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Infants' knowledge of their own species
Michelle Heron-Delaney, Sylvia Wirth and Olivier Pascalis
Phil. Trans. R. Soc. B 2011 366, 1753-1763
doi: 10.1098/rstb.2010.0371
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Phil. Trans. R. Soc. B (2011) 366, 1753–1763
doi:10.1098/rstb.2010.0371
Research
Infants’ knowledge of their own species
Michelle Heron-Delaney1, Sylvia Wirth2 and Olivier Pascalis3,*
1
University of Sheffield, Sheffield S10 2TP, UK
Centre de Neuroscience Cognitive, UMR5229, CNRS, 67 boulevard Pinel, 69675 Bron Cedex, France
3
Laboratoire de Psychologie et NeuroCognition, Université Pierre Mendès-France, Grenoble, France
2
Recognition of individuals at first sight is important for social species and can be achieved by
attending to facial or body information. Previous research suggests that infants possess a perceptual
template for evolutionarily relevant stimuli, which may include humans, dangerous animals
(e.g. snakes), but not non-dangerous animals. To be effective, such a mechanism should result in
a systematic preference for attending to humans over non-dangerous animals. Using a preferential
looking paradigm, the present studies investigated the nature of infants’ early representation of
humans. We show that 3.5- and six-month-old infants attend more to human beings than nonhuman primates (a gorilla or monkey) which are examplars of non-dangerous animals. This
occurred when infants were presented with head or body information in isolation, as well as
when both are presented simultaneously. This early preference for humans by 3.5 months of age
suggests that there is a basic representation for humans, which includes both head and/or body
information. However, neonates demonstrated a preference only for human faces over nonhuman primate faces, not for humans over non-human primates when the stimuli were presented
with both head and body simultaneously. The results show that although neonates display a preference for human faces over others, preference for the human body only develops later, in the first few
months of life. This suggests that infants have acquired some knowledge about the human body at
3.5 months of age that may have developed from their privileged experience with other humans in
the first few months of life, rather than an innate ability to detect humans in their entirety.
Keywords: face; development; species
1. INTRODUCTION
To survive in life threatening and in social situations,
individuals of any given species need to detect dangerous species, as well as conspecifics, and to discriminate
in-group and out-group members. In other words,
individuals need to categorize stimuli, the propensity
to group items into distinct morphological sets, and
to recognize them within a category. Recognition of
individuals at first sight is especially important for
social species, and may have been pivotal for the development of primate societies with strong social
relationships [1]. Some species recognize individuals
using their olfactory (e.g. hamster; [2]) or auditory
(e.g. birds; [3]) capabilities. Primates individuate
using their visual system. Recognition can be based
on several visual elements of an individual, such as
body shape or gait, but examination of the face leads
to the fastest and most accurate identification [4].
Before individual recognition occurs, categorization
occurs at a fast rate and may even facilitate it [5].
Indeed several studies have shown that categorization
occurs faster than recognition in human adults,
* Author for correspondence ([email protected]).
One contribution of 10 to a Theme Issue ‘Face perception: social,
neuropsychological and comparative perspectives’.
suggesting that it constitutes a separate process [6].
The rapidity of the process is likely to permit fast reaction to threatening stimuli like predators. Moreover,
such categorization may facilitate approaches of
non-threatening stimuli like conspecifics and further
individuation. Whereas we possess some understanding
about the development of face processing supporting
individual recognition, we know less about the development of whole body categorization. How and when do
human infants generate a representation of what a
human body looks like? In this paper, we are investigating if the representation that human infants have of
humans includes the whole body or not. Considering
the limitation of the visual system during the first few
months of life and the nature of the infant–carer interaction, which is usually face to face, it is sensible to
suggest that the whole human body representation
emerges after the face representation. Alternatively, it
is possible that, given its adaptive value, whole body representation is present from birth and is a hard-wired
ability passed through evolution.
In this paper, we will review research on adults’ and
infants’ representation of their own and other species.
We will also compare infant abilities for human face
processing to those for whole body processing. Finally,
we will present new experimental evidence of an early
representation of the human face and whole body
during infancy.
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This journal is q 2011 The Royal Society
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M. Heron-Delaney et al. Infants’ knowledge of their own species
2. REPRESENTATION OF HUMANS AND OTHER
SPECIES
Some stimuli in our environment require more attention than other stimuli as our survival may depend on
our ability to detect them. For example, harmful
stimuli such as snakes and spiders may require high
levels of attention, to avoid negative consequences
such as being bitten and poisoned. More generally,
monitoring non-human animals was of great importance to our ancestors because these animals could
potentially be either predators or food, or they
could be fellow humans of interest for social
purposes. Thus, given the adaptive value of categorizing other living animals, including humans, we can
hypothesize that humans, over evolution, developed
a system that affords privileged processing of living
things.
