The insights provided by the application of evolutionary
psychology to established fields of evidence has clinical rel-
evance in areas such as fears, anxiety, and depression (Dim-
berg & Ohman 1996; Gilbert 1992; Marks & Nesse 1994).
Certain evolved behaviours, such as the attachment behav-
iour of human and other primate infants, are accepted as
orthodoxy. But broader notions of an evolved “human na-
ture,” that is, of specialised propensities
1
that generate
functional behaviours effective in the social and material
world, are still viewed with skepticism or indifference by
many in the psychological and medical communities (An-
dersson 1993; Schaffner 1998). Among the more important
reasons for this may be concern about genetic determinism
or, at least, a distaste for invoking genetic factors (Crawford
1989; Daly & Wilson 1988; Plomin 1989; Tooby & Cos-
mides 1992), and suspicion of “Just-So Stories,” that is, post
hoc explanations from available evidence (Gould & Lewon-
tin 1979). Far from endorsing the over-simple gene expla-
nations that are sporadically revisited in psychiatry (Plomin
1989), evolutionary psychology aims to broaden our under-
standing of behaviour, in all its flexibility and contingency
in relation to the immediate environment and the individ-
ual’s learning history, by addressing the cognitive adapta-
tions that are part of our heritage (Cronin 1991).
1. The function of pain
Pain is the “final mediator” of a wide range of selection
pressures (Walters 1994): sublethal injuries that may still
threaten survival or reproduction are incurred during pre-
dation, intraspecific combat, and competition with con-
specifics. By virtue of its aversiveness, pain serves to pro-
mote the organism’s health and integrity, to the extent that
congenital absence of pain on injury significantly shortens
human life (Damasio 1999; Wall 1999): “Suffering offers us
the best protection for survival” (Damasio 1994, p. 264).
The adaptive value of experiencing pain at first seems self-
evident: distinguishing harmful from harmless situations,
prompting avoidance of harm and its associated cues, giv-
ing a high priority to escape from danger, and promoting
healing by inhibiting other activities that might cause fur-
ther tissue damage (Bateson 1991). Research in animals has
focussed mainly on immediate escape from pain (e.g.,
Rachlin 1985), but according to Wall (1979; 1999), this is
BEHAVIORAL AND BRAIN SCIENCES (2002) 25, 439–488
Printed in the United States of America
© 2002 Cambridge University Press 0140-525X/02 $12.50
439
Facial expression of pain:
An evolutionary account
Amanda C. de C. Williams
Department of Psychological Medicine, Guy’s, King’s and St. Thomas’
Medical School, University of London; and INPUT Pain Management Unit,
St. Thomas’ Hospital, London SE1 7EH, United Kingdom.
amanda.williams@kcl.ac.uk
Abstract: This paper proposes that human expression of pain in the presence or absence of caregivers, and the detection of pain by ob-
servers, arises from evolved propensities. The function of pain is to demand attention and prioritise escape, recovery, and healing; where
others can help achieve these goals, effective communication of pain is required. Evidence is reviewed of a distinct and specific facial
expression of pain from infancy to old age, consistent across stimuli, and recognizable as pain by observers. Voluntary control over am-
plitude is incomplete, and observers can better detect pain that the individual attempts to suppress rather than amplify or simulate. In
many clinical and experimental settings, the facial expression of pain is incorporated with verbal and nonverbal vocal activity, posture,
and movement in an overall category of pain behaviour. This is assumed by clinicians to be under operant control of social contingen-
cies such as sympathy, caregiving, and practical help; thus, strong facial expression is presumed to constitute an attempt to manipulate
these contingencies by amplification of the normal expression. Operant formulations support skepticism about the presence or extent of
pain, judgments of malingering, and sometimes the withholding of caregiving and help. To the extent that pain expression is influenced
by environmental contingencies, however, “amplification” could equally plausibly constitute the release of suppression according to
evolved contingent propensities that guide behaviour. Pain has been largely neglected in the evolutionary literature and the literature on
expression of emotion, but an evolutionary account can generate improved assessment of pain and reactions to it.
