that the unenculturated apes chose the intentionally dropped
marker boxes because they saw the experimenter’s action of dropping the marker as
being like those actions of intentionally dropping food in a typical ape-ape foodsharing context; and that Chantek chose the intentionally placed marker boxes
because he saw the experimenter’s action of carefully placing the marker as being
like those actions of intentionally placing food in typical human-ape food-sharing
encounters. But, of course, the animals would not have seen the experimenter’s
actions in such intentional terms if their respective past food-sharing encounters
never led to them (or others) receiving the food in the location where it was dropped
or gently placed by the donor. If the food was quickly eaten by the donor (or
miraculously disappeared) as soon as it hit the floor or lay in the recipient’s hand, for
example, the apes would not have taken such actions of dropping or placing food as
food-sharing at all, let alone as expressing the donor’s intention to share food. And
so, if Call and Tomasello’s explanation here is correct, the apes are likely to know
from previous experience that dropping or gently placing food in a food-sharing
context generally leads to the food remaining in the area that it was dropped or
placed long enough for the recipient to retrieve it. But now we are in position to give
a complementary behavior-reading answer to our second question. The unenculturated apes tended to choose the box on which the marker was intentionally dropped
because they saw the experimenter’s action as being similar to the act of dropping
food 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 because
he saw the experimenter’s action as being similar to the careful placement of food by
humans and expected a similar type of outcome, too: food in that location. We do not
see that the complementary behavior-reading explanations here are any less
plausible, or any more speculative, than Call and Tomasello’s own intentional-state
attribution explanation.
4.2 The Unwilling Versus Unable Protocol
In 2004, Call and colleagues ran another intentional-state attribution study with
chimpanzees. In that study, the subject chimpanzee was fed grapes by an
experimenter through a hole in a Plexiglas wall. Immediately following 2 to 6
motivational trials in which the chimpanzee received a grape, a test trial was run in
which the experimenter suddenly stopped feeding the chimpanzee either because
he was unable (but still willing) to do so or because he was unwilling (but still
Solving Povinelli’s Problem
459
able) to do so. In the various unable test trials, the experimenter’s willingness to
continue feeding the chimpanzee was suggested to the animal by the fact that he
had been, up to that point, feeding the chimpanzee quite regularly, and that his
sudden 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 grape
to the chimpanzee, or because his hands were currently occupied with another
task). In the various unwilling test trials, the experimenter’s sudden unwillingness
to feed the chimpanzee was suggested to the animal by the fact that he was
physically able to deliver a grape (i.e., there were no barriers, obstacles, or clumsy
behavior 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 unable
trials, chimpanzees tended to wait and remained quiet (i.e., they engaged in little to
no begging behaviors). Such “patient” behavior on the chimpanzees’ part was
suggestive of their understanding that the experimenter would eventually come to
remove the physical constraint or cease the clumsy or distracted behavior and return
to feeding them. By contrast, in the unwilling test trials, chimpanzees tended to beg
more often (as if trying to cause the experimenter to continue feeding) and ended up
leaving the testing room soon after begging (as if understanding that there was no
point to sticking around and begging if the experimenter was so bent on not
delivering the food). The researchers concluded:
The current study provides suggestive evidence that chimpanzees spontaneously (i.e., without training) are sensitive to others’ intentions. Observing the
behavior of a human not giving them food, chimpanzees demonstrated in their
spontaneous behavior that they recognized a difference between cases in which
he was not giving food because he was unwilling to or because, for various
reasons, he was unable. (p. 496).
Thus, Call et al. (2004) take their chimpanzees to be following something like the
following mindreading rules:
Unable-yet-willing: If the experimenter has been feeding me regularly and
suddenly stops because he is physically unable to continue (e.g., because of a
barrier, 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 wait
and be patient.
Unwilling-yet-able: If the experimenter has been feeding me regularly and
suddenly stops though not because he is physically unable (e.g., there are no
barriers, obstacles, etc. preventing him from feeding me), then he is no longer
willing (i.e., no longer has the intention) to feed me; in which case, the thing to
do is to beg to see if this might cause him to start feeding me again, and if this
doesn’t initially work, to just give up.
Of course, one might wonder how the chimpanzees came to know what to do in
each of the two kinds of test situations, since they could not have learned these
different response strategies from repeated exposure to the test trials (in which
chimpanzees’ were not differentially rewarded). It is plausible to suppose that the
chimpanzees might have learned these different strategies from past encounters with
460
R.W. Lurz, C. Krachun
other feeding agents (e.g., other humans or conspecifics).8 On the basis of such past
encounters, the chimpanzees could have learned that the best strategy (i.e., the one
most likely to lead to receiving food) to adopt toward an agent who appears to be
willing but temporarily unable to feed you is to sit tight and wait; whereas, the best
strategy to adopt toward an agent who appears to be unwilling though quite able to
feed 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, chimpanzees
must infer it from observed behavioral/environmental cues. As noted above, the cues
that the chimpanzees presumably used to infer that the experimenter remained
willing to feed them in the unable trials were that he had been feeding them regularly
but had suddenly stopped because he was physically unable. And the cues that the
chimpanzees presumably used to infer that the experimenter was no longer willing to
feed them in the unwilling test trials were that he had suddenly stopped feeding them
even though he remained physically able to do so. But once these observable
grounds for attributing the different intentions to the experimenter are made explicit,
a complementary behavior-reading explanation of the chimpanzees’ performance
quickly suggests itself. It seems just as plausible that instead of using the above
mindreading rules, chimpanzees simply used the following behavioral rules:
Stops-feeding-because-unable: If the experimenter has been feeding me
regularly and suddenly stops because he is physically unable to continue (e.g.,
because of a barrier, obstacle, clumsy behavior, preoccupied hands, etc.), then
the thing to do is to just wait and be patient.
Stops-feeding-though-able: If the experimenter has been feeding me regularly
and suddenly stops though not because he is physically unable (e.g., there are no
barriers, obstacles, etc. preventing him from feeding me), then the thing to do is
to beg to see if this might cause him to start feeding me again, and if this doesn’t
initially work, to just give up.
Again, one might wonder how chimpanzees could come to know such behavioral
rules, given that they did not learn them over the test trials. To this question, the same
answer that was given above can be given here. Through their past encounters with other
feeding agents, the chimpanzees could have learned that the best strategy to adopt
toward an agent who has been feeding them but suddenly stops because he is physically
unable to continue is to sit tight and wait; whereas, the best strategy to adopt toward an
agent who has been feeding them but suddenly stops though not because he is physically
unable to deliver the food is to beg, and if that doesn’t work, to give up. Thus, both the
mindreading explanation and the complementary behavior-reading explanations are
forced to provide the same type of explanation regarding how the chimpanzees came to
know the rules that they followed in the test trials.
Note also that both the mindreading and the complementary behavior-reading
explanations 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 were
tested, they would have had plenty of opportunity to learn such response strategies as a result of their
interactions with human caretakers who fed them.