Animals change their status far more frequently
than other objects in our environment (e.g. plants,
artefacts), and thus require more frequent and
constant monitoring [7]. The animate monitoring
hypothesis proposes that the human attention system
evolved to differentially monitor animals/humans
versus other objects: animals and humans should
recruit more spontaneous attention. To test this
hypothesis, New et al. [7] used a change-detection
paradigm, in which adults were exposed to alternations
between complex natural scenes and duplicates with a
single change. Adults were substantially faster and
more accurate at detecting changes in animals and
people relative to changes in other familiar objects
(e.g. plants, buildings), including vehicles, which
adults have been trained for years to monitor to
avoid collision and death. In some experiments, there
was a further advantage for human detection over
other animals, such that accuracy was better (i.e.
more changes were detected) when the target was a
person than an animal. Kirchner & Thorpe [5] have
demonstrated that human participants can detect
animal presence in pictures even if they were presented extremely rapidly. Crouzet et al. [8] have
extended this work, demonstrating that participants
can elicit a saccade towards a human face in 100 ms!
Together, these studies suggest that the human
attention system evolved to differentially monitor
animals/humans, which are highly relevant objects
in our environment and potentially essential for
our survival.
In terms of harmful or negative stimuli, Seligman’s
[9] theory of prepared learning proposes that as
adults we most readily learn to fear classes of threats,
such as snakes, that were recurrent throughout evolutionary history [10]. Thus, it appears that some
stimuli (both positive and negative) are more relevant
to survival of a species than others, and adults are
tuned to attend to those stimuli accordingly. In the
context of human development, therefore, it is of
interest to investigate when these stimuli are first perceived as highly relevant or attention-worthy during
development.
From birth, infants face the difficult task of extracting relevant information from a complex world and
deciding what is essential to learn in order to survive.
One hypothesis about how infants categorize and
Phil. Trans. R. Soc. B (2011)
process stimuli is that mechanisms which have evolved
from our ancestral history will prime our attention
towards specific categories of stimuli and will be present early in development. Evidence supporting this
hypothesis comes from the finding that infants attend
to evolutionarily relevant stimuli that are dangerous.
Five month old infants look longer at a moving
image of a spider than at reconfigured or scrambled
versions of the same image. This pattern of responding
does not occur when infants are presented with a neutral stimulus (a flower) in an analogous task [11].
Similarly, 7– 18 month olds more readily associate a
fearful voice with a movie of a snake than other
moving objects [12]. Infants were simultaneously presented with two films—one of a snake and the other of
an exotic animal—accompanied by a recording of
either a very frightened or very happy human voice.
Infants looked longer at films of snakes while listening
to a frightened human voice than while listening to a
happy voice. These studies suggest that human infants
are primed towards evolutionarily relevant stimuli, in
this case towards detecting specific animals that were
potentially dangerous throughout evolutionary history.
This allows for rapid identification and facilitates
learning about harmful objects early in life.
Not all highly relevant evolutionary stimuli are
negative. Humans are essential for the infant’s survival
as well as for social interaction and comforting,
making them a positive evolutionary stimulus. Detecting and orienting rapidly to humans is therefore crucial
for infants. Infants attend to and have knowledge
about humans from birth. Newborns prefer to look
at typical faces rather than faces where the parts have
been scrambled [13,14]. Infants prefer to look at biological motion than non-biological motion during the
first week of life [15]. The preference for humans
also extends to the auditory domain: newborns prefer human speech than non-speech analogues (which
contain similar spectral and temporal parameters as
speech) [16]. Together, the above research strongly
suggests that infants are born with mechanisms
which draw their attention towards evolutionarily
relevant stimuli (humans) and predispose them to
learn about conspecifics.
How specific is the mechanism which recruits
attention to humans? Is it species-specific, or a broad
representation which also includes non-human animals? How early do infants recognize or treat their
own species differently?
A series of studies by Quinn et al. (e.g. [17])
demonstrates that
three-month-olds
represent
humans and non-human animals (cats and horses) differently. By three months of age infants can categorize
non-human animals on the basis of perceptual features. They can form a categorical representation for
domestic cats that excludes birds, horses, dogs, tigers
and female lions, this categorization being primarily
based on face/head information [18 –20]. This has
been demonstrated using the habituation – dishabituation procedure, which involves presenting infants
with exemplars from one category (e.g. dogs) until
they reach habituation criterion. Following this, two
exemplars are presented: a novel exemplar from the
already familiarized category (e.g. a new dog) and a
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Infants’ knowledge of their own species
novel exemplar from a novel category (e.g. a cat).
If infants can differentiate the two categories they
should look longer at the exemplar from the novel category. Using this procedure, it has been shown that
there is an asymmetry in three month olds’ categorization such that when habituated to horses or cats
infants will dishabituate to a human (i.e. they differentiate the two categories). However, when familiarized
with humans, infants do not dishabituate to a cat or
horse, and instead include cats and horses in their
category for humans [17]. This indicates that infants’
representation for humans is quite broad, based on
many exemplars, and thus can include other animals.
By contrast, the representation for other animals is
much narrower, based on a prototype; thus infants
do not include humans in their representation for
other animals [17].
Interestingly, this asymmetry in categorization is
only observed when infants are presented with whole
animal stimuli, not when provided with information
from just the head or the body alone of the exemplars
[21]. This suggests that young infants’ representation
for humans is based on the overall structure of the
stimuli (i.e. a head on top of an elongated body
with appendages) and contrasts with the finding that
representations for non-human animals appear to be
based on part/featural information such as heads
[18,19]. This leads to the question of how human
faces and bodies are processed when presented
separately.
