In the mid-twentieth century, the behavioural sciences were dominated by an epistemological consensus asserting that human behaviour was almost entirely the product of socio-cultural conditioning. Influenced by the cultural determinism of Franz Boas and Margaret Mead, anthropological orthodoxy held that facial expressions were arbitrary communicative signs, functioning much like spoken dialects—learned, culturally specific, and devoid of innate biological programming. This social constructivist paradigm asserted that emotional displays varied endlessly across diverse geographical and sociological boundaries, leaving little room for universally shared affective mechanisms.
This dominant paradigm was fundamentally challenged in the late 1960s and early 1970s by the groundbreaking empirical investigations of clinical psychologist Paul Ekman and his primary research associate, Wallace V. Friesen. Building upon the long-marginalised evolutionary propositions of Charles Darwin and the theoretical affect system formulations of Silvan Tomkins, Ekman and Friesen sought to determine whether primary emotional states possessed an invariant, pan-cultural neuromuscular architecture. While their preliminary cross-cultural investigations within industrialized societies yielded high rates of cross-cultural agreement in emotional interpretation, these initial findings remained vulnerable to a formidable methodological counter-argument: the confounding variable of shared visual mass media, which could have systematically diffused Western facial communicative conventions across the globe.
To establish a definitive, unconfounded test of the evolutionary hypothesis, Ekman and Friesen conducted field expeditions into the remote interior of the Eastern Highlands of Papua New Guinea to study the Fore linguistic-cultural group. Geographically isolated, pre-literate, and largely unexposed to Western iconography, television, cinema, or commercial print media, the South Fore constituted a crucial test case for affective science. Published in 1971 in the Journal of Personality and Social Psychology, their seminal study, titled “Constants across cultures in the face and emotion,” provided compelling quantitative evidence that geographically and culturally isolated individuals could reliably identify specific emotional states from standardized facial configurations. This historical investigation altered the trajectory of affective psychology, revitalized evolutionary approaches to human behavior, and laid the empirical foundation for modern nonverbal communication science.
1. Historical and Theoretical Context of Emotion Research Pre-Ekman
1.1 The Darwinian Foundation of Evolutionary Affective Display
The conceptual origins of universal affective expression trace directly to Charles Darwin‘s 1872 evolutionary treatise, The Expression of the Emotions in Man and Animals. Following the publication of On the Origin of Species (1859) and The Descent of Man (1871), Darwin sought to demonstrate that human mental states, subjective experiences, and physiological displays were continuous with those of ancestral mammalian lineages. Rather than viewing the human face as an exclusively divine creation endowed with unique communicative musculature—a view prominently advocated by the anatomist Sir Charles Bell—Darwin argued that facial expressive behaviors represented functional, phylogenetically conserved motor patterns shaped by natural selection.
Darwin’s thesis rested on three mechanistic principles: the principle of serviceable associated habits, the principle of antithesis, and the principle of the direct action of the excited nervous system on the body. Under the principle of serviceable associated habits, complex expressive movements initially evolved because they served tangible somatic and regulatory functions during survival-critical encounters. For example, the facial configuration characteristic of disgust—marked by the retraction of the upper lip, wrinkling of the nasal bridge, and partial closure of the oral and nasal cavities—originated as an adaptive physiological reflex designed to prevent the ingestion or inhalation of noxious, pathogen-bearing substances. Similarly, the wide eyes and flared nostrils associated with fear optimized visual scanning and respiratory intake to facilitate swift evasive action.
Through comparative morphological analyses spanning domestic animals, non-human primates, and diverse human populations documented through early international questionnaires, Darwin assembled empirical observations illustrating cross-species homologies in muscular contraction. He reasoned that if facial expressions were innate biological adaptations rather than learned cultural conventions, they should be manifested uniformly across disparate human populations, including individuals who were congenitally blind and thus unable to acquire expressive gestures through imitation. Despite Darwin’s innovative use of photographic stimulus evaluation and comparative ethology, his insights were largely sidelined for nearly a century as twentieth-century social science swung decisively toward environmental determinism.
1.2 The Rise of Cultural Relativism in Mid-Twentieth-Century Anthropology
During the 1930s through the 1960s, anthropology and social psychology underwent a paradigm shift toward cultural relativism and strict environmental determinism. Spearheaded by cultural anthropologists such as Franz Boas, Margaret Mead, and Ruth Benedict, this intellectual movement conceptualized the human neonate as an infinitely malleable tabula rasa upon which local cultural conventions inscribed psychological reality. Physical gestures, emotional responses, and nonverbal expressions were categorized as semiotic codes, fundamentally analogous to linguistic syntax, exhibiting arbitrary variations across distinct societies.
This perspective reached its peak in the kinesics framework formulated by Ray Birdwhistell. Birdwhistell contended that there were no universal gestures or body motions possessing innate affective meaning. In his influential volume Kinesics and Context (1970), he argued that bodily movements and facial displays were structured by culture-specific communicative systems acquired through socialization. Birdwhistell posited that an expression such as a smile did not necessarily index internal subjective pleasure; rather, its structural configuration, social deployment, and emotional interpretation were entirely context-dependent and governed by local norms. He documented ethnographic instances where smiles were exhibited during situations of grief, embarrassment, or social appeasement, interpreting these contextual variations as proof that nonverbal behavior lacked fixed biological anchors.
Concurrently, behaviorism maintained a dominant position within experimental psychology. Researchers operating under the radical behaviorist framework of B.F. Skinner dismissed internal affective states as unobservable mentalistic epiphenomena, focusing instead on stimulus-response contingencies and operant conditioning. Emotional behaviors were understood as conditioned motor responses shaped by external reinforcement schedules. The prevailing academic consensus held that any cross-cultural similarities in facial actions were trivial byproducts of universally shared social learning environments, rather than the manifestation of evolved, neurobiological affect programs.
1.3 Silvan Tomkins and the Revival of the Primary Affect Paradigm
The revival of the biological paradigm within affective science was largely catalyzed by the theoretical contributions of Silvan S. Tomkins. In his multi-volume work Affect Imagery Consciousness (1962, 1963), Tomkins introduced an evolutionary theory of human motivation that placed the affect system at the center of human experience. He argued that affects constituted the primary biological motivating mechanism, operating as an autonomous, innate system distinct from homeostatic drives such as hunger, thirst, and sex. While drives were temporally constrained and biologically localized, affects provided generalized, amplified responses to environmental stimuli, guiding behavior through distinct emotional profiles.
