Smiles as Multipurpose Social Signals

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Review

Smiles as Multipurpose Social Signals Jared Martin,1,* Magdalena Rychlowska,2 Adrienne Wood,1 and Paula Niedenthal1 The human smile is highly variable in both its form and the social contexts in which it is displayed. A social-functional account identifies three distinct smile expressions defined in terms of their [284_TD$IF]effects on the perceiver: reward smiles reinforce desired behavior; affiliation smiles invite and maintain social bonds; and dominance smiles manage hierarchical relationships. Mathematical modeling uncovers the appearance of the smiles, and both [285_TD$IF]human and Bayesian classifiers validate these distinctions. New findings link laughter to [286_TD$IF]reward, affiliation, and dominance, and research suggests that [287_TD$IF]these functions of smiles are recognized across cultures. Taken together, this evidence suggests that the smile can be productively investigated according to how it assists the smiler in meeting the challenges and opportunities inherent in human social living.

Trends Smiles are highly variable across a number of dimensions. Predominant approaches to smile categorization do not sufficiently explain this variability. Their ubiquity and social impact make smiles a critical topic for affective and cognitive science. A social-functional analysis, categorizing smiles by how they resolve the challenges and opportunities required by social living, suggests three types of smiles: reward smiles that reinforce desired behavior; affiliation smiles that form and maintain social bonds; and dominance smiles that manage social hierarchies.

Why Study the Smile? The human smile is a potent social tool. Smiles grab perceivers’ attention [1,2], influence their brain activity [3–5], and affect their inferences about the person expressing the smile (the encoder; [5–7]). Extant research across topics as wide[28_TD$IF]-ranging as close relationships [8], group decision making [9], and negotiation [10] demonstrates that smiles can also affect the behavior of the perceiver. For example, in the absence of further information, individuals are more likely to trust a smiling stranger [11–13] and select a less personally advantageous outcome when it is delivered with a smile [14,15].

Recent evidence supports this typology: distinct morphological features communicate each functional intent and motivations to smile are predictably variable across culture based on factors related to the salient social tasks in a given culture.

Given the ubiquity of smiles in social interaction, as well as their power to regulate the behavior of observers, it is critical that affective and cognitive science be guided by a principled account of the form and function of this facial expression. The present review develops an integrative framework for advancing the science of smiles.

Current Approaches to Classifying Smiles The contraction of a single muscle, the zygomaticus major (see Glossary), is sufficient to qualify a facial expression as a smile [16]. But [289_TD$IF]although every smile includes this essential [290_TD$IF]movement, the expression can vary along many other dimensions [17]. Perhaps more than any other facial [291_TD$IF]display, smiles vary in temporal dynamics [11,18,19], accompanying emotional states [20,21], and corresponding contexts [22]. In light of this variability, a number of attempts have been made to partition the smile into theoretically productive categories, with some researchers proposing two [23], and others upwards of 50, distinct smile expressions [24]. One approach distinguishes between smiles that are due to (and accurately reflect) underlying positive affect – called true or genuine smiles – and those that do not – called false or fake smiles (see Box 1 for a historical overview [16]). The so-called Duchenne marker – the appearance of [29_TD$IF]‘crow’s feet’ around the eyes – has been most often used as a morphological indicator of [267_TD$IF]‘true’

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[281_TD$IF]Department of Psychology, University of Wisconsin – Madison, Madison, WI, USA 2 [283_TD$IF]School of Psychology, Queen’s University Belfast, Belfast, Northern Ireland, UK 1

*Correspondence: [email protected] (J. Martin).

http://dx.doi.org/10.1016/j.tics.2017.08.007 © 2017 Elsevier Ltd. All rights reserved.