3. BODY PROCESSING
(a) Adults
Recent brain imaging suggests that human bodies are
a special category of objects for adults, and that we
have specific brain areas dedicated for processing
human bodies [22]. In particular, the extrastriate
body area (EBA) in the lateral occipitotemporal
cortex [23,24] was shown to respond selectively to
images of the human body. Furthermore, the strength
of visual representations in the EBA is determined by
long-term visual experience, such that representations
are strongest for stimuli in their usual combinations
of visual field and side [25]. Another cortical area
that is active when bodies are being processed is the
fusiform body area (FBA), located in the lateral posterior fusiform gyrus. These two cortical areas appear
to have different functions within the domain of
processing bodies. The EBA seems to specialize in
processing the human body at the part level, whereas
the FBA appears to specialize in processing the body
in terms of its configuration as a whole [24,26].
Event-related potential (ERP) studies also suggest a
distinct neural representation of the human body.
Images of the human body (without a head) elicited
a negative ERP peaking at 190 ms (the N190). While
similar to the N170 for faces, the N190 differs in
both spatial distribution and amplitude, and peaks
later than the N170. Furthermore, scrambled bodies
do not elicit the N190 [27]. Taken together, these
studies suggest that there are dedicated neural mechanisms that process the human body in adulthood.
Phil. Trans. R. Soc. B (2011)
M. Heron-Delaney et al.
1755
An indicator of expertise which can be measured
behaviourally involves assessing whether objects are
processed holistically (i.e. viewed as a whole instead
of various parts) or configurally (i.e. processing spatial
relations between an object’s parts). Configural processing indicates expertise and can be assessed by
the ability to detect and recognize an object once it
has been inverted. The inversion effect indicates a disturbance in the recognition and processing of objects
which are processed configurally, since spatial relations
cannot be processed as easily once objects are presented upside down. Research indicates that human
bodies are subject to an inversion effect, such that
detection and recognition of postures and normal
human body configurations are disrupted if the
bodies are inverted, suggesting configural processing
[28,29]. This is in contrast with recognition of different houses, which are recognized equally well in
upright and inverted orientations. These studies
suggest that adults most easily detect and recognize
human bodies by analysing their configurations, and
that human bodies are a special class of objects for
adults owing to expertise and experience.
(b) Infants
Infants’ knowledge about the human body varies
widely depending on the information available to the
infants as well the nature of the task. The earliest
demonstration of knowledge related to humans is not
species specific. Infants demonstrate knowledge of biological motion from birth [15]. Newborns selectively
prefer to attend to biological motion (hen-walking
animations) and the preference is orientation specific:
newborns looked longer at upright displays than
upside-down displays of biological motion. This study
suggests that detection of biological motion is an
intrinsic capacity of the visual system, which is most
probably part of an evolutionarily relevant system
that applies to many species and predisposes animals
to attend to other animals.
The earliest evidence of specific knowledge about
the human body shape is demonstrated at four to six
months of age, when the human body model is real
and moving naturally [30]. In this study infants were
habituated to a series of different normal human
body postures and then presented with scrambled
human bodies (e.g. arms lowered to hips) on the test
trial. The human model was a real live human
moving naturally, who could be manipulated to look
both normal and scrambled with the aid of a curtain
and a second experimenter. Infants dishabituated to
the scrambled bodies, indicating recognition of typical
and scrambled bodies as distinct categories. However,
movement is crucial to early detection of violations of
the configuration of the body shape. When the same
habituation – dishabituation procedure was used with
the one key difference that the real human bodies
remained static, the earliest response to scrambled
static human bodies was at nine months of age [31].
In line with this, Zieber et al. [32] report that nine
month olds are sensitive to the relative proportions of
human body parts, while five month olds are not.
Infants were presented with pairs of photos depicting
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M. Heron-Delaney et al. Infants’ knowledge of their own species
a normal versus a proportionally distorted body
(created by lengthening the neck and torso and
shortening the legs). Nine month olds exhibited a
preference for the normal body when images were presented upright but not when they were inverted,
controlling for the possibility that infants were simply
responding to some low-level feature. Taken together,
these studies show emergence of knowledge about the
configuration of the human body shape in its static
form at nine months of age.
There is some evidence that sensitivity to the
human body shape may develop before infants
demonstrate this ability in visual tasks. Gliga &
Dehaene-Lambertz [33] report different ERP recordings for typical and scrambled bodies in threemonth-old infants. In this study, infants were
presented with images of human bodies where head
information had been removed. The mean amplitudes
for the scrambled bodies were significantly greater
than for the typical body images. This may indicate
discrimination of typical and scrambled bodies or it
may be that the infants were responding simply to
the low-level configuration information in the stimuli
(e.g. overall symmetry) without recognizing the
images as human bodies [33]. More research is
required before firm conclusions can be drawn.
4. FACE PROCESSING
(a) Adults
The core of the human neural system for face processing as revealed by functional MRI studies is of three
cortical areas: the inferior occipital gyrus (occipital
face area; OFA) that allows the creation of a global
stimulus based on the parts of a face; the middle fusiform gyrus (fusiform face area; FFA) supporting
recognition/discrimination of individuals and the
superior temporal sulcus that carries information relative to the direction of gaze, orientation and features of
the faces [34]. These three areas are more activated in
humans viewing human faces than other visual objects
[34– 36] with the lateral FFA showing the strongest
activation in the right hemisphere [37].