Keywords: adaptation; evolutionary psychology; facial expression; pain
Amanda C. de C. Williams is Senior Lecturer in Clin-
ical Health Psychology at GKT Medical School, Uni-
versity of London, UK, and a consultant clinical psy-
chologist at a specialised treatment unit for chronic
pain. Her research interests are in depression in the
context of pain, and treatment of chronic pain and ways
of assessing its outcome. Attendance at evolutionary
psychology seminars encouraged her to address dis-
crepancies between behavioural formulations of pain in
the literature and clinical experience of patients. She is
currently on the Scientific Programme Committee of
the International Association for the Study of Pain.
less important than making recovery the overriding priority
after escape. The presence and intensity of pain are often
poorly related to the degree of tissue damage, making it too
late for prevention of injury, if not for future avoidance; nei-
ther escape nor avoidance would require that pain contin-
ued well into the recovery phase, demanding attention and
not habituating to any appreciable degree in humans (Ec-
cleston & Crombez 1999). The affective dimension of pain
appears to share mechanisms with vigilance to threat
(Chapman 1995; Crombez et al. 1998), and threat itself fa-
cilitates attention to pain (Eccleston & Crombez 1999).
The gate control theory (Melzack & Wall 1965) brought
about a paradigm shift in the study and understanding of
pain. It proposed that the pain signal following tissue dam-
age is modulated at each synapse, thus throughout its trans-
mission, by the balance of signals from the periphery and
from descending pathways originating in multiple sites in
the brain. This allowed for the influence on the signals and
their transmission of memories and prior learning; beliefs,
fears, and expectations; and emotional state. However,
these affective-motivational aspects have been sidelined in
subsequent research that has established a great deal more
about the sensory-discriminative dimension of pain; for in-
stance, aspects such as its quality, location, and intensity
which are largely determined by peripheral input (Chap-
man & Nakamura 1999; Craig 1999). Although there is de-
bate on the extent of anatomical separation of sensory-dis-
criminative and affective-motivational processing of pain in
the brain, there is consensus on the importance of recog-
nising the separate processes (Price 1999; Wall 1999). Clin-
ical and scientific focus, however, remains on pain sensation
and sensory discriminative processing, for a number of rea-
sons. Almost all experimental work is performed on ani-
mals, with most attention to quantification of nociceptive
stimuli and their local effects; some attention to a restricted
range of behaviours (largely escape and avoidance); and
none to emotion and cognition. Experimental work is
largely restricted to acute pain and to peripheral and spinal
mechanisms; although brain-imaging techniques offer rich
data, its interpretation lacks adequate models (Wall 1999).
“The careful sensory neurophysiologist who strays from the
spinothalamic pathway quickly becomes lost in a huge and
complex maze of reciprocal connections” (Chapman &
Nakamura 1999, p. 114).
Although the neurophysiological model performs far
better than its predecessors in building an understanding of
pain and of methods of analgesia, it casts little light on the
evolutionary function of pain and related behaviour. Pain
undoubtedly motivates to action (Damasio 1994; Frijda
1994; Hinde 1985; Wall 1999), serving as a “lever” for deci-
sion making and for action based on drives and instincts
(Damasio 1999). Behaviour following injury shows remark-
able consistency across species (Walters 1994), modified by
contextual variables such as the nature of the threat, the
severity or imminence of injury, its location, and the costs
of active defense. On the basis of accumulating evidence
about the activity of areas of the brain concerned with mo-
tor function, Wall (1999) proposes analysis of pain by syn-
thesis: that sensory inputs are analysed, classified, and iden-
tified by premotor systems in terms of motor actions
relevant to the input, with the priorities of first escaping the
stimulus, then limiting further damage and prioritising re-
covery, and then seeking safety and relief. However, it is
harder to adduce evidence for this from laboratory studies
in which possible behaviours are constrained, often condi-
tioned rather than unconditioned, and the widespread use
of electric shock as the noxious (but not tissue-damaging)
stimulus in research with laboratory animals raises ques-
tions of generalisability to injury-related pain (Panskepp et
al. 1997; Walters 1994). Outside the laboratory, there is a
dearth of observations of the behaviour of wounded mam-
mals (Fleckness & Molony 1997; Mench & Mason 1997;
Walters 1994), and what observations exist rely on the in-
terpretation of behaviour or changes in behaviour whose
function is not fully understood (Mench & Mason 1997).