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Box 1. Brief History of [263_TD$IF]‘True’ and ‘False’ Smiles Where did the concept of a [264_TD$IF]‘false’ smile come from? Duchenne claimed that the contraction of the [265_TD$IF]orbicularis oculi, creating ‘crow’s feet’ in a smile ‘does not obey the will; it is brought into play by a true feeling, by an agreeable emotion (Figure I). Its inertia, in smiling, unmasks a false friend’ [25]. Subsequently, Ekman and Friesen, typically credited for repopularizing Duchenne’s early observations, theorized about two kinds of smiles that do not accompany positive emotion: (i) phony smiles that occur without any particular underlying feeling at all; and (ii) masking smiles that are produced intentionally in order to conceal negative emotions with the appearance of positivity [16]. The same researchers conducted empirical investigations of masking smiles: nursing students were filmed as they watched a pleasant video and then as they intentionally covered with smiles their reactions to a negative film showing amputations and burns [103]. [26_TD$IF]‘True’ smiles (of felt positive affect, i.e., Duchenne smiles) were defined as expressions that contained both zygomaticus major and orbicularis occuli activation, and masking smiles as expressions containing the contraction of zygomaticus major plus facial actions associated with any of five negative emotions. Masking smiles were more frequently displayed during the viewing of the negative film. Research using methods of electroencephalography also showed that, in both infants [55] and adults [104], the production of [267_TD$IF]‘true’ smiles – compared to other smiles containing zygomaticus major, but not orbicularis occuli activity – is accompanied by more left- than right-sided hemispheric activation, which is thought to be an indicator of positive affect. Since publication of this seminal work, studies of [268_TD$IF]‘true’ and ‘false’ smiles have for the most part contrasted expressions elicited by amusing stimuli with smiles intentionally posed for a camera often with no or unspecified instructions [14,105]. Thus, rather than [269_TD$IF]‘true’ and masking smiles, most studies focus on [270_TD$IF]‘true’ and ‘phony’ smiles, with an emphasis on the presence versus absence of the Duchenne marker (Figure I). In addition, [271_TD$IF]perceivers’ understanding of the smile meaning (i.e., critical dependent variable) has been limited largely to judgments of whether or not the smiler is experiencing genuine [27_TD$IF]positive emotion [106]. Participants’ judgments of other [273_TD$IF]underlying states have thus been inadvertently underrepresented.

Figure I. Examples of Duchenne (left) and Non-Duchenne Smiles. Contraction of the orbicularis occuli is absent in the non-Duchenne expression (right).

smiles. Several classic studies have demonstrated that this feature is present when a smile spontaneously occurs during states of happiness and that it is lacking when a smile is encoded deliberately, especially in attempts to mask negative feelings [16,25].

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Glossary Autonomic nervous system (ANS): controls many relatively automatic biological processes (e.g., sweating). It is classically divided into two branches, the sympathetic (fight and flight) and parasympathetic (rest and digest) nervous system. ANS measures are frequently collected in emotion science largely because they are thought to be involved in emotion-related biological processes over which we have limited conscious control [126]. Duchenne marker: First documented by Guillaume-Benjamin Duchenne de Boulogne [25], the Duchenne marker refers to the visible ‘crow’s feet’ around the eyes resulting from the contraction of the orbicularis oculi, or eye sphincter muscle. Historical homogeneity: nationlevel variable that quantifies the number of source countries that have contributed to a present-day population over the last 500 years [95]. Nations with high historical heterogeneity scores, like the US and Brazil, derived their populations from extensive and diverse long-history immigration trends, while homogeneous populations like those of Norway and Japan have been relatively stable. Orbicularis oculi: muscle responsible for opening and closing the eyelids, the orbicularis oculi is under both voluntary and involuntary control [127]. Social-functional theories of emotion: inspired by ethological and evolutionary approaches to psychology, social-functional theories of emotion argue that emotions – and associated facial expressions, body states, etc. – aid the individual in solving social challenges and opportunities. These challenges and opportunities occur at many levels of analysis, including within dyads and groups [42,43]. Zygomaticus major: connecting the check bone with the corners of the mouth, contraction of the zygomaticus major pulls the corners of the lips up and back [127], and is considered a necessary muscle in smile production.

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However, some individuals can exert intentional control over the Duchenne marker, producing [293_TD$IF]‘true’ smiles on command [26,27]. Further undermining the marker’s diagnostic status, smiles involving [294_TD$IF]‘crow’s feet’ do not always accompany self-reports of happiness [28,29]. These and other findings [30–32] suggest that the presence of the Duchenne marker is neither a necessary nor sufficient feature of smiles conveying positive emotion. Beyond the weakness of the Duchenne marker as a diagnostic tool, there are more general shortcomings of parsing smiles according to whether they reflect true [295_TD$IF]or faked positivity. Humans contract their zygomaticus major muscles when experiencing a variety of emotions – or even little emotion – and in contexts that [296_TD$IF]vary from pleasant to neutral to unpleasant. For example, smiles occur spontaneously during states of embarrassment, pride, misery, and discomfort, and during greetings [16,20,33–36]. There has been little scientific accounting of this diversity. The claim that [293_TD$IF]‘true’ smiles result from positive emotion limits theorizing about these expressions by inadequately characterizing the feelings, motivations, and situations with which smiles are associated.