At the physiological level, there is evidence suggesting that we process human and other species’
faces differently. Face selective electrophysiological
activity has been observed in ERP (recorded from
the scalp) studies with adults, and consists in a negative deflection, with peak latency around 170 ms
after stimulus onset (N170). This potential tends to
be of larger amplitude and shorter latency for faces
than other objects [38]. It is influenced by stimulus
inversion as the N170 is of larger amplitude and
longer latency for inverted human faces compared
with upright human faces. This inversion effect is
particular to human face stimuli and has not been
observed for animal faces [39]. At a behavioural
level, Dufour et al. [40] showed that humans recognized human faces better than monkey faces in a
two alternatives forced choice task. Likewise, studies
conducted with a visual paired comparison task
where no instruction of recognition is given, showed
that humans discriminate automatically between two
human faces but not between two macaque faces [41].
Phil. Trans. R. Soc. B (2011)
Taken together, these elements provide solid
evidence that the adult face processing system is
somewhat species specific and lacks the flexibility to
allow recognition of faces of other species at an
individual level. How does such a highly specialized
face processing system in adults develop? What is the
impact of experience on its development?
(b) Infants
From the first moments of life, newborn infants prefer
to look at human faces over almost any other form of
stimuli ([13,14,42,43], suggesting the existence of a
specialized cognitive system operating from very early
in life. Johnson & Morton [44] have proposed that
this natural orientation and attentiveness towards
faces may be driven by CONSPEC—a subcortical
system containing very basic information regarding
the visual structural characteristics of members of
one’s own species. These structural characteristics
probably include a bounded area, an asymmetrical featural pattern with more elements on the upper portion
of the bounded area, and a positive stimulus contrast
[45]. On the other hand, CONLERN refers to a cortical system which accrues and retains fine details
regarding the visual characteristics of conspecifics via
experience with such conspecifics [44]. Thus, one
perspective holds that an initial biological predisposition to attend to faces is subsequently complemented
by visual experience with conspecifics to develop face
expertise. However, it should also be noted that
the existing evidence regarding infants’ preference for
the first-order configuration of schematic faces does
not directly implicate a preference for own-species
configuration. Thus, if neural mechanisms, such as
CONSPEC and CONLERN do exist, they might
not necessarily be species specific. In addition to face
preference, human infants also demonstrate evidence
of categorization and recognition [46].
Nelson [47] hypothesized that in humans the
representation of faces at birth is broad and develops
according to the type of facial input received, tuning
towards the predominant faces in the environment.
In term of categorization, Quinn et al. [48] found
that face representation in three-month-olds is biased
towards their primary carer. Infants primarily raised
by their mother prefer to look at female faces when
paired with male faces, whereas a population of three
month olds who had been primarily raised by their
fathers show a preference for male faces. Kelly et al.
[49] further highlighted the role of experience in
early infancy, demonstrating that three-month-old
Caucasian infants prefer to look at faces from their
own racial group when paired with faces from other
racial groups in a visual preference task. Caucasian
newborns tested in an identical manner demonstrated
no preference for faces from either their own or other
racial groups.
Growing evidence suggests that greater experience
with a particular face type leads to improved face processing abilities (e.g. better recognition abilities),
whereas a lack of experience with a particular face
type leads to relatively poorer face processing abilities
(e.g. poor recognition abilities). Pascalis et al. [50]
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Infants’ knowledge of their own species
M. Heron-Delaney et al.
1757
Figure 1. The stimuli presented to infants in experiments 1, 2 and 3.
investigated the ability of six- and nine-month-old
infants to recognize faces from their own species
(human) and those from other species (rhesus macaque) using a standard infant recognition paradigm.
As expected, infants at both ages were able to demonstrate recognition with human faces. However, when
tested with the monkey faces, only the six-month-old
group showed evidence of recognition, suggesting
that the face system becomes tuned to human faces
between six and nine months of age. This perceptual
narrowing process is akin to a similar phenomenon
in the language domain whereby younger infants are
able to discriminate almost all phonemes in any language in the world but later become particularly sensitive to phonemes of the language to which they receive
the most exposure in their environment [51,52].
It is possible to maintain accurate recognition
memory for monkey faces if six-month-old infants
are exposed regularly to other species’ faces [53].
However, the exposure should be associated with consistent individuation between such faces (i.e.
repeatedly referring to each monkey face by a specific
name; [54]). Such species-specific perceptual narrowing is also evident at the intersensory level by eight
months of age. Four to six-month-old infants demonstrated an ability to match a visual image of a monkey
producing a given vocalization with its congruent
auditory call, while eight-month-olds did not [55].
This perceptual narrowing in face perception at the
broader species level has also been found at the within
species level—that is, pertaining to different human
races. Kelly et al. [56] showed that Caucasian infants
at three months of age show recognition memory for
individual faces within Caucasian, Chinese, Middle
Eastern and African races, while nine-month-olds
only showed recognition memory for own-race Caucasian faces. In addition, Kelly et al. [57] showed that
Chinese infants undergo a similar course of perceptual
narrowing in that increased exposure to Chinese faces
leads to a recognition memory only for own-race
Chinese faces at nine months of age.