Solving Povinelli’s Problem
461
concepts that apply to a perceptually heterogeneous class of behaviors and
environmental circumstances. Only the mindreading hypothesis, however, takes
chimpanzees to use these abstract behavioral concepts as their basis for ascribing the
mental state of willing (or intending) to the experimenter. Yet nothing is apparently
gained in explaining the chimpanzees’ performance by making this additional
assumption. The chimpanzees’ performance is just as well explained in terms of their
following the behavioral rules above. Thus, Call and colleagues’ study does not solve
Povinelli’s problem.9
4.3 The Imitative Learning Protocol
In the past 20 years, there has been a lot of interest regarding imitative learning in
animals, with some imitation studies being used to test for intentional-state
attribution in primates. In one such study, Buttelmann et al. (2007) adapted an
imitation protocol developed by Gergely et al. (2002) for testing human children. In
Buttelmann et al.’s study, enculturated chimpanzees observed while an experimenter
demonstrated how to illuminate a light mounted onto a box by pressing on the light
with his forehead. Half the chimpanzees observed the experimenter do so while his
hands were occupied with holding a blanket around his shoulders; the other half
observed the experimenter do so while his hands were free. In both cases, after
observing the demonstration, chimpanzees were allowed to manipulate the apparatus
on their own. Chimpanzees were more likely to turn on the light as the experimenter
had (with his head) if the experimenter’s hands were free during the demonstration
than 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 “possess
the 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 that
agents generally intend to perform actions that they (the agents) know or believe to be
most efficient in the environmental setting. In the hands-free demonstration, according
to 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 with
capuchin monkeys. In one of their experiments, however, the researchers used a spoon, rather than a
human hand, to deliver food to the animal subject during motivational trials. In contrast to their differential
responses in human-hand experiments, monkeys in the spoon experiment did not discriminate between a
spoon that was ‘unable yet willing’ and one that was ‘unwilling yet able’ to deliver food. In each case, the
monkeys 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 it
comes to humans but not to inanimate objects. However, such results are in no way inconsistent with what
one would expect from a complementary behavior-reading account of the monkeys’ behavior. For the
complementary behavior-reading hypothesis does not say that monkeys learn to follow behavioral rules
that apply to any moving object (even spoons). Rather, the hypothesis holds, just as does the mindreading
hypothesis that the researchers endorse, that monkeys follow rules that they learned from (and take to
apply specifically to) human or other animal agents. Thus, the complementary behavior-reading
hypothesis is no more expected to predict a differential response pattern from monkeys in the different
spoon 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 an
unusual body part, although no significant differences were found with apparatuses that produced sounds
(Buttelmann et al. 2007).
462
R.W. Lurz, C. Krachun
are unoccupied and yet he employs his head, (2a) he must have intended to turn on the
light 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 to
be the experimenter’s intention in using this unusual means of operation, the
chimpanzees turn on the apparatus with their head. And in the hands-occupied
demonstration, according to the mindreading rule, chimpanzees are taken to reason
that since (1b) the experimenter’s hands are occupied and could not be used and he
instead employs his head, (2b) the experimenter must have intended to turn on the
light in this unusual manner for the reason that it was the most efficient means
available. Thus (3b) desiring to imitate what they understood to be the experimenter’s
intention in using his head, chimpanzees turn on the apparatus by the most efficient
means available, which for them is their hands or mouth.
The question is whether chimpanzees could have imitated as they did not because
they desired to copy what they understood to be the experimenter’s intention in using his
head, but rather because they desired to copy what they understood to be the most
efficient means to turn on the light. This interpretation, which takes chimpanzees to
follow the behavioral rule that agents generally perform actions that are the most
efficient in the environmental setting, is consistent with interpretations of similar
performance by infants in analogous imitation studies (see Gergely et al. 2002). In the
hands-free demonstration, according to this rule, chimpanzees are taken to reason that
since (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 of
operation (otherwise, the experimenter would have used his hands). Thus (3c) desiring
to turn on the light in what they understand to be the most efficient way, chimpanzees
turn on the light with their head. In the hands-occupied demonstration, according to
the behavioral rule, chimpanzees are taken to reason that since (1d) the experimenter’s
hands are occupied and could not be used and he instead employs his head, (2d) the
experimenter 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 efficient
means to them, chimpanzees turn on the light with their hands or mouth.
The above complementary behavior-reading explanation is no less plausible or more
speculative than Buttelmann et al.’s intentional-state explanation. Both hypotheses rest
upon the common assumption that chimpanzees judge agents’ actions within a context
as being more or less usual and efficient. The significant difference between them is that
the intentional-state hypothesis credits chimpanzees with inferring the experimenter’s
underlying intention in performing the unusual mode of operation, whereas the
complementary behavior-reading hypothesis credits chimpanzees with inferring the
most 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 by
Gergely et al. 1995) to test for goal attribution in chimpanzees.11 In the study, four
infant chimpanzees were familiarized (habituated) to one of two video displays in
which 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.
Solving Povinelli’s Problem
463
block stopped moving. In the experimental video, the block is shown making a
parabolic 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 the
same parabolic jump in the air before making contact with the ball. The experimental
video, of course, was designed to make it look as if the block had the goal to make
contact with the ball by the most direct path possible (in the circumstance, jumping
over the barrier); whereas, the control video was designed to make it look as if the
block either had no goal at all or the goal of making contact with the ball by jumping
in the air first (an indirect path). After the habituation phase, the chimpanzees were
then shown two videos in which the barrier was removed. In one of the videos (old
action test), the block performed the same movements as before, a parabolic jump
and contact with the ball; in the other (new action test), the block moved in a straight
line toward the ball. Uller hypothesized that if the chimpanzees that were habituated
to the experimental video interpreted the block as having the goal of contacting the
ball in the most direct way possible, then they should find the old action test
surprising and look longer at it than at the new action test; and if the chimpanzees
that were habituated to the control video interpreted the block as having no goal or
the goal of contacting the ball by jumping, then they should find the new action test
surprising and look longer at it than at the old action test. And these are precisely the
results Uller got.
Although this may be one of the better studies of internal-goal attribution in
chimpanzees—for it tests the animal’s ability to predict an agent’s behavior rather
than perform some task (e.g., discrimination or imitation) that is only distantly
related to the agent’s action—we do not believe it shows that chimpanzees attribute
the mental state of having goals or intentions to agents, as opposed to attributing
goal-directed movement (see Gergely and Csibra 2003 for a similar interpretation of
the infant data). To see this, let us use an analogy. Water, as we know, flows from a
higher 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, we
expect water to flow around the obstacle, and when the obstacle is removed, we
expect the water to flow in a straight path. Does this understanding of hydraulics
require or involve our interpreting water as having (in its mind!) the goal or intention
to 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 moving
toward 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’s
study understand the movement of the block in the experimental video, in
teleological but not mental-state terms. They observe the block following the most
direct path possible toward the ball (as we would see the movement of water around
a barrier toward a lower level of elevation). When the barrier is removed, the
chimpanzees should (and do) expect the block’s movements to once again follow the
most direct path to the ball—in this case, moving along a straight line (again, just as
we would expect the water to flow down a straight path after the barrier was
removed). 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 toward
a certain end state (contact with ball) and following the most direct route to it. The
chimpanzees that watch the control video, on the other hand, observe the block
464
R.W. Lurz, C. Krachun
following an indirect path toward the ball, and so they should (and do) expect the
block to follow the same indirect path in the test video. Again, there is no mental
state attribution here, just attribution of goal-directed movement.
4.5 The Neurological Protocol
In the mid-nineties, researchers at the University of Parma quite unexpectedly
discovered that a set of neurons in the F5 area of the macaque monkey’s premotor
cortex not only fired when the monkey performed a goal-directed action, such as
picking up a grape, but also fired when it observed another monkey or human
performing the same goal-directed action. F5 neurons were subsequently called
“mirror-neurons,” and researchers soon began to speculate that they were used by
monkeys (as well as other primates) to interpret other agents’ actions in goal-directed
ways (Gallese et al. 1996; Rizzolatti et al. 1996).
However, some researchers in recent years have gone further and argued that the
mirror-neuronal system found in the inferior parietal lobe (IPL) of the macaque brain
is 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 selectively
depending on the type of goal-directed action it executed. Some neurons (the “graspand-eat” neurons) discharged more strongly when the monkey grasped and ate a
grape than when it grasped an object and placed it into a container (receiving a grape
from 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 a
container (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 and
grasp-and-place—were otherwise kinematically the same. When the monkey was
then allowed to observe an experimenter perform one of these two types of grasping
actions, it was discovered that its grasp-and-eat neurons were more likely to fire
when it observed the experimenter grasping a grape (without the container on table)
than when it observed the experimenter grasping an object (or grape) with the
container on the table; whereas, its grasp-and-place neurons were more likely to fire
when it observed the experimenter grasping an object (or grape) with a container on
the table than when it observed the experimenter grasping the grape (without the
container). The researchers interpreted these findings as showing that the IPL mirrorneuron system in the macaque brain “allows the monkey to predict the goal of the
observed action [i.e., eating or placing] and, thus, to ‘read’ the intention of the acting
individual” (p. 666).