Assessment methods for pain in domesticated and farm an-
imals are unstandardised (Fleckness & Molony 1997). The
extent of pain in animals soon after injury, in the escape or
active defence phase, is uncertain compared to its presence
later on when the animal is resting and protecting the in-
jured area (Wall 1979; 1999). This has led to models, such
as those of Bolles and Fanselow (1980) that locate the warn-
ing function in the emotional experience of threat and fear
in the early post-injury phase.
The emotion (affect) dimension of pain is therefore
largely absent from much pain research, but because pain
does not fit the paradigm of emotion (Ekman 1992; Izard
1991; Frijda 1994) it falls outside investigation of emotional
expression. By contrast, the definition of pain by the Inter-
national Association for the Study of Pain as “an unpleasant
sensory and emotional experience associated with actual or
potential tissue damage, or described in terms of such dam-
age” (IASP 1979) provides a central role for emotion. Out-
side the pain field it is rare to find pain described other than
as an aversive sensation associated with avoidance and es-
cape, even in evolutionary writing on adaptations: for ex-
ample, Nesse and Williams (1994) describe pain phenom-
ena in humans entirely in terms of design compromises for
defence. Imaging of pain processing in humans and clinical
lesion studies indicate distinct locations for encoding un-
pleasantness aspects (in the anterior cingulate) compared
to sensory aspects (in the somatosensory cortex) (Damasio
1994; Rainville et al. 1997), and for learned anticipation of
pain compared to actual pain (in different parts of the an-
terior cingulate cortex, the insular cortex, and the cerebel-
lum: Ploghaus et al. 1999). It is noted that all these areas
where pain is processed are close to areas involved in mo-
tor responses (Ploghaus et al. 1999; Rainville et al. 1997),
recalling Wall’s model of analysis of pain by synthesis with
possible motor responses.
In humans, emotional aspects have largely been investi-
gated by self-report, relying on consciously represented ex-
periences (such as fear) or consciously initiated action (such
as coping). The rich literature on cognitive content and
processes, including emotion, associated with pain, consists
of a bewildering array of associated concepts (such as sense
of control, beliefs about illness, coping attempts), few of
which bear any reliable relationship either with identifiable
cognitive processes or with specific behaviours. By contrast,
experimental work on attention and pain (Eccleston &
Crombez 1999), which includes methodologies that sample
nonconscious processes, complements the motivational
model: pain grabs attention, interrupts associated behav-
iour, and urges action toward mitigating it; the more intense
and threatening the pain, the more disruptive of attention
to anything else. Considerations of adaptive mechanisms
and function of pain are, however, rare in the pain field,
with some notable exceptions such as the work of Craig and
Williams: Facial expression of pain: An evolutionary account
440 BEHAVIORAL AND BRAIN SCIENCES (2002) 25:4
colleagues on facial expression, and of Crombez and Eccle-
ston and colleagues (Crombez et al. 1996; 1998; Eccleston
et al. 1997) and Chapman (1995) on cognitive mechanisms
and pain. In a broader cognitive context, theoretical mod-
els of consciousness that incorporate somatic experience
(Chapman & Nakamura 1999; Damasio 1999; Melzack
1999) provide the basis for integration of supraspinal pro-
cessing of pain, but still tend to under-represent the inter-
action of thoughts, beliefs, and emotional state with pain
(Keefe et al. 1996). An ideal model of pain would extend
the ecological validity of the current detailed understand-
ing of neurophysiology and integrate it with behavioural,
cognitive, and emotional dimensions. Models developed in
human research are diverse, with minimal theoretical inte-
gration between them or with sensory experimental work
(although cognitive and behavioural models are often com-
bined in treatment), and very little reference to function in
evolutionary terms. Some of those who have attempted in-
tegrated theories note that they struggle against the bias in
language (Sullivan 1995) and pervasive dualism (Chapman
1995; Morris 1998).
A major psychological model that has dominated pain
psychology in clinical settings for several decades is the op-
erant model proposed by Fordyce et al. (1968) described
more fully below. Although it is a theory of pain behaviour
rather than of pain, and explicitly builds on the gate control
model of pain, it is important because – consistent with
Skinnerian behaviourist principles – it asserted that pain
was unknowable, and that the proper focus of theory and
empirical work was observable and quantifiable behaviours
associated with pain (henceforth “pain behaviours”). In the
course of repeated episodes of pain, or with persistent pain,
unconditioned verbal, paraverbal, and motor responses
come under the control of external contingencies. These
behaviours contribute to or constitute aspects of disability.
