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Social smiling may be unique to humans, although spontaneous smiling is shared with other apes. The human smile is a universal nonverbal signal of affiliative communication. While human smiling can be associated with the emotional experience of happiness independent of social context (spontaneous smiling), smiling is most highly correlated with social motivations, such as signaling friendliness, approachability, and cooperativity, or else to convey politeness or appeasement in order to diffuse social tension (Kraut and Johnston 1979).
The silent bared-teeth display observed in non-human primates is thought to be either a homolog or a phylogenetic precursor to the human social smile. The bared-teeth display is similar in form to the human smile, and is used mostly as a submissive signal to deflect hostility from a dominant conspecific, although in chimpanzees it has also been observed from a dominant towards a submissive, likely to signal a non-aggressive approach (Kraut and Johnston 1979).
Spontaneous smiling in humans occurs early in infancy, often immediately following birth, and is similar in occurrence and frequency across cultures (Wormann et al. 2012). The earliest occurrence of social smiling in humans, in which the infant smiles in response to another human face, is thought to occur between two and three months of age (Watson et al. 1972). However, a cross-cultural study found frequency of social smiling to be culturally-dependent, such that social smiling occurred more frequently in a population engaged in many face-to-face interactions between caregivers and offspring than in a population in which face-to-face interaction was uncommon (Wormann et al. 2012).
Spontaneous smiling devoid of external stimulus during REM sleep have been observed in neonatal chimpanzees, and chimpanzee infants between the ages of one and two months imitate smiling facial expressions of human researchers (Tomonaga et al. 2004). However, it is thought that such imitative smiles are reflexive rather than socially motivated, as chimpanzee mothers have not been observed to smile at their infants. Beginning in the first few months of infancy, most primates display a “play face”, a relaxed open-mouthed expression that is exclusively social in context. As the name suggests, “play-face” is most often observed during play, and is thought to convey that rapid, ambiguous movements are mutual, voluntary, and non-aggressive in nature. Chimpanzee mothers have been observed to emphasize the play face of their infant in some instances by pressing her index finger on the infants lower gums to exaggerate the smile (Bard 2002). The play face is thought to be a phylogenetic precursor to human laughter rather than the human social smile, however, as it is a metacommunicative signal of bi-directional affiliative social engagement (Preuschoft 1992).
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Social smiling may be derived in humans
Spontaneous smiling is universal in human populations, and occurs in infants at birth. Social smiling is also universal in humans, but frequency is culturally-dependent.
Heterogeneity of long-history migration explains cultural differences in reports of emotional expressivity and the functions of smiles., , Proc Natl Acad Sci U S A, 2015 May 12, Volume 112, Issue 19, p.E2429-36, (2015)
A cross-cultural comparison of the development of the social smile: a longitudinal study of maternal and infant imitation in 6- and 12-week-old infants., , Infant Behav Dev, 2012 Jun, Volume 35, Issue 3, p.335-47, (2012)
Development of social cognition in infant chimpanzees (Pan troglodytes): Face recognition, smiling, gaze, and the lack of triadic interactions, , Japanese Psychological Research, Volume 46, Issue 3, p.227–235, (2004)
Primate Parenting, , Handbook of Parenting: Biology and Ecology of Parenting, Volume 2, Mahwah, NJ, p.99-140, (2002)
“Laughter” and “Smile” in Barbary Macaques (Macaca sylvanus), , Ethology, Volume 91, p.220–236, (1992)
Social and emotional messages of smiling: An ethological approach., , Journal of Personality and Social Psychology, 1979, Volume 37, Issue 9, p.1539 - 1553, (1979)