From Wikipedia, the free encyclopedia
Theory
of mind is the ability to
attribute mental states—beliefs, intents, desires, pretending, knowledge, etc.—to oneself and others and to understand
that others have beliefs, desires, and intentions that are different from one's
own. Deficits occur in people with autism spectrum disorders, schizophrenia, attention deficit
disorder, as well as neurotoxicity due to alcohol abuse. Though there are philosophical approaches to issues raised in such discussions,
the theory of mind as such is distinct from the philosophy of mind.
Defining theory of mind
Theory of mind is a theory insofar as the mind is not directly
observable. The presumption that others have a mind is
termed a theory of mind because each human can only intuit the existence of his
or her own mind through introspection, and no one has direct access to the mind
of another. It is typically assumed that others have minds by analogy with
one's own, and based on the reciprocal nature of social interaction, as
observed in joint attention, the functional use of language, and understanding of others' emotions and
actions. Having a theory of mind allows one to
attribute thoughts, desires, and intentions to others, to predict or explain
their actions, and to posit their intentions. As originally defined, it enables
one to understand that mental states can be the cause of—and thus be used to
explain and predict—others’ behavior. Being able to attribute mental states to
others and understanding them as causes of behavior implies, in part, that one
must be able to conceive of the mind as a "generator of representations". If a person does not have a complete theory
of mind it may be a sign of cognitive or developmental impairment.
Theory of mind appears to be an
innate potential ability in humans, but one requiring social and other
experience over many years to bring to fruition. Different people may develop
more, or less, effective theories of mind. Empathy is a related concept, meaning experientially
recognizing and understanding the states of mind, including beliefs, desires
and particularly emotions of others, often characterized as the ability to
"put oneself into another's shoes." Recent neuro
ethological studies
of animal behaviour suggest that even rodents may exhibit ethical or empathic
abilities. Neo-Piagetian theories of
cognitive development maintain
that theory of mind is a byproduct of a broader hypercognitive ability of the human mind to register,
monitor, and represent its own functioning.
Research on theory of mind in a
number of different populations (human and animal, adults and children,
normally- and atypically-developing) has grown rapidly in the almost 30 years
since Premack and Woodruff's paper,
"Does the chimpanzee have a theory of mind?", as have the theories of theory of mind. The
emerging field of social neuroscience has also
begun to address this debate, by imaging humans while performing tasks
demanding the understanding of an intention, belief or other mental state.
An alternative account of theory
of mind (ToM) is given within operant psychology and provides significant
empirical evidence for a functional account of both perspective taking and
empathy. The most developed operant approach is founded on research on derived
relational responding and is subsumed within what is called, "Relational Frame
Theory." According to this view empathy and perspective taking comprise a
complex set of derived relational abilities based on learning to discriminate
and verbally respond to ever more complex relations between self, others,
place, and time, and the transformation of function through established
relations.
Philosophical roots
Contemporary discussions of ToM
have their roots in philosophical debate—most broadly, from the time of
Descartes’ "Second Meditation," which set the groundwork for
considering the science of the mind. Most prominent recently are two
contrasting approaches in the philosophical literature, to theory of mind: theory-theory and simulation
theory. The theory-theorist imagines a veritable theory—"folk
psychology"—used to reason about others' minds. The theory is developed
automatically and innately, though instantiated through social interactions.
On the other hand, simulation
theory suggests ToM is not, at its core, theoretical. Two kinds of
simulationism have been proposed. One version (Alvin
Goldman's) emphasizes that one must recognize one's own mental states
before ascribing mental states to others by simulation. The second version of
simulation theory proposes that each person comes to know his or her own and
others' minds through what Robert Gordon names a logical "ascent routine",
which answers questions about mental states by re-phrasing the question as a
metaphysical one. For example, if Zoe asks Pam, "Do you think that dog
wants to play with you?", Pam would ask herself, "Does that dog want
to play with me?" to determine her own response. She could equally well
ask that to answer the question of what Zoe might think. Both hold that people
generally understand one another by simulating being in the other's shoes.
One of the differences between
the two theories that have influenced psychological consideration of ToM is
that theory-theory describes ToM as a detached theoretical process that is an
innate feature, whereas simulation theory portrays ToM as a kind of knowledge
that allows one to form predictions of someone's mental states by putting
oneself in the other person's shoes and simulating them. These theories
continue to inform the definitions of theory of mind at the heart of scientific
ToM investigation.