But we do not see how these findings show that the macaque understands that the
experimenter has a particular internal goal or intention when he grasps a grape or
object, as opposed to it merely being able to anticipate the end state (eating or
placing) of the experimenter’s initial grasping act by way of comparing the initial act
to its own past grasping behaviors in similar contexts. For one thing, the researchers
simply speculate that the monkey predicts the end state (eating or placing) of the
experimenter’s grasping act, but there is absolutely no evidence of this in the
experiment. Although its grasp-and-eat neurons fire when it observes the
experimenter grasping a grape, for example, this may indicate nothing more than
that the monkey is recollecting its own act of grasping and eating grapes under
Solving Povinelli’s Problem
465
similar conditions. However, despite the absence of any empirical support for this
speculation, it is certainly possible that the monkey uses its mirror neurons to make
such a prediction. But even on this assumption, there is no more reason to think that
the monkey predicts the end state of the experimenter’s grasping act by attributing an
intentional state to the experimenter than to think that it predicts the end state by
comparing the experimenter’s grasping act to its own past grasping acts in similar
contexts. There is no more reason, that is, to assume the intentional-state attribution
hypothesis which takes the monkey to think something like,
“When I grasp a grape in such a context, I intend to eat it; so the
experimenter’s grasping the grape in such a context indicates that he has the
same type of intention and will likely eat the grape,”
rather than the complementary behavior-reading hypothesis that takes the monkey to
think something like,
“When I grasp a grape in such a context, I typically eat it; so the
experimenter’s grasping the grape in such a context indicates that he will
likely eat it.”
For example, in observing the experimenter grasping a grape in the absence of the
container, 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 to
suppose that this association with its own past grasping-and-eating behavior in the
context of an absent container would selectively increase the firing among its graspand-eat neurons, perhaps allowing it to “predict the goal [eating grape] of the
observed action.” Likewise, in observing the experimenter grasping an object (or a
grape) in the presence of the container, the monkey may recall its own grasping of
objects 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 have
selectively increased the firing of its grasp-and-place neurons, allowing the monkey
to 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 approaches
currently used to study goal attribution in animals capable of overcoming Povinelli’s
problem. In each case, a behavior-reading hypothesis complementary to the
intentional-state hypothesis under consideration provides an equally plausible
account of the data.
5 Relevance to Children’s Ability to Attribute Internal Goals, and a Question
of Parsimony and Convergence
Call and Tomasello (2008) have responded to the above complementary behaviorreading strategy to understanding the data on internal-goal attribution in primates
with the following parity-of-reasoning argument:
12
The researchers did observe that the monkey’s grasp-and-eat neurons also fired when it observed the
experimenter grasping a grape in the context of a container, but they did not fire as strongly as its graspand-place neurons (p. 664).
466
R.W. Lurz, C. Krachun
Indeed consistent use of this explanatory strategy would also deny human
children an understanding of goals and intentions [qua mental states] because
most of the chimpanzee [and monkey] studies are modeled on child studies.
(p. 189).
Such a charge against the complementary behavior-reading strategy would be a
compelling reason, we believe, to abandon it as a way to understand the primate data
if there were not a similar controversy in developmental psychology over how to
interpret the infant studies on goal attribution. But there is a similar controversy. As
noted above, a number of researchers have argued that the data from the preverbal
infant studies are just as well accounted for by infants understanding something
about others’ external goals as by their understanding something about others’
internal goals. Povinelli’s problem is just as much a problem for these infant studies
as it is for analogous primate studies. Since parity-of-reasoning arguments are
compelling only insofar as their analogue base is more certain than their target, we
do 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 to
the research on preverbal infants, just as we are suggesting it is for research on
primates. If internal-goal attribution in primates is anything like what we suggest
below, then it follows from our account that internal-goal attribution in preverbal
infants will be most unambiguously manifested in their ability to attribute discrepant
or ‘false’ perceptions. The one study to date on false perception attribution in infants
by Song and Baillargeon (2008), unfortunately, does not overcome Povinelli’s
problem. In that study, infants at around 14.5 months showed signs of expecting a
human agent to reach for a container that deceptively appeared to contain a preferred
toy (a doll), but they did so only when the agent was absent during the baiting of the
containers (false-perception trials). When the human agent was present during the
baiting of the containers (true-perception trials), infants showed signs of expecting
the agent to reach for the container that actually contained the doll, even though it
was not the container that deceptively appeared to contain the doll. However, as
some have suggested (Hogrefe et al. 1986; Wellman 2011), infants could have
performed as they did simply because they expected the absent (‘ignorant’) agent to
behave incorrectly (e.g., to choose the non-preferred container). Alternatively, infants
could have themselves been deceived by the misleading appearance in falseperception trials but not in true-perception trials. In true-perception trials, it is
possible that infants simply failed to register the deceptive appearance of the
container as a result of their focusing their attention on the human agent and his/her
unobstructed line of gaze to the baiting process; whereas in false-perception trials,
without the distraction of the human agent, infants may have noticed and been
fooled by the deceptive appearance of the container, resulting in their wrongly
thinking that the doll was in that container. Neither of these alternative explanations
credits the infants with understanding anything about perceptual states in others.
As will be shown below, our protocol is designed to control for such alternative
explanations. Thus, on our account, consistent use of the complementary behaviorreading strategy to understand the infant data on internal-goal attribution does not
entail an outright denial that infants understand internal goals and intentions; at
most, it entails reserving our strongest assertion of internal-goal attribution in infants
Solving Povinelli’s Problem
467
until the time that children begin to attribute discrepant or false perceptions.
What we are recommending, then, is that if researchers want to demonstrate
unambiguously that infants attribute internal goals, they may wish to adopt an
experimental protocol along the same lines as the one we are proposing here
for chimpanzees.
Call and Tomasello (2008) also charge that consistent use of the complementary
behavior-reading strategy is particularly unparsimonious compared to the mindreading strategy that they favor. As our own analysis of the five studies above
illustrate, no one behavioral rule accounts for all the data. In some of the studies, the
behavioral rule invoked to explain animal performance is hypothesized to be based
on associative learning, while in other studies it is hypothesized to be based on
embodied simulation or (non-mentalistic) teleological reasoning. Of course, since the
complementary behavior-reading explanations in each case were modeled on their
competing mindreading explanations, the same point can be made for Call and
Tomasello’s mindreading strategy. There is no one mindreading rule that accounts for
all the data, either. As we saw above, in some studies, the mindreading rule invoked
to explain animal performance is hypothesized to be based on some type of
associative learning, while in other studies it is hypothesized to be based on a type of
mental simulation or a psychologically enriched version of teleological reasoning.
So both strategies (behavior-reading and mindreading) are required to use different
rules of inference and prediction in order to explain the results of the different
studies. Nevertheless, it may be said, as Call and Tomasello (2008) suggest, that at
least the data from the various studies can be given a unified explanation under the
general mindreading hypothesis that primates understand something about the
internal goals underlying others’ actions. But, of course, the complementary
behavior-reading strategy can offer a similar unifying hypothesis: primates
understand something about the external goals of others’ actions.