Central to Tomkins’s theoretical architecture was the somatic feedback hypothesis, which posited that the subjective experience of emotion was driven by feedback from the neuromuscular activation of the face. Tomkins asserted that the face was the primary organ of the affect system, possessing an extraordinarily dense concentration of motor units and sensory receptors capable of generating rapid, differentiated signals. According to his formulation, subcortical neural programs were triggered by changes in the density of neural stimulation, activating hardwired motor patterns that mobilized specific facial muscles, vascular responses, and vocalizations. He classified these primary affects into discrete categories, including interest-excitement, enjoyment-joy, surprise-startle, distress-anguish, fear-terror, shame-humiliation, and disgust-contempt.
Tomkins exercised a direct intellectual influence on the field by actively mentoring two young experimental psychologists: Paul Ekman and Carroll Izard. Recognizing that his theoretical claims remained largely speculative in the absence of rigorous empirical validation, Tomkins challenged both researchers to subject the universal affect hypothesis to systematic, falsifiable cross-cultural testing. He emphasized that resolving the debate between evolutionary universalism and cultural relativism required collecting data from pre-literate, isolated societies that had developed completely outside the sphere of Western media and globalized communicative norms.
2. Scholarly Background and Scientific Trajectory of Ekman and Friesen
2.1 Paul Ekman’s Methodological Evolution
Paul Ekman entered nonverbal communication research through the lens of clinical psychology and psychodiagnostic assessment. Having completed his clinical training at Adelphi University and an internship at the Langley Porter Neuropsychiatric Institute, Ekman was initially interested in developing objective behavioral markers to assess psychiatric patients, specifically focusing on nonverbal indicators of depression, psychomotor agitation, and deception. His early doctoral and postdoctoral research, supported by grants from the National Institute of Mental Health (NIMH), examined body movements, hand gestures, and posture as behavioral reflections of intrapsychic conflict and psychopathology.
Influenced by his interactions with Silvan Tomkins, Ekman’s focus expanded from clinical motor behaviors to the systematic investigation of facial expressions of emotion. In the mid-1960s, Ekman began designing comparative international studies to evaluate the cross-cultural recognition of emotional faces. Working alongside colleagues, he administered forced-choice judgment tasks using photographs of posed facial expressions to university students in five modern, literate nations: the United States, Japan, Brazil, Chile, and Argentina. The results revealed striking consensus across these disparate nationalities, with observers consistently mapping the same facial configurations onto specific emotion labels such as happiness, anger, fear, sadness, disgust, and surprise.
Despite the statistical strength of these initial findings, Ekman recognized an inherent methodological limitation that left his conclusions open to challenge by cultural relativists. All five nations investigated were industrialized societies saturated by shared transnational media, international magazines, Hollywood films, and commercial advertisements. Relativist critics, most notably Margaret Mead and Ray Birdwhistell, argued that the observed consensus was not proof of evolved biological universals, but rather evidence of cultural diffusion: individuals worldwide had simply learned to interpret Western facial conventions through repeated exposure to imported visual mass media. To definitively refute this counter-argument, Ekman recognized that he had to establish rigorous falsification protocols by conducting research within a society completely unexposed to global visual media.
2.2 Wallace V. Friesen’s Contribution to Nonverbal Coding
The scientific partnership between Paul Ekman and Wallace V. Friesen was critical in transforming the study of nonverbal behavior from qualitative, subjective interpretation into a rigorous, quantitatively reproducible empirical discipline. Joining Ekman at the Laboratory for the Study of Human Interaction and Conflict at the University of California, San Francisco (UCSF), Friesen brought a methodical approach to psychometric measurement, behavioral taxonomy, and statistical verification. While Ekman frequently served as the primary theoretical architect and public advocate of their research, Friesen focused on the operationalization of fine motor actions and the establishment of objective observational criteria.
Together, Ekman and Friesen developed experimental protocols to eliminate observer bias and control for stimulus artifacts. Prior to their work, much of the research on facial expressions had relied on unstandardized, low-resolution photographs displaying ambiguous or idiosyncratic poses. Friesen and Ekman standardized these materials by curating photographic sets that were calibrated against anatomical indices of emotional expression. They developed rigorous inter-rater reliability protocols, ensuring that facial displays were systematically characterized by independent observers blind to the experimental hypotheses.
Friesen was instrumental in developing the stimulus banks and testing protocols deployed in their cross-cultural field expeditions. His attention to methodological rigor was vital in adapting standardized laboratory procedures into robust, portable field methodologies capable of withstanding the challenging logistical realities of the New Guinea highlands. This systematic approach laid the groundwork for their subsequent development of the Facial Action Coding System (FACS), cementing Friesen’s reputation as a co-architect of modern affective behavioral science.
3. The Fore Linguistic-Cultural Group: Selection and Field Setting
3.1 Geographic and Sociological Isolation of the South Fore
To establish a definitive test of the universal affect hypothesis, Ekman and Friesen required an exceptional research demographic: a population that had remained entirely unexposed to Western visual media, possessed no written language, lived in severe geographic isolation, and had maintained traditional horticultural practices without integration into modern socioeconomic systems. They found these conditions among the Fore people residing in the Okapa District of the Eastern Highlands Province of Papua New Guinea.
Until the mid-twentieth century, the rugged interior highlands of New Guinea remained one of the few regions on Earth largely uncontacted by European explorers and colonial administrations. The South Fore, dwelling within steep, forested mountainous terrain, practiced subsistence slash-and-burn horticulture complemented by pig husbandry and forest foraging. Their social structure was decentralized, organized around patrilineal descent groups and fortified village hamlets. Geographically barricaded by high mountain ridges and river gorges, the South Fore had virtually no contact with external industrial societies prior to the mid-1950s, when the Australian colonial administration established minimal administrative patrols.
Crucially for the purposes of the investigation, the Fore linguistic-cultural group possessed no written script and had zero access to modern mass communication technologies. There were no commercial movie theaters, television transmissions, magazines, newspapers, or packaged consumer goods bearing pictorial advertisements. Furthermore, the vast majority of South Fore individuals had never traveled outside their immediate ancestral valleys, remaining entirely unfamiliar with English or other non-indigenous languages. They represented a genuine test case: an isolated population whose visual and communicative semiotics had developed independently of Western cultural diffusion.
