Why do we leak lubricant from the eyes to solicit comfort from others? Why do we bare our teeth and crinkle our faces to express non-aggression? The defensive mimic theory proposes that a broad range of human emotional expressions evolved originally as exaggerated, temporally extended mimics of the fast, defensive reflexes that normally protect the body surface. Defensive reflexes are so important to survival that they cannot be safely suppressed; yet they also broadcast information about an animal's internal state, information that can potentially be exploited by other animals. Once others can observe and exploit an animal's defensive reflexes, it may be advantageous to the animal to run interference by creating mimic defensive actions, thereby manipulating the behaviour of others. Through this interaction over millions of years, many human emotional expressions may have evolved. Here, human social signals including smiling, laughing and crying, are compared component-by-component with the known, well-studied features of primate defensive reflexes. It is suggested that the defensive mimic theory can adequately account for the physical form of not all, but a large range of, human emotional expression.

Infants’ smiling is considered an expression of affection, and an index of cognitive and socio-emotional development. Despite research advances in this area, there is much to explore on the ontogeny of smiling, its meaning and the context in which it is manifested early in life. This study aimed at: (a) investigating smiling patterns in these different developmental moments in early infancy, (b) analyzing patterns of association between babies’ smiles and their mothers’ affective behaviors, and (c) verifying whether babies can answer contingently, with smiles, to mothers’ affective behaviors. Participants were sixty Brazilian mother-infant dyads. Infants in three age levels (one, three, and five months of age) and their mothers were observed. They were videotaped at home, during 20 minutes in free sessions. The results indicate increase in frequency of infants’ smiling instances across ages (F(2, 59) = 9.18, p < .05), variations in the frequency of maternal behaviors accompanying the variations in infants’ smiling (F(2, 59) = 6.03, p < .05), correlations between infants’ smiling and mothers’ affective behaviors, and contingency between the behaviors of both mothers and infants. It was verified a strong association between mothers’ behavior and their babies’ smiles, emphasizing the importance of affective interactions in early stages of development.

This study investigated how extrafoveally seen smiles influence the viewers’ perception of non-happy eyes in a face. A smiling mouth appeared in composite faces with incongruent (angry, fearful, neutral, etc.) eyes, thus producing blended expressions, or they appeared in intact faces with genuine expressions. Overt attention to the eye region was spatially cued, foveal vision of the mouth was blocked by gaze-contingent masking, and the distance between the eyes and the mouth was varied. Participants evaluated whether the eyes were happy or not. Results indicated that the same non-happy eyes were more likely to be judged as happy, and more slowly to be judged as not happy, in presence more than in absence of a smile. As (a) the smiling mouth was highly salient regardless of type of eyes, (b) the influence on the eyes increased gradually as a function of eye-mouth proximity, and (c) the effect occurred in the absence of fixations on the mouth, we conclude that a salient smile radiates outwards to other face regions through a projection mechanism, thus making the eye expression look happy.

In general, the smile that orthodontists try to create at the end of treatment satisfies objective functional criteria as well as subjective esthetic criteria.

It’s obvious that in today’s society, a smile plays an important role in nonverbal communication: restoring or enhancing this smile is, in fact, the main reason patients consult their orthodontists.

After reviewing the literature, we were able to identify various factors involved in the perception of a smile and to underscore the esthetic connection between the smile and the face.

Is there a relationship between the smile and facial harmony?

What are the objective criteria that allow us to discern whether a smile is pleasant or not?

Our study confirmed that the esthetics of the face correlates with the perception of the smile and that the factors discussed in the literature pertaining to this correlation are valid.

However, the study has also made it possible to clarify the respective influence of various factors as they relate to a pleasant smile, an unattractive smile or an unsightly smile. Therefore, our study has shown that objective functional criteria are involved in the entirely subjective esthetic realm of a smile.