Smiles of reward, affiliation, and dominance.

From Martin et al.:

Abstract

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 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 human and Bayesian classifiers validate these distinctions. New findings link laughter to reward, affiliation, and dominance, and research suggests that 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.

From the text:

Extant research on smiles, as well as the descriptions of play, threat, and submissive expressions in primates, 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. The researchers combined computer graphics and psychophysics to model the facial movements – or, action units (AUs) – that, in combination with the zygomaticus major, are perceived to communicate reward, affiliation, and dominance. 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 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 movements. Finally, faces that displayed unilateral, asymmetrical activation of zygomaticus major and AUs known to be related to disgust including the nose wrinkler and upper lip raiser were perceived as more dominant.

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