3.2 Anthropological Rationale for Participant Cohort Stratification
The selection of the Fore as the primary field site was facilitated by Ekman’s collaboration with D. Carleton Gajdusek, a medical anthropologist and virologist at the National Institutes of Health. Gajdusek was conducting research in the Eastern Highlands investigating kuru, a fatal neurodegenerative prion disease endemic among the Fore, for which he would subsequently receive the Nobel Prize in Physiology or Medicine in 1976. Gajdusek’s established field research infrastructure, extensive longitudinal film archives, and deep social relationships with local Fore communities provided Ekman and Friesen with safe logistical access to this exceptionally remote region.
To construct an internally valid experimental design, Ekman and Friesen stratified their participant sample based on degree of contact with Western civilization. Working alongside field anthropologists and native bilingual guides who knew individual life histories within the villages, the researchers instituted strict screening criteria to categorize participants into two distinct cohorts: the “most isolated” traditional Fore and a comparative cohort of “Western-contacted” individuals.
The “most isolated” cohort was restricted to individuals who met rigorous exclusion criteria:
- They had never seen a movie, motion picture, or pictorial projection;
- They had never attended a Western mission school, government school, or acquired literacy in any language;
- They had never resided or worked on coastal plantations, mining centers, or outside their highland territory;
- They spoke only the indigenous Fore language or adjacent highland dialects, with zero proficiency in English or Tok Pisin (the regional Neo-Melanesian lingua franca);
- They had never lived in close proximity to European administrative officers, medical workers, or Christian missionaries.
By isolating these variables, Ekman and Friesen effectively controlled for linguistic drift, cultural diffusion, and media-based visual learning, ensuring that any systematic consensus in emotion decoding could not be attributed to Western acculturation.
4. Experimental Design and Field Research Methodology
4.1 Overcoming Cross-Linguistic and Literacy Barriers
Administering standardized psychological experiments in an illiterate, pre-industrial highland community required a fundamental overhaul of standard psychometric methods. In conventional cross-cultural testing, experimental psychologists typically presented respondents with a visual image of a face paired with a list of abstract emotion labels (such as “anger,” “sadness,” or “fear”) or utilized semantic differential Likert scales. These classical methodologies were unworkable among the South Fore, who had no tradition of literacy, possessed limited experience with paper-and-pencil tasks, and lacked lexical equivalents for some abstract English emotion nouns.
To overcome these methodological challenges, Ekman and Friesen adapted an experimental paradigm originally devised by John Dashiell in 1927 for assessing emotional comprehension in young children. Rather than requiring participants to select an abstract semantic label, the Dashiell method embedded emotional constructs into concrete, evocative narrative scenarios. Each narrative described an antecedent event designed to elicit a specific emotional state within the local ecological and cultural milieu.
The translation and back-translation of these narrative vignettes required rigorous linguistic procedures. Ekman worked closely with bilingual South Fore individuals who were fluent in Tok Pisin and indigenous dialects. Initial narrative concepts were drafted, translated into Fore, and then independently back-translated into English by separate translators to verify that the core affective themes were preserved without introducing Western linguistic idioms. Furthermore, the field experimenters were trained to maintain neutral vocal tones and avoid unconscious nonverbal cues, such as gaze steering or subtle posturing, that might bias the respondents’ choices during trials.
4.2 The Two- and Three-Alternative Forced-Choice Story Method
The operational protocol implemented by Ekman and Friesen relied on a forced-choice matching-to-sample paradigm. In a standard adult experimental trial, the participant sat before the researcher and an indigenous translator. The experimenter arranged three standardized photographic cards, measuring approximately 3 × 5 inches, mounted side-by-side upon a neutral cardboard display frame. The spatial position of the target photograph was systematically randomized across trials to prevent positional response biases.
Once the visual stimuli were displayed, the translator read a brief, standardized story conveying an emotional antecedent. The participant was instructed to listen carefully to the narrative and then point to the individual whose face showed the emotion described in the story. A typical trial sequence proceeded as follows:
- Happiness antecedent: “His (or her) friends have come, and he is happy.”
- Sadness antecedent: “His (or her) child has died, and he feels very sad.”
- Anger antecedent: “He is angry, and he is about to fight.”
- Disgust antecedent: “He is looking at something he dislikes, or something that smells bad.”
- Surprise antecedent: “He is looking at something new and unexpected.”
- Fear antecedent: “He is sitting in his house all alone, and there is no one else in the village. There is no food and no place to hide. Suddenly, a wild boar comes running into the house, and he has no weapon.”
The participant’s response was recorded through a direct physical pointing gesture toward the selected photograph, eliminating the need for any verbal or written output from the subject.
For pediatric participants, the experimental design was modified to reduce cognitive load. Rather than presenting three simultaneous photographic stimuli, child subjects were administered a two-alternative forced-choice task, comparing one target emotion against a single distractor photograph. This developmental adaptation prevented task fatigue and ensured that children could maintain attention across the testing battery.
4.3 Sample Composition and Stratification Variables
The empirical study published in 1971 evaluated a substantial sample size given the logistical constraints of the field setting. The total experimental cohort comprised 342 individuals from the Fore community, divided into systematically stratified adult, child, and acculturated comparison groups:
The primary experimental cohort was composed of 189 adult Fore participants (68 males and 121 females) who met the strict criteria for minimal Western contact. These individuals represented the critical test group, possessing no literacy, no cinema exposure, and negligible interaction with external administrative or religious entities.
To evaluate the ontogenetic development of emotional decoding, the researchers also tested 130 Fore children (67 boys and 63 girls). These children had never attended school, could not read or write Fore, English, or Tok Pisin, and had experienced no contact with Western media or travelers.
Finally, to determine whether exposure to Western culture altered recognition profiles, a comparative group of 23 Westernized adult Fore was evaluated. These individuals had attended Westernized mission or government schools, had learned to speak English, lived in commercial plantation labor settlements, or worked in active contact with European administrators.
The inclusion of balanced gender cohorts within both the isolated adult and pediatric groups enabled the researchers to test for sex differences in emotional decoding accuracy within a traditional society. The overall sample was sufficient to yield meaningful statistical power, allowing Ekman and Friesen to test whether the observed choices deviated significantly from chance expectations across all emotional categories.
5. Stimulus Selection and Standardization Protocols
5.1 The Visual Photographic Stimulus Bank
The visual materials utilized in the Fore field study required precise anatomical and technical calibration. Ekman and Friesen recognized that using unstandardized stimuli could compromise the internal validity of the experiment. Therefore, the photographic stimulus bank consisted of a carefully curated set of portraits displaying posed facial configurations that had been empirically validated in prior investigations across modern, literate societies.