Alternatively, one might argue that the point raised by Call and Tomasello is that
the alternative behavior-reading explanations discussed above appeal to very
different kinds of cognitive mechanisms (e.g., associative learning, embodied
simulation, non-mentalistic teleological reasoning); whereas Call and Tomasello
can 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 an
underlying cognitive mechanism than is complementary behavior-reading. Like
complementary behavior-reading, mindreading is a general ability or competency for
predicting other agents’ behaviors when provided with certain kinds of behavioral/
environmental cues. Animals may manifest this ability through the use of different
cognitive mechanisms or processes, such as associative learning, mental simulation,
or mentalistic teleological reasoning. Thus, what the data from the above studies
indicate, if one interprets them as showing that primates have the general ability to
predict other agents’ behavior by means of attributing internal goals, is that this
ability is realized by different cognitive mechanisms in different situations and by
different kinds of primates. Similarly, what the data indicate if one interprets them as
showing that primates have the general ability to predict other agents’ behavior by
attributing external goals, is again that this ability in primates is realized by different
cognitive mechanisms in different situations and by different kinds of primates. The
only difference between these two views is that the various cognitive mechanisms
468
R.W. Lurz, C. Krachun
understood to be employed in the manifestation of mindreading in primates all
operate on representations of mental states as well as on representations of the
various behavioral/environmental cues used to apply these mental states to other
agents; whereas, the mechanisms understood to be employed in the manifestation of
complementary behavior-reading in primates operate only on the latter kinds of
representations. The difference is not that the studies show that the ability to
mindread in primates can be understood to be realized by a single underlying
cognitive mechanism while the ability to engage in complementary behavior-reading
must be understood to be realized by an assortment of very different cognitive
mechanisms.
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-goal
attribution (and perhaps other types of mental state attribution) in primates rather
than just at individual studies, as we have done above. For in both claims about
human theory of mind and primate theory of mind, it is the convergence of a body of
evidence—both experimental and observational—that has led some researchers to
conclude that primates (in particular, chimpanzees) understand something about
others’ mental states such as internal goals.
Many researchers would certainly agree that it is important to consider the body of
evidence in favor of primate mindreading, and that the body of evidence may indeed
come to converge on the conclusion that primates understand something about mental
states in others. However, a number of prominent researchers who have examined this
body of evidence (Heyes 1994, 1998; Macphail 1998; Penn et al. 2008; Penn and
Povinelli 2007; Shettleworth 1998, 2010) have argued that there is no compelling
reason to think that it does converge on the hypothesis that primates are mindreaders
rather than on the alternative hypothesis that primates are complementary behaviorreaders. To be genuinely confident that the body of evidence converges on the former
rather than the latter hypothesis, it is argued, would require that one “show not merely
that [the theory of mind hypothesis] can be applied to diverse phenomena but that for
each of a range of phenomena it provides a better explanation than alternative,
nonmentalistic hypotheses” [emphasis added] (Heyes 1998, p. 111). Yet critical
examination of the most compelling theory-of-mind studies (both experimental and
observational) in primates has failed to show that the data are better explained by a
mindreading hypothesis than by an alternative nonmentalistic hypothesis. What are
needed to move the primate theory-of-mind debate forward, the researchers argue, are
experimental protocols that could provide such data. Until then, they hold, there is
simply no more reason to think that the body of evidence converges on the hypothesis
that primates are mindreaders than on the hypothesis that they are complementary
behavior-readers. Of course, some researchers may be presently convinced that the
body of evidence does converge on the former rather than the latter hypothesis, but a
number of researchers, as noted, are not. And it is to these skeptical researchers that
we are appealing in providing an experimental protocol to test for internal-goal
attribution in chimpanzees that overcomes Povinelli’s problem.
Thus, we do not find Call and Tomasello’s parity-of-reasoning or parsimony
arguments, or the argument from the convergence of the body of evidence, strong
enough to overcome the motivation and importance to solve Povinelli’s problem
directly by designing more discriminating tests.
Solving Povinelli’s Problem
469
6 The Appearance-Reality Mindreading Theory
In this section and the next, we sketch an experimental protocol and its theoretical
rationale that show how researchers can acquire data that would provide compelling
evidence that primates—chimpanzees specifically—represent others’ internal goals. The
experimental protocol that will be outlined below is based on an appearance-reality
mindreading (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 in
Krachun (2008), Krachun et al. (2010), and Lurz (2011a, b). According to the ARM
theory, mental state attribution, such as attributions of internal goals, evolved in the
animal kingdom for the purpose of predicting the behavior of other agents
(conspecifics, predators, or prey) in environmental settings in which the animal’s
competing behavior-reading counterparts could not. In many environmental settings,
the way distal objects (e.g., food, partially occluded objects, other animals) perceptually
appear (e.g., look, sound, smell) to an agent is a better predictor of how the agent is
likely to act toward the objects than the way the objects objectively are. Behaviorreading animals can appeal only to the latter sorts of reality-based, mind-independent
facts, such as facts about agents’ past behavior or their current line of gaze to objects in
the environment. Mindreading animals, in contrast, can appeal to the subjective ways
environmental objects perceptually appear to agents to predict their behavior.
In illusory settings where distal objects appear differently from what they really
are, the ability to predict another agent’s behavior on the basis of how objects look or
sound to the agent (contrary to the way the object might really be) has the potential
to contribute to the animal’s overall level of fitness. Understanding that a dominant
conspecific, for instance, fails to see the camouflaged insect or distant piece of fruit
in its line of gaze as an insect/piece of fruit (seeing it instead as perhaps a leaf/dark
spot on the forest floor) would enable a subordinate mindreading animal to predict
that the dominant is unlikely to try to eat the insect/fruit. Such understanding could
provide the subordinate animal with the valuable opportunity to eat the insect/fruit
later in private, free of any agonistic encounter with the dominant. Were a behaviorreading subordinate placed in the very same situation as this mindreading one it
would not be capable of availing itself of this opportunity, however. For such a
subordinate would understand only the reality-based fact that the dominant is gazing
directly at an edible insect/piece of fruit. Knowing this fact about the dominant and
the environment would not enable the subordinate to predict that the dominant is
unlikely to try to eat the insect/fruit. If anything, such a fact would likely lead the
subordinate 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 looking
directly at edible insects/fruit). This, in turn, would likely discourage the subordinate
from attempting to eat the insect/fruit now or to make plans to eat it later. Although
situations like this are not likely to occur frequently in primate natural habitats, they
may well occur often enough, and provide enough of a fitness benefit to the
mindreader, to exert a selection pressure in favor of mindreading over behaviorreading. Obviously, more detailed ecological observations of chimpanzee (and other
primate) behavior in illusory environmental settings are needed. However, there is no
a priori reason to think that such situations would not provide enough selection
pressure to favor mindreading over behavior reading.
470
R.W. Lurz, C. Krachun
Thus, the ARM theory hypothesizes that internal-goal attribution in chimpanzees (and
perhaps other primates), in so far as it exists, may have evolved as a result of chimpanzees
coming to introspect their own ability to distinguish the way environmental objects
perceptually appear to them from the way they know (believe) them to really be, and
using this introspective knowledge of perceptual appearances in illusory (as well as nonillusory) settings to predict other agents’ behavior. There is currently no direct behavioral
evidence supporting the ARM theory, other than the fact that chimpanzees, of all species
of nonhuman animal that have been studied, have shown the strongest (though equivocal)
evidence for mindreading and have also shown themselves capable of discriminating
perceptual appearance from reality (Krachun et al. 2009a). The ARM theory predicts the
co-occurrence of these two cognitive abilities in chimpanzees; the complementary
behavior-reading theory does not. What is needed, and what we aim to provide in the
final section, is an empirically more direct way to test the ARM theory—a way that
overcomes Povinelli’s problem.
Before turning to the experimental protocol designed to test the ARM theory, we wish
to make two further points that will help in clarifying the relevance of the experimental
protocol in relation to the question of whether chimpanzees attribute internal goals. The
first has to do with a particular type of simple internal goal. In many animals, the simplest
forms of internal goals are action-guiding perceptual states. Certain types of perceptual
states in animals—perceptions of predators, mates, food, species-specific warning and
mating calls, types of colors, odors, and sounds, for example—feed rather directly into
the production of predictable types of behaviors in the animals. A male stickleback, to use
one well-known example, will quite predictably attack another male upon seeing its red
underbelly. 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 aptly
calls such simple forms of perception that act as direct guides to behavior “pushmipullyu” mental representations. Pushmi-pullyu perceptual states are, thus, simple forms
of internal goals. And so if animals, such as chimpanzees, demonstrate that they can
predict the external goals of other creatures by attributing such simple behavior-guiding
perceptual states to them, then this, we believe, counts as compelling evidence that
these 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 to
other agents. A number of researchers, for example, interpret studies by Hare et al. (2000, 2001) as
providing some of the strongest evidence that chimpanzees can predict dominant conspecifics’ feeding
behaviors on the basis of whether they think the dominant sees the food in the environment or saw where
the food was last hidden. However, Heyes (1994, 1998), Lurz (2009), and Povinelli and Vonk (2006) have
persuasively argued that these studies, as well as other perceptual-state attribution studies in primates and
other animals, are unable to determine whether chimpanzees predict other agents’ behavior on the basis of
what they think the agent sees or cannot see, or simply on the basis of what they think the agent has or
does not have an unobstructed line of gaze to (either currently or in the recent past). Having a line of gaze
to 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 just
don’t see the objects). However, line of gaze is one of the principal observable cues on which an animal
might reasonably infer that another subject sees an object. The main objective of this paper is to show how
researchers can overcome this methodological problem by running a type of test in which an animal can
predict an agent’s external goal by attributing a perceptual state (internal goal) to the agent but not by
representing facts about what the agent has (or lacks) a line of gaze to.