Only a few studies have investigated the electrophysiological response elicited by own versus other
species faces during infancy. De Haan et al. [39]
have found an ‘infant N170’ in six-month-olds that
Phil. Trans. R. Soc. B (2011)
was elicited by faces at a latency of 290 ms followed
by a positivity at 400 ms. They also examined the
influence of stimulus inversion, for both human and
monkey faces and found that in adults, inversion
affected only the processing of human faces and not
monkey faces, while in six-month-olds, inversion
affected the ERPs similarly for human and monkey
faces. Around 12 months of age, the adult ERP patterns are observed [58]. These results suggest that
humans possess an evolved system for processing
faces that becomes specialized as a consequence of
exposure exclusively to faces from a single species.
This review of the literature, though not exhaustive,
shows that at birth, human infants most probably
benefit from a perceptual system that allows them
to process a variety of ecologically relevant stimuli
selected over evolution. Longitudinal examination of
this system highlights its adaptive and plastic nature
enabling recognition to be best tuned to the precise
stimuli infants encounter in their environment. These
plastic properties are observed for different species,
body regions (faces versus the rest of the body) and
for face stimuli this plasticity applies at different superordinate levels (races, gender). In the following
sections of this paper, we will present experiments conducted in our laboratory aimed at understanding the
nature of infants’ representation of humans between
3 days and nine months of age. Previous literature
suggests that infants possess a perceptual template
for evolutionarily relevant stimuli, which may include
humans, dangerous animals (e.g. snakes), but not
non-dangerous animals. Such a mechanism should
result in a systematic preference for humans over
non-dangerous animals. However, other primates
share the same general body shape and face arrangement as humans; thus it is important to determine
if any preferences for humans are human or primate
specific.
This series of studies investigated whether infants
could discriminate humans from upright non-human
primates, and if there is a preference for humans
from birth and across the first six months of life (experiment 1). Next, we investigated whether infants could
discriminate humans from non-human primates using
only face information (experiment 2) or body
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M. Heron-Delaney et al. Infants’ knowledge of their own species
information only (experiment 3). Together, these
experiments take the first step to investigate infants’
early representation of human beings in an inter-species
framework.
5. EXPERIMENT 1: HEAD AND BODY
INFORMATION AVAILABLE
This experiment investigated whether or not there is
a preference to attend to humans over non-human
primates at birth, three months and six months of age.
Table 1. Summary of results for experiments 1, 2 and 3:
percentage of looking time to each category of stimuli.
human versus non-human primate
age
whole (%)
face (%)
body (%)
neonates
3.5
months
six
months
53 versus 47
70 versus
30**
67 versus
33***
58 versus 42*
60 versus
40**
66 versus
34***
not tested
61 versus
39**
62 versus
38***
*p , 0.05, **p , 0.01, ***p , 0.001.
(a) Methods
(i) Participants
Participants were recruited from the Royal Hallamshire Hospital, Sheffield, UK. In total, 14 full-term
neonates (eight females; age range 9 – 102 h old, M ¼
60 h), fourteen 3.5-month-old infants (seven females;
age range 105 – 114 days, M ¼ 110 days) and 16 sixmonth-old infants (eight females; age range 180 – 190
days, M ¼ 184 days) were included in the final
sample. A further six neonates were excluded from
the final sample due to fussiness (N ¼ 3) or falling
asleep (N ¼ 3), and seven 3.5-month-olds were excluded due to side bias (n ¼ 6) or fussiness (n ¼ 1).
Side bias was defined as looking at one image in the
pair for 95 per cent or more of the total looking time.
(ii) Stimuli
The stimuli were four colour photographs of humans
and non-human primates (figure 1). Pairs of one
male adult and one non-human primate were shown
to infants (two pairs in total). Non-human primates
included a monkey and a gorilla. Humans were presented wearing long-sleeved jackets and trousers so
that minimal skin was exposed. The human’s clothing
was coloured to match the fur of the non-human
primate with which it was paired. The human male
had closely shaved hair on his head. Photographs
were presented against a white background. In both
pairs the human and non-human primates were presented standing and matched on height.
(b) Testing procedure
Newborns were tested in a quiet room, seated in a
semi-upright position in a padded infant car chair,
which was secured to a table, limiting movement and
ensuring safety, approximately 40 cm from a screen
(measuring 30 45 cm) onto which the paired images
were projected. Infants of 3.5- and six-months-old
were tested in an anechoic chamber at the University
of Sheffield, UK. Infants were seated on their mother’s
lap approximately 60 cm away from a screen, which
displayed the images. For all age groups the experimenter remained out of sight during testing, and
both the mother and the experimenter remained
quiet. The two pairs of photographs were presented
until 10 s of cumulative looking had been obtained
for each pair. When projected onto the screen all
images measured 18 18 cm (168 visual angle) and
were positioned side by side separated with a 9 cm
gap. If the infant spent 10 s looking away from the
projected images, the trial was terminated. Between
Phil. Trans. R. Soc. B (2011)
the two image pairings, a blank screen was presented
for 3 –5 s. Images were counterbalanced for side.
A black and white CCD camera (specialized for
low light conditions) was used to film the infant’s eye
movements. This was displayed to the experimenter,
during recording, on an ITC control monitor. Time
was recorded and displayed on the control monitor
using a Horita II (TG-50) at 25 frames per second.