The stimulus array featured photographic portraits of 24 separate Caucasian adult actors (both male and female) whose expressions represented six targeted basic emotions: happiness, sadness, anger, disgust, surprise, and fear. Neutral control portraits were also incorporated. The facial displays were systematically verified to ensure that they corresponded to specific, reproducible muscular movements, foreshadowing the anatomical framework later codified in the Facial Action Coding System.
Technical aspects of the photographs were rigorously standardized to prevent extraneous visual artifacts from influencing participant selections:
- All portraits were captured under uniform, diffuse lighting conditions to eliminate dramatic shadow casting;
- Subjects were oriented directly forward in full frontal view, maintaining a direct gaze toward the camera lens;
- All images were rendered in black-and-white, mitigating potential chromatic distractions;
- The backgrounds were neutral and devoid of environmental or context-dependent markers;
- Actor clothing was cropped or masked out to prevent socio-cultural or temporal dress cues from biasing choices.
Presenting images of Caucasian faces to Fore participants represented a conservative methodological test: if isolated New Guinea highlanders could accurately decode subtle emotional expressions on the faces of an unfamiliar racial and morphological demographic, the underlying motor patterns were highly likely to be biologically universal rather than culturally learned.
5.2 Validation of the Primary Emotional Categories Tested
The six primary emotional categories evaluated by Ekman and Friesen were grounded in distinct, highly differentiated patterns of facial muscular contraction. Prior to deploying these stimuli in Papua New Guinea, the researchers ensured that each photographic display exhibited clear somatic markers associated with specific functional states:
Happiness: Morphologically defined by the simultaneous contraction of the zygomaticus major (which draws the lip corners upward and obliquely toward the cheekbones) and the orbicularis oculi, pars orbitalis (which elevates the infraorbital cheek, narrows the eye aperture, and generates radial cutaneous wrinkles, or “crow’s feet,” at the lateral canthus). This configuration constitutes the classic, authentic Duchenne smile.
Sadness: Characterized by the medially directed elevation and convergence of the inner eyebrows, driven by the medial slips of the frontalis and the corrugator supercilii. This action produces oblique brow angles and horizontal wrinkles across the central forehead, typically paired with downward traction on the lip corners mediated by the depressor anguli oris.
Anger: Characterized by the downward and inward drawing of the eyebrows via the corrugator supercilii and depressor supercilii, creating vertical glabellar furrows. This is accompanied by tonic retraction of the upper eyelid via the levator palpebrae superioris, producing an intense gaze, alongside firm adduction and narrowing of the lips via the orbicularis oris.
Disgust: Driven by the contraction of the levator labii superioris alaeque nasi, which elevates the central upper lip and draws the lateral margins upward, causing pronounced nasolabial deepening and cutaneous wrinkling over the nasal bridge, often accompanied by the elevation of the chin via the mentalis.
Surprise: Morphologically marked by the generalized contraction of the frontalis muscle across the entire epicranial aponeurosis, elevating the brows into arched positions, widening the palpebral fissure to expose the sclera, and permitting the jaw to drop open through the passive relaxation of the masseter and temporalis muscles.
Fear: Distinct from surprise, fear involves the medial convergence and horizontal elevation of the eyebrows through the combined action of the frontalis and corrugator muscles, raising the upper eyelids, and horizontally stretching the lips laterally across the lower face via the risorius and platysma.
6. Quantitative Results and Empirical Findings
6.1 Statistical Recognition Rates Across Primary Emotions
The quantitative results gathered from the 189 isolated Fore adults confirmed the universal affect hypothesis, revealing recognition rates that significantly exceeded statistical chance across nearly every emotional category. With three photographs presented per trial, chance recognition was mathematically fixed at 33.3%. In contrast, the observed recognition rates across the isolated adult sample consistently reached high levels of statistical significance (p < 0.01).
The identification of happiness yielded near-ceiling consensus: isolated adult Fore participants correctly selected the smiling photographic stimulus in 92.3% of experimental trials when paired against diverse negative emotional distractors. This demonstrated that positive, affiliative facial displays are unambiguously interpretable across completely disconnected human populations.
Negative emotional configurations also elicited strong consensus. The facial display for anger was identified correctly in 85.3% of presentations when paired with non-hostile distractors. Disgust was accurately decoded in 83.0% of trials, and sadness was correctly recognized at an accuracy rate of 79.0%. Statistical analyses (utilizing chi-square goodness-of-fit tests) demonstrated that these recognition distributions differed significantly from random selection.
Comparisons between the isolated adult Fore cohort and the 23 Westernized Fore adults revealed no statistically significant differences in decoding accuracy. If the ability to interpret emotional displays were dependent upon socialization or exposure to Western mass media, the Westernized cohort should have systematically outperformed their isolated counterparts. The absence of such a discrepancy demonstrated that exposure to Western cinema, education, and language was not a prerequisite for the accurate interpretation of these primary facial configurations.
6.2 The Distinct Anomaly of Fear-Surprise Discrimination
Amid the high recognition rates across most emotional categories, the empirical data revealed one notable, statistically robust exception: the discrimination between fear and surprise. While Fore respondents reliably discriminated fear when it was paired with alternative negative emotions (such as anger, disgust, or sadness) or with happiness, they struggled to differentiate fear from surprise when both configurations appeared simultaneously on the display card.
When presented with the narrative describing an impending life-threatening hazard (e.g., the wild boar attack narrative) and asked to choose between a fearful face and a surprised face, participants selected the surprised photograph almost as frequently as the fearful one. Similarly, when presented with the surprise narrative, respondents frequently selected the fear photograph. In these specific trial pairings, the recognition rates fell to levels that were not statistically distinguishable from chance.
Ekman and Friesen interpreted this finding through an evolutionary and functional lens. Morphologically, fear and surprise share distinct neuro-muscular activations: both involve wide eye opening via the levator palpebrae superioris and the opening of the mouth. In ancestral environments, sudden, highly unexpected environmental events (surprise) frequently presaged acute, survival-critical danger (fear). Within Fore culture, unexpected occurrences were often linked to immediate existential threats, such as ambush by hostile raiders or predatory attacks. Ekman posited that while fear and surprise represent distinct subjective states, their facial motor programs share overlapping functional origins, accounting for their perceptual confusion within this traditional population.