A Social-Functional Framework for Smiles The dominant true–false distinction places smiles (and other expressions) on an automaticity continuum, ranging from reflexively automatic to consciously produced, but this dimension captures little of the full variability in the physical forms of smiles. A more theoretically productive approach conceptualizes smiles as a means for a displayer to impact the social environment in an advantageous manner [37]. As an example, behavioral ecologists define facial expressions as signals that communicate social motives and intentions (e.g., to approach or to attack) rather than indicators of internal emotional states [38–40]. In their view, the distinction between [297_TD$IF]‘true’ and ‘false’ smiles has arisen from an unwarranted adherence to a two-factor model of facial expressions – according to which all facial expressions are either innate and reflex-like or instrumental and dissimulative [41]. Thus, following a two-factor model, [298_TD$IF]‘true’ smiles result from positive emotions (especially happiness) and all other smiles are [29_TD$IF]‘false’. In contrast, the behavioral ecology perspective acknowledges that a wide range of smiles are [30_TD$IF]‘true’ insofar as they provide reliable predictions about a displayer’s intended social behavior, independent of the person’s supposed internal state. The recent Simulation of Smiles Model (SIMS; [35]) draws from the behavioral ecology view as well as [301_TD$IF]social-functional theories of emotion[302_TD$IF] [42,43] to propose that distinct human smiles exist in the service of meeting three basic tasks of human social living: reinforcing desired behavior [30_TD$IF](reward smile); forming and maintaining [304_TD$IF]social bonds (affiliation smile); and negotiating social hierarchies (dominance smile). The tasks are not mutually exclusive. However, to the extent that one and only one task is central to a given social interaction, the expression displayed in that interaction will be a more prototypical example of the smile that has evolved to address that task. The SIMS thus exchanges the question about whether an expression reflects a genuine positive emotion for an examination of how smiles are used to address specific social challenges and opportunities. It should be noted that while language and other nonverbal behaviors may also be used to successfully address problems of group living [44], smiles are particularly effective and efficient communication tools because they regulate social life with a minimum disruption of interpersonal equilibrium. Indeed, the flexible use of smiles may be related to the high social tolerance – or the ability to be in close proximity to others with little or no aggression – observed in humans and some other primate species [45,46]. Existing research and theory, drawn from both human and non-human animal literature, provide initial inspiration and

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evidence for the proposal that each of three fundamental social tasks can be solved with a physically distinct smile. Reinforcing Desired Behavior: The Reward Smile Research shows that some smiles can reinforce perceivers’ behavior by activating reward centers of their brains [47][305_TD$IF]. As a potent example, babies’ smiles (but not other facial expressions) activate dopamine-associated regions of their mothers’ (but not other mothers’) brains – including the ventral tegmental area, substantia nigra, and striatum – which likely serves to keep the mothers invested [306_TD$IF]([48–50], see also Box 2). Related findings document that such reward smiles (Figure 1A) reinforce specific [307_TD$IF]outcomes including decision making and social learning [15,51]. In other words, some smiles induce pleasurable states in the perceiver, and can thus be used to reinforce the behavior that gave rise to the smile in the first place. Creating and Maintaining Social Bonds: [308_TD$IF]The Affiliation Smile Other smiles, such as those often called social smiles [52], serve to invite and maintain mutually positive and beneficial social bonds [20,53]. Although some theorists define such smiles as misleading and dissimulative [11,54], a social-functional account assumes no manipulative intent of smiles that do not reflect intense positive emotion. Affiliation smiles (Figure 1B) signal the potential for mutual positivity as well as an openness to establishing or re-establishing positive bonds, and can be produced spontaneously without arising from pleasurable states such as amusement or joy. For example, babies’ smiles of greeting to strangers are not reward smiles, but neither are they intentional nor manipulative [55]. In addition, people often smile when feeling embarrassed, and research demonstrates that such smiles signal appeasement to in-group members [20]. So-called embarrassed smiles therefore have an affiliation function in the present framework.