Solving Povinelli’s Problem
471
It is also important to mention that some researchers and philosophers have been
quite skeptical about the very idea of animals attributing mental states, even simple
internal goals. In many cases, the skepticism is grounded on an assumption that
attributing mental states necessarily involves attributing states that the attributor
understands as being hidden inside the other agent’s body, as being the cause of the
agent’s outward behavior, and as having representational (i.e., truth-evaluable)
contents. But as these researchers and philosophers argue, animals do not appear
capable of representing hidden internal causes of external events or of grasping the
normative ideas of truth and veridicality that are needed to understand the idea of
mental 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 argued
against it at length elsewhere (Lurz 2011a, b). Here we simply wish to express
our rejection of this line of skepticism by noting that we do not think it at all
necessary that animals, in representing others’ internal goals, must represent them
as spatially internal, causally efficacious, representational states.14 If an animal, A,
successfully predicts what another creature, B, intends to do (e.g., eat a particular
grape among several) by understanding how some aspect of the environment
perceptually appears to B (e.g., by understanding that that particular grape looks
large to B), then A’s attribution of this perceptual state to B is an attribution of an
internal goal, a genuine psychological state. However, to do this A need not
represent B’s perceptual state as something that is spatially inside B’s body, or as
something that literally causes B to eat the grape, or even as something that has a
truth-evaluable representational content. A need only represent B’s perceptual state
as 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 A
need 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 likely
is). Nor does A need to understand that this triadic state of affairs is the cause (or
partial cause) of B’s eating the large grape (though, it may be). A may simply
know, from past occurrences, that B (or others of B’s type) will typically eat the
grape that looks large to him (or them). And neither does A need to understand that
the representational content of B’s perceptual state—the content that that
particular grape is large—is something that is made true or false by the actual
state 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 states
by virtue of which they think, reason, perceive, and attribute mental states to others. What we are claiming
is that, in their ability to attribute mental states (even discrepant or ‘false’ mental states), animals do not
need to be understood to represent mental states in such ways. Representing mental states in such terms
may be the way that we (philosophers, scientific researchers, and perhaps most people) represent them, but
it 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 triadic
relations, both in the social domain (e.g., understanding the relation of their being between individuals x and y
in dominance rank) and in the domain of tool use (e.g., understanding the relation between a demonstrator, a
tool, and the object the tool is used to manipulate).
472
R.W. Lurz, C. Krachun
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 particular
grape looks large to B.
It is important to note, however, that even though A may not represent B as being
in a representational (truth-evaluable) state, this does not mean that A cannot
attribute a discrepant or ‘false’ perception (as it is frequently called). In representing
that a grape looks large to B, A is not required to believe that the grape is large, or
even to see the grape as being large at the time of attribution. A, for example, may
have reason to believe that the grape is not actually large (e.g., A may have seen the
small grape placed behind a magnifying lens), and A may currently not see the grape
as being large (e.g., A may be currently looking at B and not at the grape). Yet at the
same time, A may have reason to represent that the grape looks large to B (e.g., A
may observe B staring directly at the grape behind the distorting lens but know that
B did not witness the grape being placed behind the lens). Therefore, A may be in a
position to attribute a perceptual state to B even though A is not currently undergoing
that type of perceptual state itself, and even though A is not currently having the type of
belief (that the grape is large) that B is likely undergoing in having that type of
perceptual state. Thus, A can attribute a discrepant (‘false’) perceptual state without
representing 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 with
chimpanzees using magnifying and minimizing lenses. The researchers first confirmed
that chimpanzees would naturally choose (by pointing to) the larger of two grapes
presented to them. Then, in the initial demo trial of the experiment, chimpanzees were
presented with a small grape displayed behind a magnifying lens (making it appear
large) and a large grape displayed behind a minimizing lens (making it appear small).
The animals consistently pointed to the magnifying lens over the minimizing lens
when requesting a grape, demonstrating that they were fooled by the distorted size of
the grapes behind the lenses. In subsequent test trials, chimpanzees were allowed to
witness the change in the apparent size of the grapes as the experimenter placed them
behind the lenses. More than half the chimpanzees chose the minimizing lens (i.e., the
apparently smaller but truly larger grape) in these trials. The results suggest, as the
researchers maintain, that some chimpanzees are capable of distinguishing between the
apparent size of an object (a grape) and its real size.17 The question is whether
chimpanzees can use this understanding of visual appearances to predict the behavior
of 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 Armstrong
1968; Thau 2002) or, on some accounts, with non-propositional, truth-evaluable entities such as positioned
scenarios (e.g., the set of different ways of filling out the space before S’s body that are consistent with the
fact 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 by
simply 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.
Solving Povinelli’s Problem
473
As a way to test for this possibility, we propose the following experimental
protocol modeled on Krachun et al.’s (2009b) competitive paradigm study with
chimpanzees and Southgate et al.’s (2007) anticipatory-looking paradigm study with
young children. The objective here is to test whether chimpanzees are capable of
anticipating (as evidenced by their looking behavior) the actions of a human
competitor by understanding how an illusory stimulus (grape) looks to the human.
The general setup is similar to Krachun et al.’s competitive paradigm (2009b). A
chimpanzee and a human competitor are in adjoining rooms divided by a clear
Plexiglas 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 on
top. Two transparent (and non-distorting) glass containers with opaque lids are
placed a meter apart on the platform, in alignment with the finger holes. On the side
of the table nearest the competitor is an opaque backdrop containing two curtained
windows, also a meter apart and aligned with the containers. The windows allow the
competitor 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 arms
during testing while allowing a clear view of his face and direction of gaze. An
experimenter (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 to
place the grapes inside the containers and to slide the containers within reach of the
contestants.
Fig. 1 General setup for experimental design
474
R.W. Lurz, C. Krachun
Pretest Trials The study begins with a pretest stage, which has three purposes: (1) to
familiarize chimpanzees with the general testing procedure; (2) to allow chimpanzees to learn that the competitor will always prefer a large grape over a small one
when choosing first; and (3) to make chimpanzees aware of the fact that the
competitor will not always be successful in obtaining the large grape. (As we will
see below, this last point is important for ensuring that the chimpanzee is motivated
to attend to the competitor’s actions in the subsequent test trials.)
In the pretest stage, non-distorting glass containers are used, and the chimpanzee
and human competitor take turns choosing between these containers. The procedure
is as follows: With the sliding platform in the middle of the table and the competitor
absent from the room, the experimenter opens the lid on each container and places a
large grape inside one container and a small grape inside the other while the
chimpanzee observes. (The placement of the large and small grape is alternated from
trial to trial in a pseudo-random order). The experimenter then closes the lids to
prevent the competitor from peering directly into the containers from above when
looking at the grapes. After the containers are baited, the competitor enters the room
and sits at the table. The chimpanzee observes the competitor enter and can clearly
see his head over the top of the backdrop. The competitor then conspicuously looks
at each grape through the sides of the glass containers before staring straight
ahead.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 platform
over to the competitor (competitor-first trials); the other type of bell (chimpanzee
bell) indicates that the experimenter is about to slide the platform over to the
chimpanzee (chimpanzee-first trials). (Competitor-first and chimpanzee-first trials
are alternated in a pseudo-random order). Besides indicating which contestant will
choose first on a given trial, the bell also serves to cue coders when they should
begin coding for anticipatory looking in the chimpanzee. After the bell is rung, the
platform is slid before the appropriate contestant, and the contestant is allowed to
make a selection of one of the grapes in the containers.