The philosophical roots of the
Relational Frame Theory account of ToM arise from contextual psychology and
refer to the study of organisms (both human and non-human) interacting in and
with a historical and current situational context. It is an approach based on
contextualism, a philosophy in which any event is interpreted as an ongoing act
inseparable from its current and historical context and in which a radically
functional approach to truth and meaning is adopted. As a variant of
contextualism, RFT focuses on the construction of practical, scientific
knowledge. This scientific form of contextual psychology is virtually
synonymous with the philosophy of operant psychology.
Theory of mind development
The study of which animals are
capable of attributing knowledge and mental states to others, as well as when
in human ontogeny and phylogeny this ability developed, has identified a
number of precursory behaviors to a theory of mind. Understanding attention,
understanding of others' intentions, and imitative experience with other people
are hallmarks of a theory of mind which may be observed early in the
development of what will later become a full-fledged theory. In studies with
non-human animals and pre-verbal humans, in particular, researchers look to
these behaviors preferentially in making inferences about mind.
Simon
Baron-Cohen identified
the infant's understanding of attention in others, a social skill found by 7 to
9 months of age, as a "critical precursor" to the development of
theory of mind. Understanding attention involves
understanding that seeing can be directed selectively as attention, that the
looker assesses the seen object as "of interest," and that seeing can
induce beliefs. Attention can be directed and shared by the act of pointing, a
joint attention behavior which requires taking into account another person's
mental state, particularly whether the person notices an object or finds it of
interest. Baron-Cohen speculates that the inclination to spontaneously
reference an object in the world as of interest ("proto-declarative
pointing") and to likewise appreciate the directed attention and interests
of another may be the underlying motive behind all human communication.
Understanding of others'
intentions is another critical precursor to understanding other minds because
intentionality, or "aboutness", is a fundamental feature of mental
states and events. The "intentional stance" has been defined by Daniel
Dennett as an understanding that others' actions are
goal-directed and arise from particular beliefs or desires. Both 2- and
3-year-old children could discriminate when an experimenter intentionally vs.
accidentally marked a box as baited with stickers. Even earlier in ontogeny, Andrew N. Meltzofffound that 18 month-old infants
could perform target manipulations that adult experimenters attempted and
failed, suggesting the infants could represent the object-manipulating behavior
of adults as involving goals and intentions. While attribution of intention (the
box-marking) and knowledge (false-belief tasks) is investigated in young humans
and nonhuman animals to detect precursors to a theory of mind, Gagliardi et al. have pointed out that even adult
humans do not always act in a way consistent with an attributional perspective. In the experiment, adult human subjects came
to make choices about baited containers when guided by confederates who could
not see (and therefore, not know) which container had been baited.
Recent research in developmental
psychology suggests that the infant's ability to imitate others lies at the
origins of both a theory of mind and other social-cognitive achievements like
perspective-taking and empathy. According to Meltzoff, the infant's innate
understanding that others are "like me" allows it to recognize the
equivalence between the physical and mental states apparent in others and those
felt by the self. For example, the infant uses his own experiences orienting
his head/eyes toward an object of interest to understand the movements of
others who turn toward an object, that is, that they will generally attend to
objects of interest or significance. Some researchers in comparative
disciplines have hesitated to put a too-ponderous weight on imitation as a
critical precursor to advanced human social-cognitive skills like mentalizing
and empathizing, especially if true imitation is no longer employed by adults.
A test of imitation by Alexandra Horowitz found that adult subjects imitated an
experimenter demonstrating a novel task far less closely than children did.
Horowitz points out that the precise psychological state underlying imitation
is unclear and cannot, by itself, be used to draw conclusions about the mental
states of humans.
Empirical investigation
Whether children younger than 3
or 4 years old may have a theory of mind is a topic of debate among
researchers. It is a challenging question, due to the difficulty of assessing
what pre-linguistic children understand about others and the world. Tasks used
in research into the development of ToM must take into account the umwelt—(the
German word Umwelt means
"environment" or "surrounding world")—of the pre-verbal
child.