Support for the concept was largely drawn from its behav-
ioural origins, specifically from positive associations of fre-
quencies of pain behaviours with availability of social rein-
forcement, and from accounts of treatment (Fordyce et al.
1968; 1981; Sternbach 1968). However, there are relatively
few tests of the covariation of pain behaviours with social
and instrumental reinforcement (Keefe & Dunsmore
1992; Turk & Matyas 1992), and clinical practice rarely es-
tablishes by observation the contingent relationships of be-
haviours targeted in treatment. Instead, the availability of
potential social reinforcement is taken to support the like-
lihood that pain behaviours observed in clinical settings are
operantly controlled. It seems to be widely overlooked that
many pain behaviours such as vocalisation of distress,
guarding, limping, and avoiding activity, are shared with
many other species in acute and chronic pain where there
is no suggestion that they are actually under operant con-
trol, although in some circumstances certain behaviours
can be so manipulated.
Behaviourism aims to deduce the working of the mind
from behaviour, but in the field of pain, the external con-
tingencies whose balance and likelihood determine the fre-
quency of behaviour become, by oversimplification and in
the absence of a theory of motivation, the reasons for that
behaviour. It has, however, contributed to the development
of an interactional and socially contextual formulation of
pain behaviour. Cognitive theory in pain represents behav-
iour as driven by beliefs and their emotional tone (such as
the fear-laden certainty that the processes causing pain
threaten physical integrity); expressive behaviour is con-
ceptualised as an epiphenomenon of internal experience, or
more often left to behavioural explanations. By contrast
with behaviourism, evolutionary psychology aims to under-
stand behaviour through a theory of mind, incorporating
cognitive, emotional, and motivational dimensions. Its fo-
cus is “behavioral control mechanisms [that] are organized
to process adaptively significant information and respond
thereto” (Daly & Wilson 1991, p. 163). But the two per-
spectives do not necessarily provide contrasting predictions
of observed behaviour. Environmental influences, from
cultural norms to interactions with particular individuals,
act on behavioural tendencies that are, already, the out-
come of contextual propensities to action, with reference to
the balance of costs and benefits (for the future of the indi-
vidual’s genes, not the individual) of possible behaviours in
a particular situation. Where possible, descriptions gener-
ated by behavioural and evolutionary theories are con-
trasted, but at this stage of development of the theory, no
opposing hypotheses are put up for test. The challenge is
not to all application of behavioural theory to the field of
pain behaviour, but to its weakness in explaining such be-
haviour particularly in clinical setting, and to its shortcom-
ings in relation to the interactive nature of pain expression,
as in the systematic biases evident in certain observer judg-
ments of pain.
This paper first addresses in greater detail the unsatis-
factory aspects of the global pain behaviour construct to de-
scribe clinically relevant behaviours, particularly those that
affect clinical decisions and pain sufferers’ well-being. It
then sketches the tenets of evolutionary psychology applied
to propensities for behaviour and the function of that be-
haviour. Germane areas of work include that of facial ex-
pression of emotion, as proposed by Prkachin (1997), and
particularly the work of Ekman and colleagues and of Frid-
lund. These are then applied to the evidence on facial ex-
pression of pain, studied systematically, and with close ref-
erence both to theory and to clinical dilemmas, by Craig
and LeResche and their colleagues. Although these re-
searchers make reference – particularly in work on infants’
expression, in consistency of expression (Prkachin 1992b),
and in reviews of the field (Craig et al. 1992; LeResche &
Dworkin 1984) – to a communicative facial expression as
the product of evolution and as a cue for help-giving (Anand
et al. 1999; Prkachin et al. 1983), their research questions
have not primarily been informed by evolutionary psychol-
ogy. Their studies constitute an invaluable canon in the field
largely overlooked by those studying facial expression of
emotion, and this article relies substantially on their find-
ings, reinterpreted in a broader context. The major issues
are the extent to which facial pain expression, wittingly or
unwittingly, constitutes a reliable communication of pain;
the factors influencing suppression and amplification; the
issue of simulation of pain expression; and detection and
the biases affecting observer judgment. This article con-
cludes with areas of particular difficulty for the approach
such as application in chronic pain, ecological validity of
methodology, and some clinical issues that urgently require
addressing.