6.3 Pediatric Recognition Findings
The data collected from the cohort of 130 Fore children yielded remarkable developmental findings. Evaluated using the two-alternative forced-choice methodology, the children demonstrated decoding accuracies that equaled or exceeded the performance profiles of the adult respondents across all tested emotions.
The pediatric recognition rates across primary emotional categories were striking:
- Happiness: 97.7% recognition accuracy;
- Sadness: 89.6% recognition accuracy;
- Anger: 89.2% recognition accuracy;
- Disgust: 88.5% recognition accuracy;
- Surprise: 85.0% recognition accuracy;
- Fear: 80.8% recognition accuracy (when paired with anger or disgust distractors).
These high accuracy rates among young, uneducated children who had spent their entire lives within traditional highland hamlets provided critical evidence against cultural transmission theories.
From an ontogenetic perspective, these findings demonstrated that the capacity to interpret facial emotional configurations is present early in human cognitive development, emerging without formal pedagogical instruction or exposure to global visual media. The child data reinforced the conclusion that the neurocognitive mechanisms responsible for decoding facial affect are innate features of human biology that mature early in ontogeny, rather than complex semiotic codes that require years of cultural socialization to interpret.
7. The Production Study: Filming Fore Facial Displays
7.1 Elicitation Methodology for Spontaneous and Posed Expressions
To establish that the cross-cultural consensus was truly bidirectional, Ekman and Friesen conducted a reciprocal “production study” designed to test whether Western observers could accurately decode facial expressions generated by members of the Fore community. If facial affect configurations are genuinely universal, the motor displays produced by isolated New Guinea highlanders should be recognizable to individuals within modern Western societies.
Working in naturalistic highland environments, Ekman directed an elicitation protocol using a 16mm cinematographic camera. Rather than instructing participants to contract specific facial muscles—an impossible task given language barriers—Ekman employed a behavioral role-play method. He presented Fore participants with concrete emotional vignettes and instructed them to display the exact facial expressions they would show if they were the individuals living through those scenarios. The elicitation prompts included:
- “Show me the face you would make if you learned that your child had just died;”
- “Show me the face you would make if you were sitting alone, and a hostile enemy warrior stood before you with a raised spear;”
- “Show me the face you would make if you were walking down a jungle path and smelled the rotting carcass of a pig.”
In addition to these posed expressions, the researchers documented spontaneous, unprompted facial interactions among village members using unobtrusive telephoto lenses. They recorded candid moments of interpersonal greetings, competitive gaming, humorous village interactions, and grief rituals. The resulting 16mm cinematographic footage preserved thousands of feet of high-fidelity visual evidence of nonverbal behavior captured in the absence of Western mass media influence.
7.2 Judgments by Western Observers of Fore Facial Displays
Upon returning to the United States, Ekman and Friesen edited the 16mm footage into brief, standardized stimulus video clips. These unedited facial expressions of Fore men, women, and children were projected to large cohorts of American university undergraduate students who had never encountered New Guinea highlanders and had no knowledge of indigenous Melanesian cultural traditions.
The American observers performed a forced-choice judgment task, identifying the primary emotional state displayed in each Fore film clip from an array of English emotion terms. The empirical findings demonstrated clear cross-cultural concordance: the American students accurately decoded the emotional intentions of the Fore participants at rates far exceeding chance. Happiness, sadness, anger, and disgust displays produced by the Fore were recognized with high statistical consensus.
Notably, the reciprocal production study replicated the fear-surprise overlap observed in the highlands. When American undergraduates viewed Fore facial displays elicited under the fear narrative prompt, they frequently categorized them as surprise, and vice versa. This bidirectional symmetry confirmed that the ambiguity was not a translation artifact or a cultural misunderstanding among the Fore, but rather a cross-cultural perceptual overlap between the facial configurations of fear and surprise. The successful decoding of Fore expressive behaviors by American observers demonstrated that the production and recognition of primary facial expressions are biologically grounded, reciprocal communication systems.
8. The Neurocultural Theory of Emotion and Display Rules
8.1 Differentiating Universal Facial Affect Programs from Cultural Variations
To reconcile their empirical findings of universal facial expressions with the extensive anthropological evidence documenting cross-cultural variations in emotional behavior, Ekman formulated the Neurocultural Theory of Emotion. This theoretical framework posits a dual-system model that distinguishes between innate biological mechanisms and culturally acquired expressive norms.
The biological component of this model is the Facial Affect Program (FAP). Grounded in subcortical central nervous system structures—most notably the amygdala, the basal ganglia, the hypothalamus, and the cranial motor nuclei within the brainstem—the Facial Affect Program contains hardwired, phylogenetically evolved neural commands that link specific emotional states to discrete, coordinated contractions of facial musculature. When a primary emotion is triggered, the FAP dispatches an involuntary efferent motor command through the seventh cranial nerve (the facial nerve), mobilizing the characteristic muscular configurations of anger, sadness, fear, disgust, surprise, or happiness.
The activation of the Facial Affect Program is automatic, rapid, and operates independently of conscious cognitive appraisal. However, this innate program rarely operates in pure isolation within adult social life. Because humans possess sophisticated cortical regulatory capacities, the motor output of the FAP is continually modulated, suppressed, or masked by higher-order cognitive systems located within the prefrontal cortex. This theoretical distinction explained why prior anthropologists had observed cultural divergence: they had documented the socially regulated, cortically managed surface displays of emotion, rather than the underlying, automated operations of the primary affect program.
8.2 The Concept and Operation of Cultural Display Rules
The socio-cultural component of Ekman’s model is encapsulated in the concept of cultural display rules. Display rules are socially learned, culturally variable norms that dictate the management and modification of nonverbal expressions depending on the social context, the presence of specific interlocutors, and prevailing status hierarchies. These rules are acquired early in socialization and function as semi-automatic behavioral filters that intercept or alter the outward somatic signals generated by the Facial Affect Program.
Ekman categorized four primary mechanisms through which cultural display rules modify facial expressions:
- De-intensification: Reducing the physical magnitude of an emotional display (e.g., showing a mild frown rather than an explosive display of rage);
- Intensification: Exaggerating the somatic manifestation of an internal affective state (e.g., amplifying expressions of grief during formal bereavement rituals);
- Neutralization: Completely suppressing facial motor output to display a blank, neutral expression (e.g., maintaining a “poker face” under acute competitive stress);
- Masking: Concealing an authentic, involuntarily triggered negative emotion beneath the deliberate motor display of a distinct affective expression (most commonly, covering anger or distress with a polite social smile).