Because our dependent measure is anticipatory looking, it is crucial that
chimpanzees be motivated to attend to the competitor’s actions in the upcoming test
trials. We create this motivation at the pretest stage in two ways: First, on competitorfirst trials, the competitor will actually obtain the larger, preferred grape only half the
time (success trials). The other half of the time (failure trials), the competitor will be
unsuccessful in obtaining either grape, so that both containers will remain baited when it
is 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 place
opaque coverings completely over the containers before sliding them towards the
chimpanzee. Thus, when it is choosing second, the chimpanzee can only know which is
the best container to choose by observing the competitor’s choice and noting whether he
was successful in obtaining the large grape.
The competitor-first trials thus proceed as follows: On half the trails (success
trials), the competitor extends an arm through the curtained window in front of the
container 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 the
grapes directly but only by looking at them through the sides of the glass containers.
Solving Povinelli’s Problem
475
grape (allowing the lid to fall back into place), retracts his arm back through the
window with grape in hand, and conspicuously eats the grape—all in full view of the
observing chimpanzee. On the other half of trials (failure trials), the competitor
extends an arm through the curtained window in front of the container with the large
grape but fails to retrieve the grape. On these trials, the competitor stops short of
reaching the large grape in the container and behaves as if his arm were stuck in the
window. After a few short thrusts of his arm (in an apparent attempt to free it), the
competitor retracts his arm through the window, leaving the large grape in the
container. (The order of success trials and failure trials are alternated in a pseudorandom order.) After the competitor has acted in one of these two ways toward the
container with the large grape, the experimenter places opaque coverings completely
over each container and moves the platform toward the chimpanzee. Once the
platform is in place before the chimpanzee, the chimpanzee is allowed to choose one
of the two containers (by pointing to it through a finger hole). Since the containers
are covered, the chimpanzee cannot make its choice by seeing the grape or grapes
inside the containers. Rather, to make its preferred choice (which we assume will be
the remaining small grape in competitor-first success trials and the large grape in
competitor-first failure trials) the chimpanzee must attend to the selection process of
the competitor. Thus, once again, the purpose of using the opaque coverings in the
competitor-first trials is to force the chimpanzee to attend to the competitor’s selection
behavior in order to make a successful selection when it is allowed to choose. As noted
above, making sure that the chimpanzee is attentive to the competitor’s selection
behavior in competitor-first trials will be important for securing anticipatory-looking
responses from chimpanzees during the testing phase of the experiment.
On chimpanzee-first trials, the chimpanzee bell is rung immediately after the
containers are baited, the platform is slid before the chimpanzee, and the chimpanzee
is allowed to indicate the grape of choice (presumably the large grape) by pointing to
it through the aligned finger hole. The experimenter then removes the selected grape
from its container and gives it to the chimpanzee to eat. The platform is then slid into
place before the competitor, and the competitor is allowed to choose. The competitor
successfully retrieves and eats the remaining gape. The opaque coverings are not
used in the chimpanzee-first trials.
This procedure is repeated a number of times, and it is expected that after playing
the game for a while, the chimpanzee will come to anticipate the following sequence
of events in competitor-first trials: (1) The competitor bell rings, (2) the platform is
slid in front of the competitor, and (3) the competitor extends an arm through the
curtained window in front of the container with the large grape (and in some but not
all trials succeeds in retrieving the large grape). Researchers can determine whether
the 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 bell
ring, as well as which window/container it looks to most often in the interval
between the bell ringing and the competitor extending an arm through a window.
(Video recordings of which window/container the chimpanzee looks at can be made
by having small recording cameras mounted above each window).
Test Trials The test trials are exactly like the pretest trials except that on some of
these test trials (size-distorting trials), the non-distorting glass containers are replaced
476
R.W. Lurz, C. Krachun
with identical looking size-distorting glass containers with opaque lids. One of these
new containers magnifies the size of objects placed inside, and the other minimizes
their size. Since the competitor is absent when the grapes are placed inside the
containers (as in the pretest stage), he does not observe the change in apparent size
of the grapes as they are placed inside the containers, as the chimpanzee does. After
the grapes are placed, the competitor enters the room, sits at the table, looks at each
grape through the sides of the containers, stares straight ahead, and the trial proceeds
as in the pretest. Thus on competitor-first test trials, the competitor bell is rung, the
platform is slid in front of the competitor, the competitor succeeds or fails to retrieve
a grape, the platform is slid in front of the chimpanzee, and the chimpanzee is
allowed to select a container. And on chimpanzee-first test trials, the containers are
baited, the chimpanzee bell is rung, the platform is slid before the chimpanzee, and
the chimpanzee is allowed to select a container.
Predictions The question is whether, on competitor-first size-distorting test trials, the
chimpanzee, upon hearing the ringing of the competitor bell, anticipates (a) the
competitor reaching an arm through the curtained window before the magnifying
container (which contains the small grape) or (b) the competitor reaching an arm
through the curtained window in front of the minimizing container (which contains
the large grape). If the chimpanzee expects (a), then it should look first to (or most often
at) the window in front of the magnifying container upon hearing the competitor
bell; 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 above
experiment should come to understand, as a result of their experience during the pretest
trials, that their competitor is disposed to reach for the grape that looks to him to be the
large grape. If chimpanzees also understand (as in Krachun et al. 2009a) that in the
size-distorting test trials, the grape inside the magnifying container looks large but is
truly small, and if they can use this knowledge to understand that the small grape
looks 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 anticipate
the competitor’s arm reaching through the curtained window in front of the
magnifying container. If chimpanzees do in fact predict the competitor’s behavior by
such perceptual-appearing attribution, then, on Millikan’s pushmi-pullyu theory of
action-guiding perceptual states, we can interpret the chimpanzees’ perceptualappearing attribution here as a type of internal-goal attribution (i.e., as the attribution
of having the goal to reach for/eat the large-looking grape).
The question is whether a complementary behavior-reading hypothesis could
reasonably give the same prediction. The answer, we submit, is no. For what the
perceptual-appearing/internal-goal attribution hypothesis above predicts (in accordance 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 the
predicted anticipation of the competitor’s reaching. Positive results from later trials may reveal only that
chimpanzees have learned to anticipate such selection behavior from the competitor through their
experience in previous trials.
Solving Povinelli’s Problem
477
occurrence (a) on competitor-first size-distorting trials, but that they will (c) provide
evidence of their ability to discriminate appearance from reality in chimpanzee-first
size-distorting trials by selecting the minimizing container (large grape) over the
magnifying container (small grape). In other words, the chimpanzees should choose
the minimizing container when choosing first but should expect the competitor to
choose the magnifying container when he is choosing first (we call this the min-mag
response pattern).
In contrast, a complementary behavior-reading hypothesis would predict either one
of two patterns of responding: 1) Chimpanzees will anticipate (a) in competitor-first size
distorting trials but will not perform as (c) states. That is, they will expect their
competitor to choose the magnified grape when he chooses first, but they will also
choose the magnified grape themselves when choosing first (a mag-mag response
pattern). 2) Chimpanzees will perform as (c) states but anticipate (b) rather than (a) on
competitor-first size-distorting trials. That is, they will choose the minimized grape
themselves when they choose first, but they will also expect their competitor to choose
the 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 complementary
behavior-reading hypothesis in this case would predict that chimpanzees would
come to understand, based on their experiences in the pretest stage, that the competitor
is disposed to reach for the grape that is in fact the largest. With this reality-based
understanding of the competitor’s behavioral disposition, the complementary behaviorreading hypothesis would predict that chimpanzees, in the competitor-first sizedistorting trials, should expect the competitor to reach for the container that, according
to the chimpanzees’ understanding of what is real, has the truly larger grape in it (the
min-min pattern, assuming that the chimpanzee distinguishes appearance from reality
in such case). Thus, the only way that chimpanzees on this hypothesis would be
expected to anticipate situation (a) in competitor-first size-distorting trials, in which the
competitor reaches an arm through the curtained window in front of the magnifying
container, would be if chimpanzees (mistakenly) understood the grape in the
magnifying container to be a large grape. But that would mean that chimpanzees
were 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 larger
grape, and would therefore be expected to select the magnifying container in those
size-distorting trials when they themselves were choosing first (the mag-mag pattern).