False-belief
task
One of the most important
milestones in theory of mind development is gaining the ability to attribute false belief: that is, to
recognize that others can have beliefs about the world that are diverging. To
do this, it is suggested, one must understand how knowledge is formed, that
people’s beliefs are based on their knowledge, that mental states can differ
from reality, and that people’s behavior can be predicted by their mental
states. Numerous versions of the false-belief task have been developed, based
on the initial task done by Wimmer and Perner (1983).
In the most common version of the
false-belief task (often called the ‘Sally-Anne’
task), children are told or shown a story involving two characters.
For example, the child is shown two dolls, Sally and Anne, who have a basket
and a box, respectively. Sally also has a marble, which she places in her
basket, and then leaves to take a walk. While she is out of the room, Anne
takes the marble from the basket, eventually putting it in the box. Sally
returns, and the child is then asked where Sally will look for the marble. The
child passes the task if she answers that Sally will look in the basket, where
she put the marble; the child fails the task if she answers that Sally will
look in the box, where the child knows the marble is hidden, even though Sally
cannot know, since she did not see it hidden there. In order to pass the task,
the child must be able to understand that another’s mental representation of
the situation is different from their own, and the child must be able to
predict behavior based on that understanding. The results of research using
false-belief tasks have been fairly consistent: most normally-developing
children are unable to pass the tasks until around age four. (Notably, while
most children, including those with Down
syndrome, are able to pass this test, in one study, 80% of children
diagnosed with autism were unable to do so.)
Appearance-reality
task
Other tasks have been developed
to try to solve the problems inherent in the false-belief task. In the
"appearance-reality", or "Smarties" task, experimenters ask
children what they believe to be the contents of a box that looks as though it
holds a candy called "Smarties."
After the child guesses (usually) "Smarties," each is shown that the
box in fact contained pencils. The experimenter then re-closes the box and asks
the child what she thinks another person, who has not been shown the true
contents of the box, will think is inside. The child passes the task if she
responds that another person will think that there are "Smarties" in
the box, but fails the task if she responds that another person will think that
the box contains pencils. Gopnik & Astington (1988) found that children
pass this test at age four or five years.
Other
tasks
The "false-photograph"
task is another task that serves as a measure of
theory of mind development. In this task, children must reason about what is
represented in a photograph that differs from the current state of affairs.
Within the false-photograph task, there is either a location or identity
change. In the location-change task, the child is
told a story about a character that puts an object in one location (e.g.,
chocolate in a green cupboard) and takes a Polaroid photograph of the scene.
While the photograph is developing, the object is moved to a different location
(e.g., to a blue cupboard). The child is then asked two control questions,
"When we first took the picture, where was the object? Where is the object
now?" The subject is also asked a false-photograph question, "Where
is the object in the picture?" The child passes the task if she correctly
identifies the location of the object in the picture and the actual location of
the object at the time of the question.
In order to make tasks more
accessible for other animals, young children, and autistic individuals, theory
of mind research has begun employing non-verbal paradigms. One category of
tasks uses a preferential looking paradigm, with looking time as the dependent
variable. For instance, Woodward found that 9-month-old infants preferred
looking at behaviors performed by a human hand over those made by an inanimate
hand-like object. Other paradigms look at rates of imitative behavior, the
ability to replicate and complete unfinished goal-directed acts, and observations of rates of pretend play.
Theory of mind deficits
The theory of mind (ToM)
impairment describes a difficulty someone would have with perspective taking.
This is also sometimes referred to as mind-blindness. This
means that individuals with a ToM impairment would have a hard time seeing
things from any other perspective than their own. Individuals who experience a theory of mind
deficit have difficulty determining the intentions of others, lack
understanding of how their behavior affects others, and have a difficult time
with social reciprocity. ToM deficits have been observed in people
with autism
spectrum disorders,
people with schizophrenia, people
with attention deficit disorder, persons under the influence of alcohol and
narcotics, sleep-deprived persons, and persons who are experiencing severe
emotional or physical pain.
Autism
In 1985 Simon
Baron-Cohen, Alan M.
Leslie and Uta Frith published research which suggested that
children with autism do not employ a theory of mind, and suggested that children with autism have
particular difficulties with tasks requiring the child to understand another
person's beliefs. These difficulties persist when children are matched for
verbal skills (Happe, 1995, Child Development) and have been taken as a key
feature of autism.