However, this article comes with a warning. Of major
concern to the author and to colleagues in the pain field is
the risk that description of an evolved pain expression in
acute pain might be mistaken as offering a tool for distin-
guishing “real” from “faked” or “imagined” pain across all
Williams: Facial expression of pain: An evolutionary account
BEHAVIORAL AND BRAIN SCIENCES (2002) 25:4 441
circumstances. The patient as an unreliable reporter of ver-
ifiable internal events is blamed when the two do not cor-
respond, often resulting in treatment failure (Armstrong
1984). Psychoanalytic, personality, psychopathological, and
humanistic schools of psychology provide models that share
with psychiatry certain dualistic assumptions about deviant
expression of unmet needs through the unverifiable com-
plaint of pain. Similar misconstructions, to the considerable
detriment of patients, of behavioural signs of distress as
pathognomic of malingering arose following the publica-
tion of work describing physical manoeuvres that indicated
patient distress rather than pain intensity (Waddell et al.
1980). This article emphatically cannot provide a means of
identifying intentional simulation of pain.
2. Pain behaviour
Variants of the operant model are barely represented in the
pain field, which has developed with considerable consis-
tency from the original propositions by Fordyce (1976).
Pain behaviours are by definition observable, and many of
the contingencies for pain behaviours consist of social rein-
forcement such as sympathetic attention or help, but can
also be the socially sanctioned avoidance of unpleasant
events such as burdensome duties (Fordyce 1976). This
model identified as targets for treatment both the pain be-
haviours and their governing contingencies. For example,
the patient whose pain elicited the sympathy of family
members only when he drew attention to it, found that at-
tention withdrawn and given instead when he communi-
cated about other things than about his pain. Several ex-
perimental and clinical studies provide evidence of change
in the frequency of pain report (Block et al. 1980; Flor et
al. 1995; Linton & Gotestam 1985; White & Sanders 1986)
and in observed activity (Flor et al. 1987a; Ritchie 1976) in
relation to actual reinforcement, or the presence compared
to the absence of a source of social reinforcement. Although
these studies may explain substantial variance in the main-
tenance of one or more pain behaviours, and possibly, the
production of those behaviours in a social setting, we can
deduce little about etiology or function of these behaviours.
By contrast, two studies found a substantial proportion of
variance in pain behaviour during medical examination to
be attributable to medical findings such as number of pre-
vious surgeries or limitation of joint movement (Keefe et al.
1984; Novy et al. 1996), and several attempted replications
or part-replications of the study by Block et al. (1980) have
produced results inconsistent with operant predictions
(Lousberg et al. 1992; Paulsen & Altmaier 1995; William-
son et al. 1997).
More broadly, by design there was no place for emotion
within the model, nor for cognitive content or process in
generating or moderating behaviour, although learning his-
tory was identified as an influence on current behaviour.
With the growth of cognitive psychology, within and outside
the pain field, the associations between cognitive variables
such as beliefs and expectations, emotions such as fear and
distress, and behaviours including pain behaviours, are un-
der investigation. The importance of escape and avoidance
when threatened with increasing pain are now revisited in
a cognitive-behavioural formulation (Vlaeyen & Linton
2000). Of particular importance are vigilance to threatened
injury or reinjury, sustained through the recovery period
(Walters 1994), and fear-laden beliefs about threat and risk,
which bias interpretation of ambiguous stimuli (such as in-
creased pain or associated symptoms) and maintain vigilant
attention to pain-related cues (Craig 1999; Vlaeyen & Lin-
ton 2000).
The concept of pain behaviour and its use have been
subject to criticism (see, e.g., Keefe & Dunsmore 1992;
Melzack 1985; Turk & Matyas 1992; Wall 1985), including
the neglect of possible adaptive properties of various pain
behaviours (Turk & Matyas 1992), but this has not had the
impact it merits on the theory and investigation of pain be-
haviour. Despite the diversity of pain behaviours (verbal
complaint; vocal, facial and postural behaviour; and partic-
ular activities or their avoidance) suggesting differences in
function (Prkachin 1986; 1997), treatment and correla-
tional studies generally represented pain behaviour as a
unidimensional construct. Facial expression is the excep-
tion to this tendency, and excellent and detailed studies of
facial expression of pain allow re-examination of the data
using an evolutionary perspective. The relationship of facial
expression to other behaviours remains unexplored in pain.