Box 2. Developmental Trajectory of Social-Functional Smiles ‘The movements of expression in the face [ . . . ] serve as the first means of communication between the mother and her infant; she smiles approval, and thus encourages her child on the right path.’ (Darwin, The Expression of the Emotions in Man and Animals, p. 365 [56]). Classic findings in developmental psychology [107,108] confirm Darwin’s intuition that smiles are potent signals that reinforce and encourage desired behavior in infants. Given the importance of the reward function of smiles early in ontogeny, one might hypothesize that reward smiles are the first of the three social-functional smiles to develop. Recent research supports this conclusion, and highlights the importance of an infant’s environment for the development of positive displays. In a study of 536 pairs of 6–12 month-old twins, infants’ smiles were significantly predicted by familial smiling but not by expressions of disapproval, frustration, or other negativity [109]. Similar findings show that smiling and other positive displays from 8–12-year-old children are more likely to be sustained when a caregiver reciprocates the positivity [110]. Together, these findings suggest that the behavior-reinforcing reward smile develops early, perhaps within the first 3 months of life [111]. What about affiliation and dominance smiles? Further research indicates that these functional smiles may develop during childhood and adolescence, respectively. Positive facial displays when helping others – related to the affiliative function of [274_TD$IF]creating and maintaining social bonds – are expressed by children as young as 2 years of age [112]. The early appearance of affiliation smiles may reflect humans’ strong propensity toward cooperation and fairness [113]. Furthermore, asymmetrical expressions – a critical element of dominance smiles [60] – are seen in adults, but not children between the ages of 3 and 8 years [114]. This finding suggests that dominance smiles may not be deployed until later in development, likely emerging during adolescence. Displays of dominance behavior may appear latest because they are related to even more complex constructs, such as hierarchy and its disruption. An important caveat to these findings is that culture may play a large role in how and when each smile develops and is used. For example, if a society is more hierarchical, dominance smiles may emerge earlier. Recent [275_TD$IF]work suggests that crosscultural differences in smiling may emerge as early as 12 weeks of age; perhaps due to differences in parent–infant co-smiling and imitation [115]. Future work is necessary in order to determine both the developmental trajectory of the three functional smiles [35] as well as the extent to which it is influenced by culture.

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(A)

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Figure 1. Examples of Each Social-Functional Smile. Reward (A), affiliation (B), and dominance (C). The expressions are depicted by an experienced actor. The actor was coached to make each type of smile guided by the morphological features isolated as diagnostic of each smile category [60].

Managing Social Hierarchies: [308_TD$IF]The Dominance Smile Finally, some smiles solve the interpersonal task of asserting dominance over another person. Status negotiation can involve situating oneself within an existing hierarchy or seeking to alter it by challenging higher[309_TD$IF]-status – or subduing lower[310_TD$IF]-status – others. Signals of dominance and superiority could also occur within egalitarian relationships when one person wants to convey disapproval or gain distance from the other’s transgression. The concept of a derisive smile was first described by Charles Darwin in his discussion of gestures of sneering and defiance, as occur in confrontations of status [56]. The empirical literature has since documented smiles that are associated with threat, criticism, and prideful feelings [57–59]. Thus, like smiles of affiliation, dominance smiles (Figure 1C) are held to be spontaneous signals that do not have to (but sometimes might) reflect a positive emotion. Unlike affiliation smiles, which signal the potential for positive interaction with the receiver, dominance smiles communicate an intention to instantiate, destabilize, or maintain asymmetric hierarchical relations between displayer and receiver – an orientation which is often desirable for one but not both interactants. Furthermore,

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dominance smiles likely reduce the probability of violent social encounters to a greater extent than more overtly threatening displays, such as expressions of disgust and anger.

Morphological Features of Social-Functional Smiles Extant research on smiles, as well as the descriptions of play, threat, and submissive expressions in primates [61], provide some hints about the possible stereotypical appearances of reward, affiliation, and dominance smiles. In humans, a data-driven approach was recently used to investigate the dynamic patterns that convey each of the three social-functional smile meanings to receivers [60]. The researchers combined computer graphics and psychophysics [62] to model the facial movements – or, action units (AUs [63]) – that, in combination with the zygomaticus major, are perceived to communicate reward, affiliation, and dominance[31_TD$IF]. Specifically, on each of 2400 trials, bilateral or unilateral zygomaticus major plus a random sample of between one and four other facial AUs were selected from a set of 36. The dynamic movement of each AU was determined by randomly specifying values of each of six temporal parameters. The [312_TD$IF]facial animation was then presented on one of eight face identities. Participants rated the extent to which each animation matched their personal understanding of a display signaling reward, affiliation, or dominance. Methods of reverse correlation were used to quantify facial movements that predicted the ratings. Results showed that eyebrow flashes – involving the inner and outer brow raiser – and symmetry of contraction of the zygomaticus major were rated as rewarding by participants. In addition to the facial actions that signaled reward, ratings of affiliation were predicted by activation of the lip pressor; one of the smile control [31_TD$IF]movements [17]. Finally, faces that displayed unilateral, asymmetrical [314_TD$IF]activation of zygomaticus major and AUs known to be related to disgust [315_TD$IF]including the nose wrinkler and upper lip raiser were perceived as more dominant (Figure 2A) [64]. The patterns of facial movements that signaled each function were then used to create animations of prototypical reward, affiliation, and dominance smiles. A new group of human observers and a Bayesian classifier were highly accurate in associating the animations with the corresponding smile category labels. Both participants and the classifier were particularly accurate in discriminating between dominance and reward smiles. They were worse at discriminating reward from affiliation smiles, reflecting perceptual or semantic similarity between the two latter categories (Figure 2B), although they still performed above chance. Finally, animations of the three smile prototypes were rated by another group of participants as systematically conveying the corresponding social[316_TD$IF]-functional motives. Observers viewed the reward and affiliation smiles as communicating high levels of positivity and social connection (with reward smiles receiving higher positivity ratings) and low dominance. Models of dominance smiles were perceived as conveying low levels of positivity and social connection, and high superiority. In summary, each of the three smiles is associated with a specific configuration of [317_TD$IF]AUs. These movements can be correctly identified by observers as instances of reward, affiliation, and dominance smiles, and interpreted as communicating distinct social motives. In subsequent investigations, findings based on perceptions of smiles encoded by computergenerated, animated faces were extended to human encoders. In particular, actors were coached to produce smiles associated with [318_TD$IF]reinforcement, bonding, and hierarchy negotiation situations and involving movements identified in the reverse correlation studies. Perceivers accurately categorized these realistic expressions according to the intended social motives and, importantly, perceived them as smiles (see Supplemental Materials, [60]), ruling out the possibility that the dominance smiles are classified as negative, nonsmile expressions. In summary, validation studies with both animated and human encoders [60] have provided