Many individuals classified as
having autism have severe difficulty assigning mental states to others, and
they seem to lack theory of mind capabilities. Researchers who study the relationship between
autism and theory of mind attempt to explain the connection in a variety of
ways. One account assumes that theory of mind plays a role in the attribution
of mental states to others and in childhood pretend play. According to Leslie, theory of mind is the capacity to mentally
represent thoughts, beliefs, and desires, regardless of whether or not the
circumstances involved are real. This might explain why individuals with autism
show extreme deficits in both theory of mind and pretend play. However, Hobson
proposes a social-affective justification, which suggests that a person with autism
deficits in theory of mind result from a distortion in understanding and
responding to emotions. He suggests that typically developing human beings,
unlike individuals with autism, are born with a set of skills (such as social
referencing ability) which will later enable them to comprehend and react to
other people’s feelings. Other scholars emphasize that autism involves a
specific developmental delay, so that children with the impairment vary in
their deficiencies, because they experience difficulty in different stages of
growth. Very early setbacks can alter proper advancement of joint-attention
behaviors, which may lead to a failure to form a full theory of mind.
It has been speculated that ToM exists on a continuum as
opposed to the traditional view of a concrete presence or absence. While some
research has suggested that some autistic populations are unable to attribute
mental states to others, recent evidence points to the possibility of
coping mechanisms that facilitate a spectrum of mindful behavior.
Alcohol
use disorders
Impairments in theory of mind, as
well as other social-cognitive deficits are commonly found in people suffering
from alcoholism due to the neurotoxic effects of alcohol on the brain, particularly
the prefrontal
cortex region of
the brain.
Brain mechanisms
In
typically developing humans
Research on theory of mind in
autism led to the view that mentalizing abilities are subserved by dedicated
mechanisms that can (in some cases) be impaired while general cognitive
function remains largely intact. Neuroimaging research has supported this view, demonstrating
specific brain regions consistently engaged during theory of mind tasks. Early PETresearch on theory of mind, using
verbal and pictorial story comprehension tasks, identified a set of regions
including the medial
prefrontal cortex (mPFC),
and area around posteriorsuperior temporal sulcus (pSTS), and sometimes precuneus and amygdala/temporopolar
cortex (reviewed
in).
Subsequently, research on the neural basis of theory of mind has diversified,
with separate lines of research focused on the understanding of beliefs,
intentions, and more complex properties of minds such as psychological traits.
Studies from Rebecca
Saxe's lab at MIT, using a false belief versus false photograph task
contrast aimed to isolate the mentalizing component of the false belief task,
have very consistently found activation in mPFC, precuneus, and
temporo-parietal junction (TPJ), right-lateralized. In particular, it has been proposed that the
right TPJ (rTPJ) is
selectively involved in representing the beliefs of others. However, some debate exists, as some
scientists have noted that the same rTPJ region has been consistently activated
during spatial reorienting of visual attention; Jean
Decety from the
University of Chicago and Jason Mitchell from Harvard have thus proposed that
the rTPJ subserves a more general function involved in both false belief
understanding and attentional reorienting, rather than a mechanism specialized
for social cognition. However, it is possible that the observation of
overlapping regions for representing beliefs and attentional reorienting may
simply be due to adjacent but distinct neuronal populations that code for each.
The resolution of typical fMRI studies may not be good enough to show that
distinct/adjacent neuronal populations code for each of these processes. In a
study following Decety and Mitchell, Saxe and colleagues used higher-resolution
fMRI and showed that the peak of activation for attentional reorienting is
approximately 6-10mm above the peak for representing beliefs. Further
corroborating that differing populations of neurons may code for each process,
they found no similarity in the patterning of fMRI response across space.
Functional imaging has also been
used to study the detection of mental state information in Heider-Simmel-esque
animations of moving geometric shapes, which typical humans automatically
perceive as social interactions laden with intention and emotion. Three studies
found remarkably similar patterns of activation during the perception of such
animations versus a random or deterministic motion control: mPFC, pSTS, fusiform face area (FFA),
and amygdala were selectively engaged during the ToM condition. Another study presented subjects with an
animation of two dots moving with a parameterized degree of intentionality
(quantifying the extent to which the dots chased each other), and found that
pSTS activation correlated with this parameter.