In short, on the complementary behavior-reading hypothesis, if chimpanzees
understand the difference between appearance and reality, they should show the minmin response pattern in our proposed protocol, which is not what the perceptualappearing/internal-goal attribution hypothesis predicts. But if chimpanzees do not
understand the difference between appearance and reality, they should show the mag-mag
response pattern, which is also not what the perceptual-appearing/internal-goal attribution
hypothesis predicts. Either way, a complementary behavior-reading hypothesis cannot
provide the same predictions as the perceptual-appearing/internal-goal hypothesis. What
this means is that this approach has the power, in principle and in practice, to overcome
Povinelli’s problem.
An added virtue of the protocol just described is that its design can be varied in a
number of ways while retaining the underlying logic that enabled it to overcome
478
R.W. Lurz, C. Krachun
Povinelli’s problem. This flexibility in its design will be of value to researchers who
might 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 described
above, chimpanzees’ ability to predict the competitor’s selection behavior is
measured through anticipatory looking. In alternative protocols, other anticipatory
responses could be used instead of (or in addition to) anticipatory looking. For
example, researchers might use an experimental setup that requires chimpanzees to
be ‘in the right place at the right time’ in competitor-first trials; otherwise, they lose
their opportunity to receive any reward. This could be achieved with a few small
changes to the original experimental setup and procedure, as described below.
First, rather than having an experimenter slide the containers within reach of the
competitor so that he can make a choice, the containers would be mounted onto a
spring-loaded, swivel mechanism that allowed the competitor himself to pull the
containers within reach. Further, the mechanism would be designed such that when
the competitor pulled a container within reach, the other container would
automatically move within reach of the chimpanzee, allowing the chimpanzee to
choose 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 chimpanzee
for a limited amount of time; once the competitor finished retrieving his chosen
grape and released the container, both containers would swivel back to their original
positions in the center of the platform. Of course, the setup would have to make it
impossible for the chimpanzee to quickly shift from one position to another in front
of the Plexiglas window. This could be achieved by erecting a clear Plexiglas divider
between the finger holes that chimpanzees needed to move around in order to reach
one hole or the other.
Thus, in order to take advantage of their time-limited opportunity to choose the
remaining grape in competitor-first trials, chimpanzees would have to move into
position in front of the appropriate finger hole before the competitor made his
selection. Their anticipatory moving behavior would indicate which container they
expected the competitor to choose (and note that in this protocol, the competitor
would always be successful in retrieving the chosen grape). As in the original
protocol, chimpanzees’ predictions of the competitor’s choice of container in sizedistorting versus non-distorting test trials would provide insights into their
understanding of the competitor’s subjective perception/internal goals. If chimpanzees are mindreaders, according to the ARM theory, then they are expected to select
the grape that really is large (i.e., the grape in the minimizing container) on
chimpanzee-first size-distorting trials, but to predict that the naive competitor will
select 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 the
other hand, chimpanzees are complementary behavior-readers, then on sizedistorting trials they are expected to produce either a min-min response pattern
(showing the capacity to make the appropriate appearance-reality discrimination) or
a mag-mag response pattern (showing the inability to make the appropriate
appearance-reality discrimination). Either way, a complementary behavior-reading
hypothesis cannot provide the same predictions as the perceptual-appearing/internalgoal hypothesis under consideration. Thus, this version of the experimental protocol
also has the power, in principle and in practice, to overcome Povinelli’s problem.
Solving Povinelli’s Problem
479
8 Conclusion
A persistent methodological problem in primate social cognition research has been how
to determine experimentally whether primates represent the internal goals of other
agents or just the external goals of their actions. This is an instance of Daniel Povinelli’s
more general challenge that no experimental protocol currently in use is capable of
distinguishing genuine mindreading animals from their complementary behaviorreading counterparts. Current experimental approaches to test for internal-goal
attribution in primates do not solve Povinelli’s problem, nor do these approaches solve
the problem when accompanied by parity-of-reasoning and parsimony arguments. To
overcome Povinelli’s problem as it pertains to testing internal-goal attribution in
primates, a new type of experimental approach is needed. The two versions of the sizedistorting protocol presented here have the power to discriminate between chimpanzees
that represent others’ internal goals and those that can (at most) represent the external
goals of others’ actions. Both versions of the protocol are designed to utilize certain
competencies of chimpanzees (e.g., the ability to distinguish appearance from reality
and to make tactical choices in competitive situations) that these animals have exercised
separately in different experimental settings. The question is whether chimpanzees will
exercise these competencies together in a single competitive setting for the purpose of
predicting a competitor’s action in terms of how a target object visually appears to the
competitor. If chimpanzees can do this, then researchers will have finally found what
they have been looking for: a genuinely compelling reason for saying that chimpanzees
understand something about the internal goals of other agents. Povinelli’s problem has a
solution, the rest is now up to researchers and, of course, chimpanzees.
Acknowledgements We wish to thank the anonymous reviewers for their insightful and helpful
comments on an earlier draft of this paper, and to thank Kristy Lee for drawing Fig. 1.
References
Armstrong, D. 1968. A materialist theory of the mind. London: Routledge and Kegan Paul.
Behne, T., M. Carpenter, J. Call, and M. Tomasello. 2005. Unwilling versus unable: Infants’ understanding
of intentional action. Developmental Psychology 41: 328–337.
Bermúdez, J.L. 2009. Mindreading in the animal kingdom. In The philosophy of animal minds, ed. R.
Lurz, 145–164. Cambridge: Cambridge University Press.
Buttelmann, D., M. Carpenter, J. Call, and M. Tomasello. 2007. Enculturated chimpanzees imitate
rationally. Developmental Science 10: F21–F38.
Call, J., and M. Tomasello. 1998. Distinguishing intentional from accidental actions in orangutans,
chimpanzees, and human children. Journal of Comparative Psychology 112: 192–206.
Call, J., and M. Tomasello. 2008. Does the chimpanzee have a theory of mind? 30 years later. Trends in
Cognitive Sciences 12: 187–192.
Call, J., B. Hare, M. Carpenter, and M. Tomasello. 2004. ‘Unwilling’ versus ‘unable’: Chimpanzees’
understanding of human intentional action. Developmental Science 7: 488–498.
Carruthers, P. 1998. Language, thought and consciousness. Cambridge: Cambridge University Press.
Csibra, G., G. Gergely, S. Biro, O. Koos, and M. Brockbank. 1999. Goal attribution without agency cues:
The perception of “pure reason” in infancy. Cognition 72: 237–267.
Davidson, D. 2001. Subjective, intersubjective, and objective. Oxford: Oxford University Press.
Fogassi, L., P. Ferrari, B. Gesierich, S. Rozzi, F. Chersi, and G. Rizzolatti. 2005. Parietal lobe: From action
organization to intention understanding. Science 308: 662–667.
480
R.W. Lurz, C. Krachun
Gallese, V. 2007. Before and below ‘theory of mind’: Embodied simulation and the neural correlates of
social cognition. Philosophical Transactions of the Royal Society B 362: 659–669.
Gallese, V., L. Fadiga, L. Fogassi, and G. Rizzolatti. 1996. Action recognition in the premotor cortex.
Brain 119: 593–609.
Gallup, G. 1982. Self-awareness and the emergence of mind in primates. American Journal of
Primatology 2: 237–248.