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Figure 2. Morphological Features Distinguish Social-Functional Smiles. Facial movements diagnostic of the three smile types with Bayesian classifier and human detection performances [60] (A) Facial expression models of three smile types. Color-coded face maps show [259_TD$IF]AUs conveying reward, affiliation, and dominance smiles. Red indicates that a high number of observers (maximum = 43) associated an AU with a given smile type – see color bar on the left. (B) Bayesian classifier performance. The color-coded matrix shows the average probability of each smile type classification (for every smile type) by a Bayesian classifier. Red indicates high probability and blue indicates low probability (see color bar to right). Diagonal squares show high classification accuracy for each smile. (C) Human detection performance. The color-coded matrix shows the performance of human observers in detecting each smile type from the smile models. Color-coded squares show for each smile type (input stimulus), the proportion of yes responses (pooled across observers) associated with each set of smile models (output response). Red indicates high proportions of responses. Diagonal squares (i.e., hits) show that each smile type is detected with high accuracy. Abbreviations: AU, action unit.

evidence that perceivers systematically distinguish between smiles that signal reward, affiliation, and dominance.

Evolutionary Origins of Social[319_TD$IF]-Functional Smiles A social-functional framework invites investigation of the possible evolutionary origins of the smile display. [320_TD$IF]Smiles may have gained their communicative meaning from their effect on the voice. According to the size code hypothesis, across species, signals of appeasement, submission, and nonthreat involve creating the impression of smallness [65]. A submissive dog, for instance, makes himself appear small by tucking his tail and ears and cowering close to the ground [66]. Animals can also convey smallness and harmlessness via voice modulation that increases pitch and alters the spacing of formants (which changes the vowel sound), giving the impression of a smaller vocal tract and therefore smaller, less threatening body size [67,68].