The film was subsequently digitized to be analysed
frame by frame on a computer using specialized software. An independent observer recoded 25 per cent
of the data for reliability. Both observers were blind
to condition. The average level of inter-observer
agreement was high (Pearson r ¼ 0.98 for 3.5- and
six-month-olds; r ¼ 0.85 for neonates).
(c) Results
Preliminary examination of the data revealed no
significant gender differences, so the data were combined for further analysis. A paired-samples twotailed t-test conducted on the total time spent looking
at the humans versus non-human primate stimuli
revealed that overall 3.5- and six-month-old infants
attended more to the humans than the non-human primates, t(13) ¼ 3.34, p ¼ 0.007 and t(15) ¼ 6.62, p ,
0.001 (table 1). In contrast, a paired-samples t-test
revealed that neonates did not look significantly
longer at the humans when compared with the nonhuman primates, t(13) , 1, p ¼ 0.530 (table 1).
A 2 3 (stimuli: human versus animal age: neonate, 3.5-, six-month-old) mixed model ANOVA was
computed on total looking time to the stimuli. There
was a significant main effect of stimuli F1,41 ¼ 41.68,
p , 0.001, h2 ¼ 0.49; however, this was subsumed
by a significant stimuli age interaction, F2,41 ¼
5.42, p ¼ 0.008, h2 ¼ 0.20. Follow-up independent
sample t-tests with a Bonferroni correction, adjusted
for 3 comparison (a ¼ 0.017) were conducted. t-tests
were computed on difference scores, which were calculated by subtracting the infant’s looking time to
the non-human primate from their looking time to
the human (thus positive scores reflect greater looking
to the human stimuli). Independent group t-tests
revealed that the difference scores for the 3.5- and
six-month-old infants were significantly different
from those of the neonates, t(26) ¼ 2.65, p ¼ 0.014
and t(28) ¼ 3.58, p ¼ 0.001, respectively. This indicates that while the 3.5- and six-month-old infants
had large difference scores due to preferentially attending to the humans, the neonates did not. There was no
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Infants’ knowledge of their own species
significant difference in the difference scores for the
3.5- and six-month-olds, t(28) , 1, p ¼ 0.641. This
is because both age groups had large difference
scores indicating greater looking to the human than
non-human primate stimuli.
The results of this experiment confirm that 3.5- and
six-month-old infants preferentially attend to humans
over other primates when the stimuli are presented
with head and body information. This ability is not
demonstrated at birth in the present study, at least
for humans in their entirety. This suggests that infants’
preference for humans at 3.5 months is at least partly
due to their rich experience with humans over the
first few months of life, relative to the very minimal
experience they have with non-human primates. The
null preference for the newborn infants is not surprising as newborns primarily see faces, with exposure to
only a select few other body parts (hands, breasts).
Given that a newborn’s vision is blurred beyond
40 cm and that their vertical field is approximately
1208, it allows a clear image of only 85 cm height,
which is smaller than an adult human body. This
means that newborn infants have very limited viewing
of human bodies in their entirety and this condition
likely accounts for the null preference observed. This
finding is consistent with research involving older
infants, which indicates that knowledge of human
faces and bodies follows different developmental trajectories, with the former evident from birth and the
latter not emerging until later in the first year [31].
Detailed body knowledge is not as important as
facial information, at least initially, from an evolutionary perspective. Facial information is more useful in
this respect because it enables us more easily to recognize individuals, identify gender and infer emotional
information such as mood states.
Therefore, some body parts may be more important
than others for discriminating humans versus nonhuman primates, at least early in development. One
might hypothesize that facial information is integral,
given that newborn infants demonstrate knowledge
about faces [13]. In the current study, the head
region of the pictures represented only a small proportion of the body, making it difficult for neonates
to use facial information to make the discrimination.
Experiments 2 and 3 test whether head or body information is necessary for newborn, 3.5- and six-monthold infants to discriminate humans versus non-human
primates.
6. EXPERIMENT 2: HEAD INFORMATION ONLY
This experiment investigated whether or not there is
a preference to attend to humans over non-human
primates in newborns and at 3.5 and six months of
age when only head information is available. Infants
demonstrate knowledge about faces from birth [13];
however, knowledge about the configuration of the
human body shape does not emerge until nine
months of age [31]. Thus, it may be that infants rely
on head information to discriminate human from
non-human primates, and that a preference for
human faces is responsible for the results obtained in
experiment 1.
Phil. Trans. R. Soc. B (2011)
(a)
M. Heron-Delaney et al.
1759
(b)
Figure 2. (a) A human face and (b) a monkey face, as used in
experiment 2 for neonates.
(a) Method
(i) Participants
Eighteen healthy, full-term newborn babies (11 boys
and seven girls, mean age 2.64 days, s.d. ¼ 1.3) were
selected from the maternity ward of the Jessop Wing
of the Royal Hallamshire Hospital, Sheffield, UK
and tested. A further 37 babies were selected but
removed from the study for the following reasons:
four babies failed to complete testing, 11 babies had
a strong side bias, 12 babies changed their state
during testing such that they were no longer alert,
and 10 babies because of an error on the part of the
experimenter. Infants of 3.5 and six months old were
recruited in an identical manner to experiment 1.