A separate body of research has
implicated the posterior superior temporal sulcus in the perception of
intentionality in human action; this area is also involved in perceiving
biological motion, including body, eye, mouth, and point-light display motion
(reviewed in). One
study found increased pSTS activation while watching a human lift his hand
versus having his hand pushed up by a piston (intentional versus unintentional
action). Several studies have found increased pSTS
activation when subjects perceive a human action that is incongruent with the
action expected from the actor’s context and inferred intention: for instance,
a human performing a reach-to-grasp motion on empty space next to an object,
versus grasping the object; a human shifting eye gaze toward empty space
next to a checkerboard target versus shifting gaze toward the target; an unladen human turning on a light with his
knee, versus turning on a light with his knee while carrying a pile of books; and a walking human pausing as he passes
behind a bookshelf, versus walking at a constant speed. In these studies, actions in the
"congruent" case have a straightforward goal, and are easy to explain
in terms of the actor’s intention; the incongruent actions, on the other hand,
require further explanation (why would someone twist empty space next to a
gear?), and apparently demand more processing in the STS. Note that this region
is distinct from the temporo-parietal area activated during false belief tasks. Also note that pSTS activation in most of the
above studies was largely right-lateralized, following the general trend in
neuroimaging studies of social cognition and perception: also right-lateralized
are the TPJ activation during false belief tasks, the STS response to
biological motion, and the FFA response to faces.
Neuropsychological evidence has provided support for
neuroimaging results on the neural basis of theory of mind. A study with
patients suffering from a lesion of the temporoparietal junction of the brain (between the temporal
lobe and parietal
lobe) reported that they have difficulty with some theory of mind
tasks. This shows that theory of mind abilities are
associated with specific parts of the human brain. However, the fact that the medial prefrontal cortex and temporoparietal junction are necessary
for theory of mind tasks does not imply that these regions are specific to that
function. TPJ and mPFC may subserve more general
functions necessary for ToM.
Research by Vittorio
Gallese, Luciano Fadiga and Giacomo Rizzolatti (reviewed
in) has
shown that some sensorimotor neurons, which
are referred to as mirror
neurons, first discovered in the premotor
cortex of rhesus
monkeys, may be involved in action understanding. Single-electrode
recording revealed that these neurons fired when a monkey performed an action
and when the monkey viewed another agent carrying out the same task. Similarly, fMRI studies with human participants have shown
brain regions (assumed to contain mirror neurons) are active when one person
sees another person's goal-directed action. These data have led some authors to suggest
that mirror neurons may provide the basis for theory of mind in the brain, and
to support simulation theory of mind reading (see above).
However, there is also evidence
against the link between mirror neurons and theory of mind. First, macaque
monkeys have
mirror neurons but do not seem to have a 'human-like' capacity to understand
theory of mind and belief. Second, fMRI studies of theory of mind typically report
activation in the mPFC, temporal poles and TPJ or STS, but these brain areas are not part of the
mirror neuron system. Some investigators, like developmental psychologist Andrew
Meltzoff and
neuroscientist Jean
Decety, believe that mirror neurons merely facilitate learning through
imitation and may provide a precursor to the development of ToM. Others, like philosopher Shaun
Gallagher, suggest that mirror-neuron activation, on a number of counts,
fails to meet the definition of simulation as proposed by the simulation theory
of mindreading.
In autism
Several neuroimaging studies have
looked at the neural basis theory of mind impairment in subjects with Asperger
syndrome and high-functioning autism (HFA). The first PET study of theory of mind
in autism (also the first neuroimaging study using a task-induced activation
paradigm in autism) employed a story comprehension task, replicating a prior study in normal
individuals. This study found displaced and diminished
mPFC activation in subjects with autism. However, because the study used only
six subjects with autism, and because the spatial resolution of PET imaging is
relatively poor, these results should be considered preliminary.
A subsequent fMRI study scanned
normally developing adults and adults with HFA while performing a "reading
the mind in the eyes" task—viewing a photo of a human’s eyes and choosing
which of two adjectives better describes the person’s mental state, versus a
gender discrimination control. The authors found activity in orbitofrontal cortex, STS, and amygdala in normal
subjects, and found no amygdala activation and abnormal STS activation in
subjects with autism.
A more recent PET study looked at
brain activity in individuals with HFA and Asperger syndrome while viewing
Heider-Simmel animations (see above) versus a random motion control. In contrast to normally developing subjects,
those with autism showed no STS or FFA activation, and significantly less mPFC
and amygdala activation. Activity inextrastriate regions V3 and LO was identical across the two
groups, suggesting intact lower-level visual processing in the subjects with
autism. The study also reported significantly less functional connectivity
between STS and V3 in the autism group. Note, however, that decreased temporal
correlation between activity in STS and V3 would be expected simply from the
lack of an evoked response in STS to intent-laden animations in subjects with
autism; a more informative analysis would be to compute functional connectivity
after regressing out evoked responses from all time series.