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Species with the ability to flexibly change the shapes of their mouths (like primates) can further exaggerate the spectral properties of vocalizations, conveying smallness/submissiveness or largeness/dominance as needed [69]. Indeed, smiles themselves influence vocal output. An open mouth with retracted lips – a smile – modulates the formants of a vocalization [70] and serves as a visual signal of nonthreat, playfulness, and/or fear in a range of species [71]. Since most mammals communicate intention and influence [321_TD$IF]conspecifics’ behavior primarily via the voice and body – not the face – this suggests a possible voice[32_TD$IF]-modulating origin of the smile, fear grimace, and other variations of retracted-lip facial displays that convey social surrender. Listeners can hear when a speaker is smiling, and even distinguish between different types of smiles based on auditory input alone [72,73]. What might be the vocal origins of the distinct social[316_TD$IF]-functional smiles proposed here? The panting play face observed across species may be homologous to the reward smile, closely linked to positive states like playfulness, joy, and amusement [74]. Smiles that accompany intense joy – in the present framework, reward smiles – involve greater mouth opening [75], which allows for a more widely and loudly broadcasted vocalization, and may increase the breadth of influence of the reward signal. Affiliation smiles lack the strong positive affective associations of reward smiles, and are used to convey a nonthreatening, prosocial, bonding orientation. Given their overlap with signs of appeasement, we might expect affiliation smiles to further modulate the formants of the voice to exaggerate the illusion of smallness and meekness. Upward vowel modulation involves a tenser mouth than the relaxed play face (e.g., similarly to producing the vowel/i/), resulting in a display that bears resemblance to the submissive bared-teeth display in nonhuman primates and to the affiliation smile in the present framework [76]. Ritualized variations of such facial displays in egalitarian primate societies may have developed to signal different social intentions beyond play and submission [77,78]. For instance, a uniquely human dominance smile may be the result of agonistic aggression (i.e., a snarl) blending with or being masked by a harmony-maintaining smile. Dominance smiles may also be accompanied by retraction of the head, which might lengthen the vocal tract and lower the pitch relative to other smiles. Preliminary perceiver-based research suggests that distinct variations of laughter can convey the same social signals as smiles [79]. Participants judged the extent to which 400 laughter samples accomplished the social tasks of reward, affiliation, and dominance. Spontaneity, a dimension frequently examined in laughter research, was included for comparison as a fourth measure. Judgments that a laugh was spontaneous or rewarding were both associated with acoustic features previously identified as diagnostic of spontaneity, such as increased pitch and decreased voicing [80]. Besides differing on a handful of acoustic variables, it seems that spontaneity and reward may be conveyed by similar features, which fits with the known origins and rewarding effects of spontaneous laughter on displayers and recipients [81]. Affiliation and dominance were also predicted by distinct acoustic properties, although the patterns were different for male and female expressers, perhaps unsurprising given the strong sexual dimorphism in human vocalizations [68]. Females sounded more affiliative when their laughter was quieter, while the opposite was true for males; this suggests that the link between perceived submission/meekness and affiliation [32_TD$IF]may be stronger for females than for males. Supporting the size code hypothesis, perceptions of female affiliation and dominance were influenced by formant positioning. [324_TD$IF]Furthermore, higher-pitched laughter sounded more affiliative and less dominant. In summary, this study demonstrated [325_TD$IF]that perceivers rely on systematic acoustic patterns to infer social intentions of reward, affiliation, and dominance from laughter.

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Future work must examine whether distinct smiles are involved in the production of laughs that convey reward, affiliation, and dominance, which would lend support to the hypothesis of voice[32_TD$IF]-modulating origin of smiles. As Darwin first suggested [56], features of human facial expression that have arguably functional origins, such as the vision-enhancing wide eyes associated with the fear display [82,83] can become social signals in themselves. This is how silent variations of the three functional smiles may still serve their social functions [84], regardless of whether they are accompanied by vocalizations.

The Functions of Smiles [326_TD$IF]across Cultures Although smiles appear to be universal in humans [85], there exist crosscultural differences in beliefs about when and how to display smiles [86], perceptual biases in decoding smiles [87], and social norms governing smile display [88,89]; all of which contribute to crosscultural misunderstandings. For example, in East Asian countries, research suggests that norms restrict overt smiles to a greater extent than in [327_TD$IF]Western countries [90,91]. Cheerfulness and smiling, highly valued in some cultures, may thus be perceived as superficial or manipulative in other cultural contexts [92]. Importantly, misinterpretations may also result from crosscultural variation in the salience of the social tasks for which different smiles are adaptively deployed [93]. Similar to other evolutionarily inspired theories in psychology [94], the logic of a social-functional account of smiles [35] suggests that the fundamental tasks of human social living are universal insofar as every human social environment, regardless of culture, presents a basic set of challenges and opportunities (e.g., reinforcing desired behavior). However, due to a confluence of social forces – one of which is a regional history of immigration [95] – these basic tasks vary in frequency and urgency across cultures. Therefore, cultures with environments that make certain social tasks similarly salient should endorse similar motives for smiling [96–98]. This conclusion is suggested by results of a recent study [93] in which respondents from nine countries rated the extent to which different feelings and motives (e.g., a person wants to manipulate or control you, cares about you, or is in a good mood) commonly produced a smile expression in their countries. Causes of smiling could be described by a three-factor structure interpretable in terms of smile functions that are consistent with an intention to reinforce behavior, form and maintain social bonds, and manage status. In addition, the researchers assessed the extent to which different country-level variables accounted for crosscultural differences in endorsement of smile functions. In particular, they focused on the dimension of historical heterogeneity, which quantifies the number of source countries that, over 500 years, contributed to the present-day population of a nation [95], thereby shifting the salient social tasks demanded by increasingly diverse social environments. Separate research has documented that people from more heterogeneous cultures endorse norms of greater overall emotional expressivity and transparency, and produce more crossculturally recognizable facial expressions [93,99]. A cluster analysis, used to explore groupings in [328_TD$IF]data, has revealed that respondents can be meaningfully [329_TD$IF]clustered into two groups differing in the endorsement of smile functions (Figure 3). Country-level historical heterogeneity is the best predictor of the proportion of respondents assigned to each group – even controlling for other potentially relevant variables such as residential mobility, present ethnic diversity, and collectivism–individualism. Specifically, historically diverse countries such as the US or New Zealand, whose populations arose from large migration flows, [30_TD$IF]endorse the affiliation function of smiles to a greater extent than ethnically[31_TD$IF]stable countries, such as Indonesia or Japan (Figure 3). The opposite is true for the hierarchy management motives, which are endorsed more strongly in historically homogeneous