Other behaviours, although they may augment or modify
the message in the face (Argyle 1988), may have other func-
tions: cancer patients who viewed their videotaped behav-
iour in the presence of a spouse (Wilkie & Keefe 1991;
Wilkie et al. 1992) mainly explained it as attempts to reduce
pain (e.g., by rubbing or support), or to control the display
of pain to attain superordinate goals such as remaining ac-
tive or not embarrassing others.
For observers, pain as a symptom presents particular dif-
ficulties, the more acute when, like clinicians, the observers
must make a judgment and decision based on their obser-
vation, because the problem presented may consist entirely
of self-report. There may be no consistent or proportional
evidence for a pain problem from other sources such as
clinical signs or investigations, because the changes are in
neurophysiological function rather than anatomical struc-
ture, as described earlier. Many injury-related pains outlast
healing; others are not attributable to injury, as in much
headache, visceral pain, and neuropathic pain. One of the
strengths of research on pain behaviour is that it addresses
the difficult issue of skepticism of observers about the cred-
ibility of pain, almost completely neglected by other psy-
chological perspectives. Observers generally trust nonver-
bal expression more than self-report (Craig et al. 1992;
Poole & Craig 1992). All behaviour is influenced by social
context: The important distinctions for the complaint of
pain are between subjective pain experience and its ob-
servable expression, and between influences on behaviour
arising from learned contingencies of response but not rep-
resented in conscious experience, and the conscious and
deliberate intention to manipulate those responses by par-
ticular behaviours. For example, an influential paper by
Waddell and colleagues (1980), which identified five signs
on physical examination any three of which indicated dis-
tress worthy of psychological assessment, described one of
the signs as “overreaction during examination” which “may
take the form of disproportionate verbalization, facial ex-
pression, muscle tension” (p. 119). Unless the physical ex-
amination is always conducted below the threshold of pain,
which rather vitiates its purpose, some facial expression of
pain is likely to be observed. It is interesting that of the five
signs, this was the most influential in surgeons’ judgments,
even though it was the most subjective; and that contrary to
Williams: Facial expression of pain: An evolutionary account
442 BEHAVIORAL AND BRAIN SCIENCES (2002) 25:4
the authors’ expectations, these signs failed to distinguish
patients with medicolegal or compensation cases (i.e., with
the prospects of gain from the judgment that they had se-
vere pain) from those without.
Although operant formulations did not generally equate
pain experience with pain behaviour (Fordyce et al. 1985),
they directed attention towards presumed social and mate-
rial benefits of being observed to be in pain (such as family
and medical attention, compensation and disability pay-
ments, and suspension of onerous duties) and away from
the costs and losses of being in pain, of more interest to
those who studied subjectivity and emotion. The explana-
tion of patients’ behaviour – the complaint of pain and un-
willingness to resume normal activities – by reference to as-
sumed benefits is adopted widely in clinical medicine, as an
apparently scientific way to express skepticism, and as a way
to explain the lack of association between the patient’s com-
plaint and the lack of confirming medical evidence that is
common in chronic pain.
The fact that facial expression conveys information does
not necessarily imply either conscious control before or
during the expression, or intentional signaling. Unfortu-
nately, definitions of pain behaviour, including facial ex-
pression, tend to use ambiguous terminology, which may
appear to describe deliberate communication: “The ob-
servable phenomena of pain consist essentially of some kind
of signal sent by the patient that he is in pain” (Fordyce et
al. 1968, p. 179, emphasis added); the “observable means of
communicating pain and suffering” (Turk et al. 1985). The
expressive individual may or may not be aware of this com-
munication. In fact, the only time we can be certain that ex-
pression is intentional is when it is posed, with or without
the associated experience, under instruction.
As described in the first section of this article, operant
mechanisms act on unconditioned pain behaviours. But
where the implicit assumption is of a tabula rasa at birth,
operant mechanisms become the exclusive explanation for
pain, invoked with minimal or no evidence. Prkachin et al.