This theoretical framework received empirical confirmation in a classic experimental study conducted by Wallace Friesen in 1972 involving American and Japanese university students. When seated alone in a darkened room watching stress-inducing, visually distressing surgical films, both Japanese and American participants produced identical facial expressions of disgust, fear, and distress, driven by the automated activation of the Facial Affect Program. However, when an authority figure (an older experimenter dressed in a laboratory coat) entered the room to observe them watching the distressing films, the Japanese students immediately neutralized and masked their negative facial displays beneath polite smiles, adhering to traditional Japanese display rules prohibiting the public expression of negative emotions before authority figures. In contrast, the American students continued to show overt expressions of distress. This definitive experiment confirmed that while the underlying biological affect programs are universal across cultures, social display rules dictate their overt manifestation within varied social contexts.
9. Methodological Critiques and Scholarly Controversies
9.1 James Russell and the Critique of Forced-Choice Paradigms
Despite the wide influence of the 1971 Fore study, its methodological foundations and theoretical conclusions encountered sustained academic criticism, most prominently from psychologist James A. Russell. In a series of critical papers, notably his 1994 publication in Psychological Bulletin, Russell argued that the apparent cross-cultural consensus reported by Ekman and Friesen was largely an artifact of their experimental methodology, rather than proof of universal, discrete emotional categories.
Russell’s primary methodological critique targeted the forced-choice response format. By restricting participants to a predetermined list of emotion terms or a small set of photographic options, the experimental paradigm eliminated cognitive nuance and prevented participants from providing alternative interpretations. Russell demonstrated that when forced-choice options are provided, participants can use processes of elimination to select the experimenter’s intended label, artificially inflating consensus rates. He argued that when researchers deploy open-ended, free-response methodologies—asking participants simply to describe what the individual in the photograph is experiencing—cross-cultural consensus drops markedly, with respondents often describing situational contexts, social actions, or physiological states rather than discrete emotion labels.
Furthermore, Russell criticized the ecological validity of the stimulus materials. The portraits deployed by Ekman and Friesen were static, highly posed, apex photographs depicting extreme muscular contractions rarely observed in daily social life. Rather than demonstrating universal recognition of natural facial displays, Russell argued that Ekman had merely shown that disparate cultures could decipher exaggerated, stereotyped caricatures of Western emotional archetypes. In place of the basic emotions paradigm, Russell proposed dimensional models of affect, most notably the Circumplex Model of Affect, which posits that emotional states are structured along two fundamental, neurobiologically continuous dimensions: valence (pleasure versus displeasure) and arousal (high versus low activation). Under this view, cultural groups universally recognize broad states of valence and activation, but categorize discrete emotions according to local cultural and linguistic frameworks.
9.2 Critiques from Behavioral Ecology: Alan Fridlund
A distinct and influential theoretical critique emerged from behavioral ecology, advanced most forcefully by evolutionary psychologist Alan Fridlund. In his 1994 book Human Facial Expression: An Evolutionary View, Fridlund challenged the fundamental assumption that facial movements function as involuntary readouts of internal emotional states. Drawing upon modern evolutionary biology, ethology, and signaling theory, Fridlund argued that natural selection would actively select against organisms that automatically broadcasted their internal feelings to conspecifics, as doing so would provide competitors with tactical advantages.
Fridlund proposed the Behavioral Ecology View (BECV) of facial displays, asserting that facial movements are not emotional readouts, but rather dynamic, context-sensitive social signals designed to negotiate interpersonal interactions. Under the BECV, facial displays communicate social motives and future behavioural trajectories rather than internal affective states:
- An “anger” face does not broadcast an internal feeling of rage; it serves as a conditional threat display signaling, “Back off, or I will attack;”
- A “sad” face does not represent an automated leak of internal grief; it operates as an appeasement or help-seeking signal communicating, “Provide comfort or support;”
- A “fear” face is not an involuntary physiological panic response; it functions as a submission display or an alert signal communicating, “I am ready to yield, or look out behind you;”
- A “smile” is not a mechanical readout of subjective joy; it is an affiliative token communicating an intention to cooperate and play.
Fridlund supported this framework by demonstrating strong audience effects in facial behavior: individuals rarely display pronounced facial movements when completely alone, unless they are mentally imagining social interactions. Facial displays correlate far more strongly with the presence of an actual or imagined audience than with internal physiological or subjective emotional states. Viewed through the lens of the Behavioral Ecology View, Ekman’s Fore findings did not demonstrate universal emotional readouts, but rather confirmed the existence of a pan-human repertoire of nonverbal social negotiation tokens shaped by natural selection to regulate communicative interactions.
9.3 Methodological Counter-Rebuttals from Ekman and Colleagues
Ekman, Friesen, and their colleagues responded to these critiques with counter-analyses, new empirical studies, and methodological defenses. Addressing Russell’s criticisms regarding forced-choice artifacts, Ekman pointed to early pilot studies and subsequent replications utilizing open-ended, free-response formats. These studies demonstrated that even when participants were free to generate their own descriptive terminology, the semantic meanings of their responses clustered around the basic emotional categories identified in their original experiments.
To refute claims that facial expressions are mere social caricatures lacking biological specificity, Ekman, Levenson, and Friesen (1983) published experimental research in Science demonstrating that the voluntary contraction of specific facial configurations produced distinct, involuntary patterns of activity in the autonomic nervous system (ANS). When participants were directed to contract specific muscles to construct anger, fear, sadness, or disgust expressions—without being instructed which emotion they were portraying—their bodies exhibited differentiated physiological profiles, including significant variations in heart rate, skin conductance, peripheral blood flow, and finger temperature. This autonomic patterning confirmed that facial motor configurations were intimately coupled to subcortical physiological response systems, rather than serving as arbitrary social gestures.
Furthermore, Ekman highlighted comparative studies documenting identical facial expressive movements in congenitally blind infants and children who could not have acquired these motor behaviors through visual observation. Ethological studies of non-human primates revealed clear phylogenetic homologies in facial actions: the “silent bared-teeth display” observed in chimpanzees corresponds morphologically to the human affiliative smile, while the “relaxed open-mouth display” matches human play expressions and laughter. These convergent lines of evidence—spanning comparative ethology, developmental psychology, autonomic psychophysiology, and cross-cultural replications—provided sustained support for the core conclusion that primary facial expressions possess an evolutionary, biologically anchored architecture.