Gergely, G., and G. Csibra. 2003. Teleological reasoning in infancy: The naïve theory of rational action.
Trends in Cognitive Sciences 7: 287–292.
Gergely, G., Z. Nadasdy, G. Csibra, and S. Biro. 1995. Taking the intentional stance at 12 months of age.
Cognition 56: 165–193.
Gergely, G., H. Bekkering, and I. Király. 2002. Rational imitation in preverbal infants. Nature 415: 755.
Hare, B., J. Call, B. Agnetta, and M. Tomasello. 2000. Chimpanzees know what conspecifics do and do
not see. Animal Behavior 59: 771–785.
Hare, B., J. Call, and M. Tomasello. 2001. Do chimpanzees know what conspecifics know? Animal
Behavior 61: 139–151.
Heyes, C. 1994. Cues, convergence, and a curmudgeon: A reply to Povinelli. Animal Behavior 48: 242–
244.
Heyes, C. 1998. Theory of mind in nonhuman primates. The Behavioral and Brain Sciences 21: 101–148.
Hogrefe, G., H. Wimmer, and J. Perner. 1986. Ignorance versus false belief: A developmental lag in
attribution of epistemic states. Child Development 57: 567–582.
Humphrey, N. 1976. The social function of intellect. In Growing points in ethology, ed. P. Bateson and R.
Hinde, 303–317. Cambridge: Cambridge University Press.
Hurley, S., and M. Nudds. 2006. Rational animals? Oxford: Oxford University Press.
Krachun, C. 2008. Mentalizing capacities in chimpanzees (Pan troglodytes) (Unpublished doctoral
dissertation). Carleton University, Ottawa.
Krachun, C., J. Call, and M. Tomasello. 2009a. Can chimpanzees discriminate appearance from reality?
Cognition 112: 435–450.
Krachun, C., M. Carpenter, J. Call, and M. Tomasello. 2009b. A competitive nonverbal false belief task for
children and apes. Developmental Science 12: 521–535.
Krachun, C., M. Carpenter, J. Call, and M. Tomasello. 2010. A new change-of-contents false belief test:
Children and chimpanzees compared. International Journal of Comparative Psychology 23: 145–165.
Lurz, R. 2009. If chimpanzees are mindreaders, could behavioral science tell? Toward a solution of the
logical problem. Philosophical Psychology 22: 305–328.
Lurz, R. 2011a. Belief attribution in animals: On how to move forward conceptually and empirically.
Review of Philosophy and Psychology 2: 19–59.
Lurz, R. 2011b. Mindreading animals. Cambridge, MA: MIT Press.
Macphail, E. 1998. The evolution of consciousness. Oxford: Oxford University Press.
Meltzoff, A. 1995. Understanding the intentions of others: Re-enactment of intended acts by 18-month-old
children. Developmental Psychology 31: 838–850.
Miles, H.L. 1990. The cognitive foundation for reference in a signing orangutan. In “Language” and
intelligence in monkeys and apes: Comparative developmental perspectives, ed. S.T. Parker and K.R.
Gibson, 551–539. Cambridge: Cambridge University Press.
Millikan, R. 2004. Varieties of meaning: The 2002 Jean Nicod lectures. Cambridge, MA: MIT Press.
Peacocke, C. 1992. Scenarios, concepts, and perception. In The contents of experience, ed. T. Crane, 105–
135. Cambridge: Cambridge University Press.
Penn, D., and D. Povinelli. 2007. On the lack of evidence that nonhuman animals possess anything
remotely resembling a ‘theory of mind’. Philosophical Transactions of the Royal Society B 362: 731–
744.
Penn, D., K. Holyoak, and D. Povinelli. 2008. Darwin’s mistake: Explaining the discontinuity between
human and nonhuman minds. The Behavioral and Brain Sciences 31: 109–178.
Perner, J., and M. Doherty. 2005. Do infants understand that external goals are internally represented? The
Behavioral and Brain Sciences 5: 710–711.
Phillip, W., J. Barnes, N. Mahajan, M. Yamaguchi, and L. Santos. 2009. ‘Unwilling’ versus ‘unable’:
Capuchin monkeys’ (Cebus apella) understanding of human intentional action. Developmental
Science 12: 938–945.
Povinelli, D. 2001. On the possibilities of detecting intentions prior to understanding them. In Intentions
and intentionality, ed. B. Malle, L. Moses, and D. Baldwin, 225–250. Cambridge, MA: MIT Press.
Povinelli, D., and T. Eddy. 1996. What young chimpanzees know about seeing. Monographs of the Society
for Research on Child Development 61: 1–152.
Solving Povinelli’s Problem
481
Povinelli, D., and J. Vonk. 2006. We don’t need a microscope to explore the chimpanzee’s mind. In
Rational animals? ed. S. Hurley and M. Nudds, 385–412. Oxford: Oxford University Press.
Povinelli, D., H. Perilloux, J. Reaux, and D. Bierschwale. 1998. Young and juvenile chimpanzees’ (Pan
troglodytes) reactions to intentional versus accidental and inadvertent actions. Behavioural Processes
42: 205–218.
Povinelli, D., C. Prince, and T. Preuss. 2005. Parent-offspring conflict and the development of social
understanding. In The innate mind (vol. 1), ed. P. Carruthers, S. Laurence, and S. Stich, 239–253.
Oxford: Oxford University Press.
Repacholi, B., and A. Gopnik. 1997. Early reasoning about desires: Evidence from 14- and 18-montholds. Developmental Psychology 33: 12–21.
Rizzolatti, G., L. Fadiga, V. Gallese, and L. Fogassi. 1996. Premotor cortex and the recognition of motor
actions. Cognitive and Brain Research 3: 131–141.
Rochat, M., E. Serra, L. Fadiga, and V. Gallese. 2008. The evolution of social cognition: Goal familiarity
shapes monkeys’ action understanding. Current Biology 18: 227–232.
Shettleworth, S. 1998. Cognition, evolution, and behavior. Oxford: Oxford University Press.
Shettleworth, S. 2010. Clever animals and killjoy explanations in comparative psychology. Trends in
Cognitive Sciences 14: 477–481.
Sirois, S., and I. Jackson. 2007. Social cognition in infancy: A critical review of research on higher order
abilities. The European Journal of Developmental Psychology 4: 46–64.
Song, H., and R. Baillargeon. 2008. Infants’ reasoning about others’ false perceptions. Developmental
Psychology 44: 1789–1795.
Southgate, V., A. Senju, and G. Csibra. 2007. Action anticipation through attribution of false belief by 2year-olds. Psychological Science 18: 587–592.
Thau, M. 2002. Consciousness and cognition. Oxford: Oxford University Press.
Tomasello, M., and J. Call. 1997. Primate cognition. Oxford: Oxford University Press.
Tomasello, M., M. Carpenter, J. Call, T. Behne, and H. Moll. 2005. Understanding and sharing intentions:
The origins of cultural cognition. The Behavioral and Brain Sciences 28: 675–735.
Uller, C. 2004. Disposition to recognize goals in infant chimpanzees. Animal Cognition 7: 154–161.
Vonk, J., and D. Povinelli. 2006. Similarity and difference in the conceptual systems of primates: The
unobservability hypothesis. In Comparative cognition: Experimental exploration of animal
intelligence, ed. E. Wasserman and T. Zentall, 363–387. Oxford: Oxford University Press.
Vonk, J., and F. Subiaul. 2009. Do chimpanzees know what others can and cannot do? Reasoning about
‘capability’. Animal Cognition 12: 267–286.
Wellman, H. 2011. Developing a theory of mind. In The Wiley-Blackwell handbook of childhood cognitive
development, 2nd ed, ed. U. Goswami. Oxford: Blackwell.
Woodward, A.L. 1998. Infants selectively encode the goal object of an actor’s reach. Cognition 69: 1–34.
Woodward, A.L. 1999. Infants’ ability to distinguish between purposeful and non-purposeful behaviors.
Infant Behavior & Development 22: 145–160.