(2001) comment critically on the wealth of speculation
about family members’ potential influence on the devel-
opment of pain compared to the dearth of hypotheses or
studies of how family members actually process pain in-
formation. The lack of research, even in animals, on
unconditioned pain responses, contributes to this bias in
the field. It remains to be seen whether definitive tests can
be designed where the processes can be observed working
separately or together. None of the studies reviewed in this
paper were undertaken with the intention of distinguishing
between contrasting predictions. Nor do any do so coinci-
dentally, other than the experimental finding that subjects
show more facial expression of pain when alone, than when
they believe themselves observed (Kleck et al. 1976) or are
observed by neutral strangers (Badali 2000): In the latter
study, self-report showed the same bias, but tolerance to
pain was the same across situations. At times, predictions
by different models coincide, for example, in anticipating
that the person in pain will show more vivid expression of
pain, whether through the release of suppression of pain ex-
pression in the presence of someone identified as a help-
giver (evolutionary explanation), or on the basis of previous
reinforcement of the expression by that person or class of
persons (operant explanation).
Nevertheless, this article reviews evidence collected in
the investigation of facial expression of pain in the light of
evolutionary theory. To varying extents in the different ar-
eas, evolutionary theory provides a superior explanation, or
provides one where operant theory is silent. The theory of
evolved pain expression predicts a distinct and specific ex-
pression of pain, across the lifespan, across pain stimuli, and
across cultures. By contrast, operant theory would antici-
pate pain expression shaped by contingencies that might be
specific to cultures and life stages. Although evolved pain
expression can be proposed only in the context of co-evo-
lution of its accurate detection by observers and its inter-
pretation in terms of approximate severity, there is no liter-
ature on the operant processes by which accurate detection
would be learned, and, again, it could be culture-specific.
In evolutionary terms, the ability to suppress pain expres-
sion would potentially be of survival advantage (as when
hiding pain from antagonists); but for the observer, whether
ally or antagonist, the ability to detect pain expression de-
spite attempts to suppress it would potentially have survival
advantage. The position is not so clear for the ability to am-
plify or simulate pain expression: Although it might bring
advantages for the individual in pain but at a fairly trivial
level of successful social cheating, for the onlooker, the abil-
ity to detect that pain has been amplified, or simulated in
its absence, could have important implications for spending
of precious resources. Operant theory would make far
stronger predictions about amplification and simulation:
On the assumption that more often than not pain expres-
sion brings benefits, the ability to simulate and amplify
would be rewarded, unlike the ability to suppress. Again, it
is not clear what operant theory would predict for the ob-
server’s detection of amplified and simulated pain expres-
sion.
3. Evolutionary psychology
An account is proposed that draws on evolutionary studies
of social behaviours. Certain pain behaviours, or rather the
propensities that underlie them, are likely to have been sub-
ject to natural selection insofar as they conferred advantage
on the individuals who used them, across contexts from the
presence of potential help-givers to that of known antago-
nists. Of necessity, vigilance to observed pain cues in others
coevolved. The individual expressing pain would derive
benefit if expression of pain were reliably followed by ac-
tions by observers that promoted recovery and survival;
protection from danger; and aid in obtaining basic require-
ments (Prkachin 1997; Prkachin et al. 1983). If the person
in pain might survive rather than not, and the cost to
helpers is low, selection advantage follows as with other
help and exchange of information – the currency of kin or
reciprocal altruism. Although the exploitation of others’
help achieves maximum payoff in a single encounter, indi-
viduals’ sensitivity to possible “social cheating” (i.e., to the
other taking a benefit without paying the costs) protects
them from repeated exploitation over a series of interac-
tions (Cosmides & Tooby 1992; 1997). The following para-
graphs expand this hypothesis and review supporting evi-
dence.
Recent development of evolutionary theory in the light
of genetics (neo-Darwinism) combines Darwin’s under-
standing of selection pressures with their application at a
genetic level, not on the individual or group (Dawkins
1976). Selection operates at the level of function, not at the
Williams: Facial expression of pain: An evolutionary account
BEHAVIORAL AND BRAIN SCIENCES (2002) 25:4 443