10. The Facial Action Coding System (FACS) as an Empirical Outgrowth
10.1 From Field Observation to Objective Anatomical Measurement
A primary limitation of early nonverbal research was the absence of an objective, reliable measurement system for quantifying facial movements. Prior to the late 1970s, researchers relied on subjective, qualitative descriptions of facial expressions (e.g., “a look of terror” or “a subtle smirk”), which introduced substantial observer bias and made cross-laboratory replication difficult. Recognizing that further progress required an objective, anatomically based measurement system, Ekman and Friesen dedicated nearly a decade to developing the Facial Action Coding System (FACS), published in 1978.
FACS was built upon the anatomical discoveries of Swedish anatomist Carl-Herman Hjortsjö, who had documented the somatic consequences of individual facial muscle contractions in his 1969 work, Man’s Face and Mimic Language. Ekman and Friesen adapted Hjortsjö’s anatomical framework by systematically cataloging every visible muscular movement in the human face. Over years of self-experimentation, they learned to contract every individual muscle group in their own faces, verifying muscular involvement through electromyographic (EMG) recordings, and recording the resulting changes in surface facial morphology, including skin displacement, furrow formation, and contour modifications.
FACS deconstructs all observable facial behaviors into discrete, elementary units termed Action Units (AUs). Rather than interpreting an expression in emotional terms, FACS requires trained coders to break down a facial display into its constituent anatomical components:
- AU 1: Inner Brow Raiser (contracting the medial slips of the frontalis);
- AU 2: Outer Brow Raiser (contracting the lateral slips of the frontalis);
- AU 4: Brow Lowerer (contracting the corrugator supercilii and depressor supercilii);
- AU 6: Cheek Raiser (contracting the orbicularis oculi, pars orbitalis);
- AU 12: Lip Corner Puller (contracting the zygomaticus major);
- AU 15: Lip Corner Depressor (contracting the depressor anguli oris);
- AU 20: Lip Stretcher (contracting the risorius).
By measuring the intensity, onset latency, apex duration, and decay asymmetry of individual Action Units, FACS provided an objective, reproducible descriptive tool that separated morphological measurement from subjective emotional inference.
10.2 Dissecting the Duchenne Smile and Deceptive Affect Displays
The development of FACS enabled the objective differentiation between genuine emotional expressions and voluntary, deceptive facial displays. Drawing upon nineteenth-century neurophysiological observations by Guillaume-Benjamin Duchenne de Boulogne, Ekman and Friesen utilized FACS to operationalize the structural distinction between authentic expressions of positive affect and voluntary, polite social smiles.
An authentic smile of genuine enjoyment—now universally designated as the Duchenne smile—is defined anatomically by the simultaneous activation of two distinct Action Units:
- AU 12 (Lip Corner Puller): The contraction of the zygomaticus major, which pulls the corners of the mouth upward toward the cheeks;
- AU 6 (Cheek Raiser): The contraction of the outer orbital slip of the orbicularis oculi, which elevates the cheeks, narrows the eye aperture, and creates radial wrinkles around the eye orbits.
Crucially, while the majority of individuals can easily contract the zygomaticus major on command (producing a non-Duchenne, voluntary social smile characterized by AU 12 alone), the lateral orbital portion of the orbicularis oculi (AU 6) is not under voluntary cortical control for most people. Consequently, when an individual feigns enjoyment or masks a negative emotional state, AU 6 is typically absent, providing an objective anatomical marker that distinguishes simulated smiles from authentic positive affect.
FACS also provided the technical framework for identifying micro-expressions—transient, involuntary facial movements lasting from 1/25th to 1/5th of a second. Micro-expressions emerge when an individual attempts to suppress an involuntarily activated emotional state. While the conscious prefrontal cortex directs motor commands to conceal or neutralize the expression, the subcortical Facial Affect Program fires briefly through the facial nerve, causing transient muscle twitches that betray the underlying emotion before the conscious suppression takes effect. The discovery and systematic measurement of micro-expressions transformed nonverbal communication research, establishing new empirical paradigms for the detection of interpersonal deception and the investigation of concealed emotional states.
11. Contemporary Replications, Neuroimaging, and Cross-Cultural Affective Science
11.1 Recent Cross-Cultural Re-Examinations in Indigenous Populations
In recent decades, modern affective scientists have revisited isolated indigenous populations using updated ethnographic, psychological, and eye-tracking methodologies to re-evaluate Ekman’s universal affect framework. Notable among these contemporary investigations are the field studies conducted by Carlos Crivelli, James Russell, and colleagues among the Trobriand Islanders of Papua New Guinea.
Working in the Milne Bay Province, Crivelli et al. tested whether Trobriand adolescents and adults matched facial expressions to emotions using both forced-choice and open-ended narrative tasks. Their findings, published in the Proceedings of the National Academy of Sciences (PNAS, 2016), revealed a notable divergence: when presented with the classic Western facial display of fear (characterized by wide eyes and an open, gasping mouth, corresponding to AUs 1+2+5+20), Trobriand participants consistently interpreted the face not as an expression of fear or submission, but as an active, aggressive display of threat and hostility. The authors argued that the gasping face serves as a defensive threat display within Trobriand culture, directly challenging the assumption that specific facial motor configurations hold universal emotional meanings across all populations.
These findings have been integrated into the theoretical framework developed by Lisa Feldman Barrett, known as the Theory of Constructed Emotion. Barrett contends that emotions are not genetically pre-programmed, modular brain circuits that fire automatically in response to specific triggers. Instead, she posits that emotions are complex cognitive concepts constructed in real time by the brain, which operates as a predictive organ utilizing past experience, interoceptive signals, and language to make sense of sensory inputs. Meta-analyses conducted by Barrett and colleagues argue that facial movements exhibit tremendous variation across different cultures, contexts, and individuals. Under this perspective, the Fore and Trobriand studies demonstrate that while facial motor outputs are biologically constrained, their communicative meanings are dynamically constructed through local conceptual, linguistic, and cultural systems.
11.2 Neurobiological and Computational Corroboration
While contemporary anthropological studies emphasize expressive variation, advances in cognitive neuroscience and computational vision have revealed conserved neurobiological substrates underlying basic emotional expressions. Functional neuroimaging (fMRI) investigations have consistently shown that specific facial displays evoke preferential activation in conserved subcortical neural circuits across diverse racial and cultural groups.