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Heterogeneity cluster Homogeneity cluster

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Figure 3. Social-Functional Typology of Smiles: Cultural Evidence. (A) Map of historical heterogeneity. Darker countries are more heterogeneous, meaning their present-day populations originate from a greater number of source countries (values on legend refer to number of source countries) [93]. Map generated at http://gunn. co.nz/map and based on data from World Migration Matrix (http://www.brown.edu/Departments/Economics/Faculty/Louis_Putterman/world%20migration% 20matrix.htm). (B) Clustering the motivations for smiling in nine countries. Participants from Canada, France, Germany, India, Indonesia, Israel, Japan, New Zealand, and the US indicated feelings and motives conducive to smiling in their culture. A cluster analysis revealed two groups: participants in [260_TD$IF]the Heterogeneity Cluster endorsed bonding motives to a greater extent than participants in [261_TD$IF]the Homogeneity Cluster. The opposite was true for hierarchy-related motives for smiling, endorsed more by participants in [261_TD$IF]the Homogeneity Cluster. Country-level historical heterogeneity was the best predictor of cluster membership. Bar graphs represent proportions of respondents in each cluster by country.

countries. The importance of long-history migration was recently supported by an independent research team who showed that historical heterogeneity was the best predictor of smiling in a sample of 866 726 participants from 31 countries [100]. In summary, it appears that social norms, structures, and messages are more easily predictable in homogeneous cultures with historically well-established institutions and codes. Such societies may encourage the use of dominance smiles to negotiate one’s status without disturbing the social group. In contrast, heterogeneous cultures historically have involved extensive crosscultural contact and therefore [32_TD$IF]increased social uncertainty. In such contexts, cues to trustworthiness and cooperation, such as affiliation smiles, may have been of [3_TD$IF]critical importance. Cultural variations in the endorsement of smile functions could thus reflect adaptations to different ecological environments in the distant past.

Concluding Remarks Imposing order on a heterogeneous category of facial expression, such as smiles, is a difficult business. Researchers sometimes strive to associate a particular expression with the displayer’s underlying emotional state. Whether emotions correspond to particular facial displays and bodily states frequently enough that the face can serve as a proxy for inferring others’ mental states is hotly debated [96,101]. Still, the relationship between emotions and facial displays in adult humans is weak enough to lead some researchers to conclude there are no predictable emotionally derived facial displays [37]. Taking the current social-functional approach to predicting the causes and consequences of facial expressions sidesteps the epistemic dilemma of the emotion expression debate. This framework takes a page from animal research and the behavioral ecology view, categorizing facial displays according to their social outcomes [102]. The approach concedes that facial and

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bodily displays may be accompanied by specific emotion states, while also allowing that sometimes they may not. A particular behavior, situated in a relevant context, should consistently serve a social function, no matter the displayer’s mental state. The smile, as a particularly flexible and common signal, serves as a useful case study for developing such an approach. The research reviewed here suggests the generativity of a social-functional account of smiles. The approach explains variance in the morphology, social contexts, social consequences, and cultural significance of smiles. It can complement other frameworks that focus on the internal state of the displayer or on the social information communicated by the facial expression. The present social-functional approach to smiles also lends itself to comparative psychology and ethology, as it does not rely on access to the displayer’s self-reported psychological state. Now that evidence has accrued for distinct smiles serving specific social functions, future work should focus on two major tasks [34_TD$IF](see also Outstanding Questions). First, although previous research documents facial movements that are associated with the social functions of smiles [60], it remains to be seen whether similar features of smiles are displayed by human beings naturally and without direction. An ethological approach, unobtrusively sampling facial expressions from social interactions ‘in the wild’, in which a single dominant social function is likely to be operative, could go a long way in addressing this concern. Second, further work is required in order to document the consequences of the social[316_TD$IF]-functional smiles as they occur in vivo (e. g., Box 3). A challenge in this endeavor will be identifying instances of smiles serving one distinct social function. Ideally, the work reviewed here [60] which identifies the morphological features of prototypical conceptions of reward, affiliation, and dominance smiles will assist researchers in both endeavors.