A subsequent study, using the
incongruent/congruent gaze shift paradigm described above, found that in
high-functioning adults with autism, posterior STS (pSTS) activation was
undifferentiated while watching a human shift gaze toward a target and toward
adjacent empty space. The lack of additional STS processing in the
incongruent state may suggest that these subjects fail to form an expectation
of what the actor should do given contextual information, or that information
about the violation of this expectation doesn’t reach STS; both explanations
involve an impairment in the ability to link eye gaze shifts with intentional
explanations. This study also found a significant anticorrelation between STS
activation in the incongruent-congruent contrast and social subscale score on
the Autism Diagnostic
Interview-Revised, but not scores on the other subscales.
In 2011, an fMRI study
demonstrated that right temporoparietal junction (rTPJ) of
higher-functioning adults with autism was not selectively activated more for
mentalizing judgments when compared to physical judgments about self and other. rTPJ selectivity for mentalizing was also
related to individual variation on clinical measures of social impairment;
individuals whose rTPJ was increasingly more active for mentalizing compared to
physical judgments were less socially impaired, while those who showed little
to no difference in response to mentalizing or physical judgments were the most
socially impaired. This evidence builds on work in typical development that
suggests rTPJ is critical for representing mental state information,
irrespective of whether it is about oneself or others. It also points to an
explanation at the neural level for the pervasive mind-blindness difficulties in autism that are evident
throughout the lifespan.
Non-human theory of mind
As the title of Premack and
Woodruff's 1978 article "Does the chimpanzee have a theory of mind?"
indicates, it is also important to ask if other animals besides humans have agenetic endowment and social environment that allows them to acquire a
theory of mind in the same way that human children do. This is a contentious
issue because of the problem of inferring from animal
behavior the
existence of thinking, of the
existence of a concept of self or self-awareness, or of
particular thoughts. One difficulty with non-human studies of ToM is the lack
of sufficient numbers of naturalistic observation, giving insight into what the
evolutionary pressures might be on a species' development of theory of mind.
Non-human research still has a
major place in this field, however, and is especially useful in illuminating
which nonverbal behaviors signify components of theory of mind, and in pointing
to possible stepping points in the evolution of what many claim to be a
uniquely human aspect of social cognition. While it is difficult to study
human-like theory of mind and mental states in species whose potential mental
states we have an incomplete understanding, researchers can focus on simpler
components of more complex capabilities. For example, many researchers focus on
animals' understanding of intention, gaze, perspective, or knowledge (or
rather, what another being has seen). Call and Tomasello's study that
looked at understanding of intention in orangutans, chimpanzees and children
showed that all three species understood the difference between accidental and
intentional acts. Part of the difficulty in this line of research is that
observed phenomena can often be explained as simple stimulus-response learning,
as it is in the nature of any theorizers of mind to have to extrapolate
internal mental states from observable behavior. Recently, most non-human
theory of mind research has focused on monkeys and great apes, who are of most
interest in the study of the evolution of human social cognition. Other studies
relevant to attributions theory of mind have been conducted using plovers and dogs, and have shown preliminary evidence of
understanding attention—one precursor of theory of mind—in others.
There has been some controversy
over the interpretation of evidence purporting to show theory of mind
ability—or inability—in animals. Two examples serve as demonstration: first,
Povinelli et al. (1990) presented chimpanzees with the choice of two
experimenters from which to request food: one who had seen where food was
hidden, and one who, by virtue of one of a variety of mechanisms (having a
bucket or bag over his head; a blindfold over his eyes; or being turned away
from the baiting) does not know, and can only guess. They found that the
animals failed in most cases to differentially request food from the
"knower." By contrast, Hare, Call, and Tomasello (2001) found that subordinate chimpanzees were able
to use the knowledge state of dominant rival chimpanzees to determine which
container of hidden food they approached. William Field and Sue Savage-Rumbaugh have no
doubt that bonobos have developed ToM and cite their communications with a well
known captive bonobo, Kanzi, as
evidence.