The human amygdala exhibits rapid, automated sensitivity to fearful facial configurations, even when stimuli are presented subliminally or outside conscious awareness. Neuroimaging demonstrates that this amygdalar activation is driven primarily by the wide scleral exposure characteristic of fear expressions (AU 5), which operates as an unconditioned threat-detection cue that mobilizes the sympathetic nervous system before cortical appraisal processes can occur. Similarly, the anterior insula and adjacent gustatory cortex are activated during both the sensory experience of foul tastes and odors and the observation of facial expressions of disgust (AUs 9 and 10), supporting Darwin’s hypothesis regarding the shared physiological origins of disgust and chemical avoidance reflexes.
Simultaneously, computer vision researchers have leveraged FACS and large-scale cross-cultural image datasets to develop Automated Facial Expression Recognition (AFER) systems driven by deep convolutional neural networks. Modern artificial intelligence models trained on millions of naturalistic, in-the-wild facial expressions from diverse global populations can classify primary emotional expressions with high accuracy across varied ethnicities and ages. Evolutionary game theory models have complemented these findings, demonstrating that reliable nonverbal signaling systems provide adaptive survival benefits within cooperative social species. These convergent discoveries suggest that human facial expressions are anchored by deeply conserved evolutionary, neural, and computational substrates that operate beneath cultural variations.
12. Epistemological Legacy and Broader Societal Impact
12.1 Impact on Clinical Psychiatry, Law Enforcement, and National Security
The empirical discoveries that emerged from the Fore expeditions have exercised a profound influence across practical applications, including clinical psychiatry, medical diagnostics, forensic interviewing, and counter-terrorism security protocols. In clinical settings, the objective measurement paradigms established by Ekman and Friesen provided psychiatrists and neurologists with standardized behavioral markers to evaluate psychopathology.
In psychiatry, the quantitative measurement of facial Action Units transformed the assessment of flat or blunted affect in schizophrenia and major depressive disorder. Rather than relying on subjective clinical evaluations, clinicians can use facial coding to measure reductions in spontaneous facial expressiveness, track responses to antipsychotic or antidepressant treatments, and identify subtle nonverbal indicators of suicide risk. In pediatric neurology and developmental psychology, facial coding systems are utilized to detect early nonverbal communicative deficits in infants, supporting the early screening of autism spectrum disorders (ASD), which frequently involve difficulties in producing and decoding mutual emotional displays.
In forensic science and law enforcement, Ekman’s findings on micro-expressions led to the development of specialized training curricula, such as the Micro Expression Training Tool (METT). These programs have been deployed across international intelligence agencies, federal law enforcement bodies (including the FBI, CIA, and Scotland Yard), and airport security frameworks (such as the Transportation Security Administration’s SPOT program). These applications aim to train security officers to identify transient, concealed micro-expressions displayed by individuals under severe investigative stress.
However, the deployment of nonverbal behavioral profiling in legal, forensic, and security environments has provoked significant ethical, legal, and civil rights concerns. Methodological critics have noted that detecting a micro-expression indicating fear or anxiety does not prove deception or hostile intent; an innocent individual may experience intense fear when questioned by law enforcement officers. Legal scholars and civil libertarians have cautioned against the uncritical adoption of behavioral lie-detection protocols in criminal justice proceedings, emphasizing that facial expressions reflect internal physiological arousal rather than objective guilt or veracity.
12.2 The Intersection of Affective Universals and Modern Artificial Intelligence
The scientific paradigm established by the 1971 Fore study has acquired renewed importance within the domain of modern artificial intelligence, specifically in the development of affective computing and human-computer interaction (HCI). Automated facial analysis platforms, pioneered by researchers such as Rosalind Picard, rely on the discrete Action Unit taxonomies originally codified by Ekman and Friesen.
Commercial AI applications now process facial expressions across numerous industries:
- Market research and user experience (UX): Tracking consumer engagement, frustration, and attention during software testing and product evaluation;
- Automotive safety: Monitoring driver alertness and emotional distraction to prevent motor vehicle collisions;
- E-learning platforms: Adjusting instructional pacing based on student confusion, boredom, or comprehension;
- Automated hiring algorithms: Evaluating candidate video interviews for organizational compatibility.
The growing commercial deployment of these automated technologies has sparked a major debate over algorithmic bias and scientific validity. Many automated emotion recognition systems were trained on Western-normative, posed photographic datasets that presuppose the universal validity of basic emotion categories. When these algorithmic models are deployed globally across diverse racial, ethnic, and cultural demographics, they often misclassify expressions, misinterpreting culturally normative facial movements or misconstruing context-dependent displays as indicators of negative affect.
In response to these controversies, multidisciplinary panels of neuroscientists, computer vision researchers, and psychologists have advocated for a more nuanced perspective on emotional expressions. While the 1971 Fore study established that specific facial motor patterns are biologically universal human capabilities, contemporary affective science demonstrates that the translation of these motor patterns into social and emotional meaning depends on physical context, personal history, and cultural norms. Recognizing both the biological commonalities of the human face and the cultural nuances of emotional display remains essential for the ethical development of artificial intelligence, cross-cultural communication, and behavioral science.
Conclusion
Paul Ekman and Wallace V. Friesen’s 1971 investigation among the Fore people of Papua New Guinea represents a foundational moment in the history of the behavioural sciences. By identifying an isolated, pre-literate population devoid of mass media exposure, their field research provided a decisive empirical test that challenged the absolute cultural relativism that dominated mid-twentieth-century anthropology and psychology. The finding that isolated highlanders could reliably identify standardized facial configurations of happiness, sadness, anger, disgust, surprise, and fear established that primary facial expressions are not arbitrary cultural gestures, but rather evolved biological adaptations shared by all humans.
The enduring significance of the Fore study extends beyond its empirical findings to the methodological revolutions it catalyzed. The research led directly to the development of the Facial Action Coding System (FACS), provided the empirical foundation for the Neurocultural Theory of Emotion, and offered a balanced framework reconciling biological universals with cultural display rules. While subsequent scholarly debates have introduced critical nuances—highlighting the influence of context, the validity of dimensional affect models, and cultural variability in display functions—the core finding of the Fore expedition remains robust: across our diverse species, human beings share a common nonverbal expressive foundation that links all humanity to a shared evolutionary heritage.
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