Box 3. Smiles Regulate Receivers’ Physiology A further test of a social-functional account of smiles rests in assessing the effect of receiving smiles on behaviorally relevant biological systems. Activity in the [276_TD$IF]autonomic nervous system[27_TD$IF](ANS) fluctuates in response to the environment and provides input to every major bodily system [116,117]. Since facial expressions can up- and downregulate ANS activity in receivers [118]; the ANS represents one such behaviorally relevant system. Although no extant research addresses the physiological effects of receiving the social-functional smiles outlined here, limited research does speak to the biological repercussions of observing smiles in meaningful and believable social contexts. For example, when exposed to smiles as positive feedback during social evaluation – compared to negative feedback in the form of head shaking and crossed arms – participants evince a challenge (compared to a threat) cardiovascular response, efficiently mobilizing biological resources [117,119] that may facilitate behavioral engagement. Although it remains for future research to illuminate the psychological mechanisms through which smiles influence physiological activity – be they social appraisals [120] or other forms of social information [121] – a number of preliminary hypotheses can be derived about the effects of functional smiles on [278_TD$IF]ANS activity. Reward, affiliation, and dominance smiles should regulate receivers’ ANS activity in the service of instantiating their proposed social-functional outcomes. For example, receiving reward and affiliation smiles (compared to dominance smiles) should produce a state of relative calm, involving reduced activity in the sympathetic branch of the ANS [122]. In contrast, in order to effectively negotiate status in social hierarchies, receiving dominance smiles (compared to reward and affiliation smiles) should produce a state of heightened vigilance [123] as exhibited by increased sympathetic nervous system activity. The specific effects of the affiliation smile (compared to reward or dominance smiles) on receivers’ ANS activity are likely complex, since a signal of social approachability and openness to bonding may be ambiguous in certain contexts ([279_TD$IF]e.g., novel situations) or from certain displayers ([280_TD$IF]e.g., enemies). Given the potential ambiguity, the effect of the affiliation smile may heavily depend on a receivers’ social sensitivity and ability to decode facial expressions. One particularly compelling avenue for future research is to consider individual differences in social sensitivity – perhaps as indexed by high-frequency heart rate variability, a measure of activity in the parasympathetic branch of the ANS [124,125] – as potential moderators of the relationship between social-functional smiles and receivers’ physiological responses to those smiles, particularly receivers’ sympathetic nervous system responses.

Outstanding Questions Which perceptual features of reward, affiliation, and dominance smiles are processed first? In what way is perception of reward, affiliation, and dominance smiles influenced by the social characteristics (sex, race, ethnicity, etc.) of the displayer [35_TD$IF]or the receiver? Can we trace the physical appearance of the three smiles to their functional origins or to homologues in nonhuman primates? How are human smiles and laughter different and similar in the social problems they solve? To what extent might the morphological features of functional smiles, identified in previous research, vary across cultures? What methods (statistical and experimental) might allow for researchers to assess both similarity and variability in social-functional smiles simultaneously across cultures? Is there a consistent [36_TD$IF]developmental ordering of reward, affiliation, and dominance smiles? Can insights into the social functions of smiles be gleaned by tracing the trajectory of smile type development between cultures and environments? Might imitation be a necessary step in the development of later-developing types of smile expression? What individual differences in physiological activity ([37_TD$IF]e.g., ANS activity, oxytocin levels[38_TD$IF]) might predispose an individual to express a certain type of social-functional smile? How? By changing the salience of social-functional tasks? Do individual differences in perceivers’ physiological activity (e. g., testosterone levels) shift classification of ambiguous smiles into one social[316_TD$IF]-functional category versus another? How do reward, affiliation, and dominance smiles impact the behavior of receivers? Through predictions of displayers’ future behavior? Do the biological systems that are influenced when perceiving reward, affiliation, and dominance smiles relate to receivers’ behaviors toward the expresser?

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Acknowledgments The authors would like to thank Alan J. Fridlund for his helpful comments during the revision process. The authors would further like to thank Alicia Waletzki and Holden Wegner for their assistance in reviewing the literature. This work was supported by the National Institutes of Mental Health (grant number T32MH018931-26 to J.D.M); the U.S.

Israeli

Binational Science Foundation (grant number 2013205 to P.M.N.); and the National Science Foundation (grant number 1355397 to P.M.N.).

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