In total, 15 full-term 3.5-month-old infants (six females; age range 104 – 114 days, M ¼ 108 days) and
16 six-month-old infants (eight females; age range
180 – 190 days, M ¼ 185 days) were included in the
final sample. A further four 3.5-month-olds were
excluded due to side bias (n ¼ 2) or fussiness (n ¼ 2).
(ii) Stimuli
Neonates were presented with two pairs of stimuli
composed of a full face (i.e. external as well as internal
features were displayed), black and white photograph
of an adult male or a female, depicted from the
crown of the head to the jaw, and a full face, black
and white photograph of a monkey’s face, depicted
from the crown of the head to the jaw (figure 2). For
3.5- and six-month-olds the stimuli were photographs
depicting the four faces of the human and non-human
primate stimuli in experiment 1 (figure 1). Two pairs
of photographs (each containing a human and nonprimate human face) were presented to infants. The
human/gorilla faces were 14 10 cm and the human/
monkey faces were 10.7 8.5 cm.
(iii) Procedure
Infants were tested in a quiet room, seated in a semiupright position in a padded infant car chair, which
was secured to a table, limiting movement and ensuring safety, approximately 30 cm from a screen
onto which the paired images were projected. Presentation of the images on the left- and right-hand side
was counterbalanced. All aspects of stimuli presentation were identical to study 1. The average level of
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1760
M. Heron-Delaney et al. Infants’ knowledge of their own species
inter-observer agreement was high: Pearson r ¼ 0.87
for neonates and r ¼ 0.98 for 3.5- and six-month-olds.
(b) Results
A paired-samples two-tailed t-test conducted on total
looking time revealed that overall neonates, 3.5- and
six-month-old infants attended more to the human
faces than the non-human primate faces, t(17) ¼
2.24, p ¼ 0.03, t(14) ¼ 4.33, p ¼ 0.001 and t(15) ¼
4.93, p , 0.001, respectively (table 1).
The results of this experiment confirm that infants
preferentially attend to humans over other primates
when only face information is available. Therefore,
body information is not necessary for infants to discriminate between the two species. Further, it appears that
the presence of body information actually eliminates
the preference for human faces in newborns.
The neonatal preference for human faces indicates
that infants have learned something about human
faces during the first few days of life. However, the
stimuli used in this study do not allow us to determine
if the preference is based on a difference in contrast:
the human eyes have more contrast between the
sclera and the iris than the monkey eyes. This finding
needs to be interpreted cautiously until further studies
can rule out low-level perceptual features as an explanation for the present results. However, if infants are
indeed making the discrimination based on facial
information, this is consistent with previous research
showing that infants have a mechanism which draws
them towards human faces or allows newborn infants
to encode something about the appearance of the
human face and subsequently prefer images which
most closely match this template [13]. A neonatal
preference for human faces is also consistent with previous research showing knowledge of human faces
develops before knowledge about the human body
shape [59].
7. EXPERIMENT 3: BODY INFORMATION ONLY
This experiment investigated whether or not there is a
preference to attend to humans over non-human primates at 3.5 and six months of age when only body
information is available. The greater amount of time
spent looking at humans when the whole body is present (experiment 1) does not rule out the possibility
that the presence of the face drives the preference. It
is possible that infants’ early representation for
humans is restricted to a limited number of features
and does not take into account the whole body. This
would be consistent with research showing that infants
do not have detailed knowledge about human bodies
until at least nine months of age [31].
We did not test neonates in this condition as no preference was observed when neonates were presented
with head and body information simultaneously.
(a) Method
(i) Participants
Participants were recruited in an identical manner to
experiment 1. In total, 14 full-term 3.5-month-old
infants (six females; age range 104 –114 days, M ¼
107 days) and 16 six-month-old infants (eight females;
Phil. Trans. R. Soc. B (2011)
age range 180 – 190 days, M ¼ 184 days) were included in the final sample. A further seven 3.5-montholds were excluded due to side bias (n ¼ 5) or fussiness
(n ¼ 2).
(ii) Stimuli
The stimuli were the same four colour photographs
of humans and non-human primates used for experiment 1, the key difference being that the heads were
occluded using a grey strip so that only body
information was available (figure 1).
(iii) Procedure
Infants were tested in an identical manner to the 3.5and six-month-olds in experiment 1. The average level
of inter-observer agreement was high (Pearson r ¼
0.97).
(b) Results
A paired-samples two-tailed t-test conducted on total
looking time revealed that overall 3.5- and sixmonth-old infants attended more to the human
bodies than the non-human primate bodies, t(13) ¼
2.96, p ¼ 0.010 and t(15) ¼ 5.27, p , 0.001 (table 1).
The results of this experiment confirm that infants
preferentially attend to human bodies over nonhuman primates’ bodies even when face information
is unavailable. Therefore, infants’ demonstrate basic
knowledge about the human body shape from 3.5
months of age, at least in terms of preferring to
attend to it over other non-human primate bodies.
This suggests that development of knowledge about
the human body shape is gradual, with detailed knowledge about the configuration of the body shape
developing later in the first year of life [31]. Thus,
while young infants may not have a detailed representation of the human body (i.e. know where parts go)
they still recognize a human body when compared
with a non-human primate.
