This paper proposes that human expression of pain in the presence or absence of caregivers, and the detection of pain by observers, 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 amplitude 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 contingencies 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.

The facial expression of pain is the end product of a complex process that is, in part, emotional. The evolutionary study of facial expression must account for the social nature of human consciousness and should address the questions of why empathy exists, the adaptive importance of empathy, and whether facial expression is a mechanism of empathy and second-person consciousness.

Human pain experience and expression evolved to serve a range of social functions, including warning others, eliciting care, and influencing interpersonal relationships, as well as to protect from physical danger. Study of the relatively specific, involuntary, and salient facial display of pain permits examination of these roles, extending our appreciation of pain beyond the prevalent narrow focus on somatosensory mechanisms.

We argue that pain behaviour cannot be wholly accounted for within the operant model of Fordyce (1976). Many pain behaviours, including facial expression, are not socially reinforced but are evolutionarily predetermined. We urge researchers to take into consideration other learning accounts. Building on the idea that pain sufferers learn to suppress the expression of pain, we begin the development of a framework for a relational understanding of pain complaint.

What differentiates expressions of pain from other facial expressions? Which facial features convey the most information in an expression of pain? To answer such questions we can explore the expertise of human observers using psychophysical experiments. Techniques such as change detection and visual search can advance our understanding of facial expressions of pain and of evolved mechanisms for detecting these expressions.

Williams argues that humans have evolved special purpose adaptations for eliciting medical attention from others, such as a specific facial expression of pain. She also recognises that such adaptations would almost certainly have coevolved with adaptations for providing and responding to medical care. The placebo response may be one such adaptation, and any evolutionary account of pain must also address this important phenomenon.

A functional interpretation of facial expressions of pain is welcome. Facial expressions of pain may be useful not only for communication, such as inviting help. They may also be of direct use, as parts of writhing pain behavior patterns, serving to get rid of pain stimuli and/or to suppress pain sensations by something akin to hyperstimulation analgesia.

Williams and the many studies she considers appear to assume that voluntary amplification in facial expression of pain implies dissimulation. In fact, the behavioral ecology model of pain expression is consistent with amplification when subjects in pain are in the presence of others disposed to render aid, and that amplification may well be voluntary.

The proposal that there are specific adaptations for the expression and detection of pain appears premature on both conceptual and empirical grounds. We discuss criteria for the validation of a pain facial expression. We also describe recent findings from our lab on coping styles and pain expression, which illustrate the importance of considering individual differences when proposing evolutionary explanations.

Facial expressions of pain may be best conceptualized as an example of an evolved propensity to communicate distress, rather than as a distinct category of facial expression. The operant model goes beyond the evolutionary account, as it can explain how the (facial) expression of pain can become maladaptive as a result of its capability to elicit attention and caring behavior in the observer.

I agree with Williams that evolutionary theory provides the best account of the pain expression. We may disagree as to whether pain has an emotional dimension or includes discrete basic emotions as integral components. I interpret basic emotion expressions that occur contemporaneously with pain expression as representing separate but highly interactive systems, each with distinct adaptive functions.

Women typically report more pain than men, as well as exhibit specific sex differences in the perception and emotional expression of pain. We present evidence that sex is a significant variable in the evolution of facial expression of pain.

Facial responses to pain might be the result of evolution but Williams' interesting “Just So” story provides no convincing evidence for her hypothesis. Contrary to her hope, casting facial action in an evolutionary perspective will probably not reduce the common practice of health care professionals blaming patients for their problems; instead, it may discourage appropriate treatment.

Behaviorists accept, but go beyond, Williams' notion that there is an evolutionary origin to some unlearned pain behaviors. A behavior-analytic developmental model is a better fit for explaining the totality of pain behaviors. This model focuses on respondent-operant interactions and views much pain behavior as “mands” (i.e., demands). Behaviorally based explanations from the crying and social referencing literature support this model.

No adequate evidence exists for the evolution of facial pain expression and detection mechanisms, as opposed to social-learning processes. Although brain affective/emotional processes, and resulting whole body action patterns, have surely evolved, we should also aspire to monitor human suffering by direct neural measures rather than by more indirect indices.

An automated system for monitoring facial expressions could increase the reliability, sensitivity, and precision of the research on the relationship between facial signs and experiences of pain, and it could lead to new insights and diagnostic methods. This commentary examines whether the research on facial expression of pain, as reported by Williams, provides a sufficient basis for machine understanding of pain-associated facial expressions.

A monolithic model that ignores functional and topographic distinctions among its components has dominated clinical accounts of pain behavior. This model contributes to a pretence of knowledge that affects the treatment of sufferers. This commentary addresses the role of the target article in correcting knowledge-pretence and introduces a complementary caveat about evolutionary psychology concepts.

An ethological analysis suggests that effort and protection actions, which are expressions of distress, are comparable with pain expressions. Distress occurs with uncontrollable pain, and the expressions are ritualized pain responses with exaggerated features and lower thresholds. Pain is a sensory-motor feeling state with aversive motivational (hedonic) value. Distress is an emotional state of failure of pain responses to control the pain.

Evolutionary explanations of pain expression require modeling social adaptations in a context where the role of health professionals as potential caregivers, conflicts with their status as relative strangers. As signals of help elicitation or of alarm, facial pain displays and responses to displays, particularly in the upper face, are expected to conform to this evolutionarily novel clinical context.

As a product of natural selection, pain behavior must serve an adaptive function for the species beyond the accurate portrayal of the pain experience. Pain behavior does not simply refer to the pain experience, but promotes survival of the species in various and complex ways. This means that there is no purely respondent or operant pain behavior found in nature.