8. GENERAL DISCUSSION OF EXPERIMENTS 1–3
This series of experiments shows that at 3.5 months of
age, infants attend more to human beings than nonhuman primates (a gorilla or monkey) when presented
with head or body information in isolation, as well as
when both are presented simultaneously. The preference for humans observed by 3.5 months of age
suggests that infants already have a representation for
humans which is flexible and accurate enough to
allow recognition of humans even with the head information missing. This flexibility appears between birth
and three months, as neonates show a preference only
for the human head.
The fact that neonates are only tuned to faces and
not human bodies might have two main reasons.
First, face to face interaction is a privileged way of
communication with the carers, thus prioritizing face
rather than body processing. Second, neonates have
very little opportunity to view a human body in its
entirety because of limited vision at birth, allowing
infants to see clearly an image only 85 cm in height
from a 40 cm distance. This does not match a full
adult height, and once newborn infants are more
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Infants’ knowledge of their own species
than 40 cm away from a person the image will become
blurred. The infant could use eye movements to
expand their visual field; however, given that newborns
spend the vast majority of their time in the supine position, and have limited body and neck mobility, it is
unlikely that the newborn infant would have experience of viewing human bodies in their entirety.
These limitations fade as infants get older, allowing a
full and flexible representation of the body to
emerge. The exact timeline of this emergence remains
unknown as there was no longitudinal day to day
evaluation of this skill, but we can hypothesize that
the representation builds gradually as infants have
more and more interactions with other humans.
While a general body representation appears to be
learned in the first three months, neonates already
demonstrate a preference for human faces over nonhuman primate faces. This is consistent with previous
research showing that from birth, infants encode
something about the appearance of the human face
and subsequently prefer images which most closely
match this template [13]. The early face processing
system will allow infants to recognize others and possibly to convey and interpret emotions [46], which is
essential for bonding and attachment with their
carers. The exact mechanisms supporting this face
preference remain unclear and could include both
fast learning based on early interactions in the postpartum period as well as innate system selected by
evolution [46]. It has been suggested that infants
have a broad face processing system that narrows
with experience and support the hypothesis that
perceptual systems tune to stimuli present in the
environment [47,60]. This is supported by the fact
that infants show individual recognition abilities for
human and non-human primates until six months of
age, a faculty which can be maintained until nine
months with longer exposure to the other species
[50,53,54]. This is not in contradiction with our current results, as within species face recognition is one
level down from discrimination at the species level.
In other words, discriminating between two samples
from two species is less difficult than discriminating
between two samples within the same species. Our
finding denotes an adaptive behaviour that is consistent with studies demonstrating knowledge of human
faces and biological motion in the first few days of
life [13,61]. Our results are also in line with the theoretical framework of the human first hypothesis [62]
which proposes that infants possess information
about their conspecifics and use it to identify and
count objects.
Other species present a very similar adaptative
system from birth [63]. In adult rhesus monkeys,
faces processing is also a species specific system (see
[64] for a review). However, this species preference
can be reversed in favour of humans if monkeys first
experience human faces [63].
One intriguing finding of our experiment is that
although neonates discriminated humans versus nonhuman primates based on face information alone,
this behaviour disappeared when neonates were presented with whole body information. Rather than
reflecting an absence of discrimination of humans
Phil. Trans. R. Soc. B (2011)
M. Heron-Delaney et al.
1761
per se, this lack of preference is most probably owing
to the small size of the face when presented with the
body and/or to body information being distracting,
both preventing newborns from extracting sufficient
facial information to support discrimination. However,
these results show that newborns have not yet learned
a template of the human body that withstands size
reduction, and enables them to recognize humans
when scale information is altered. To decide on this
issue, experiments using stimuli projected in actual
size may be needed.
In contrast, 3.5-month-olds discriminate and prefer
a human body when compared with a non-human
primate body. Their human representation is strong
enough to overcome both size reduction and altered
information, as infants could discriminate human
headless bodies from a monkey’s body presented in
the same fashion. This quite robust discrimination
could be supported by a coarse representation of
human bodies which includes conspecifics, rather
than by a detailed one which would include knowledge
about the location of individual body parts. This
interpretation is supported by the findings that knowledge about the configural structure of the human
body was not demonstrated until nine months of age
[31]. Similarly, infants tested for their knowledge
about the relative proportions of human body parts
did not show accurate discrimination of body proportions until nine months [32]. Both of these tasks
are more challenging, and require more detailed
knowledge than simply recognizing that stimuli are
human or not, as was required in the current studies.
Nonetheless, at 3.5 months of age infants are able to
discriminate a human from a non-human primate
who is perceptually similar in terms of overall body
shape and facial structure. It is unclear which features
allow infants to make the discrimination; it could be
colour, texture or subtle differences in overall shape
and proportion. It would be of interest to run the
current experiments with 3.5- and six-month-olds
using eye tracking, to establish which features they
use to discriminate human and non-human primates
in each condition.
Whereas our results allow us to draw conclusions on
the presence of overall body representation at an early
age, future research should test for neonatal preferences for other human body parts such as hands,
which are presented within infants’ visual field on a
regular basis. This would further our understanding
on infants’ perceptions of relevant stimuli.
Preparation of this paper was supported by NIH grant NIH
R01 HD046526. Michelle Heron-Delaney was supported by
ESRC grant RES-000-22-3246.
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