The physiological studies of facial expressions caused by smiling were first investigated by Duchenne (1862), a French neurologist. He triggered muscular contractions in human face with electrode. Here is the picture (Duchenne on the right) obtained from Wiki
Duchenne (1862) reported that the emotional smile that was believed to come from the soul involved facial movement in the eye region called orbicularis oculi, whereas deliberate smiles include activation of the zygomatic major. (See the difference below. That’s me). The genuine smile was later coined as “Duchenne smile” to honor his work. People who showed Duchenne smiles were pereceived as being more expressive, natural outgoing and sociable (Frank et al., 1993). Recent study by (Mehu, Little, & Dunbar, 2007) also showed that viewing Dunchene smiles in facial stimuli significantly increased participants’ perception of generosity and extroversion of the face more than viewing nonDuchenne smiles.. (Ok I got it! Some smiles are more genuine than the others.) But how do all the scientific studies on smiles link to our courtship and mate attractiveness? Well, the ultimate explanation of smiles could be traced all the way back to Darwin.
Darwin (1872/ 1998) was one of the first people to discuss that emotional expressions were adaptive and had an important communication purpose to others. With regards to the smile, Darwin suggested that its function might signal submissive gestures that prevent acts of aggression from dominant individuals. Therefore the mental states of the smiling person could be translated as “I am harmless, cool, friendly and happy, ” and the social status of the person being smiled at could be taken as, “Look. I am a dominant one.” What about in mate selection?
An individual who is wealthy, physically attractive (healthy), caring, warm and trustworthy has a high mate value and would be an ideal male partner to pass on our genes (Fletcher et al., 1999). Masculinity in men, another valuable biological trait, is also linked to mate qualities such as competitiveness (Carre’ & McCormick, 2008), strength (Windhager et al., 2011) and health (Thornhill & Gangestad, 2006). Keeping that in mind, let’s look at some findings from the studies on various mate trait perception of smiles.
Okubo et al. (2015) found that manipulating male face stimuli (smiling or neutral faces) have different effects in how female perceive. Female participants, who are East Asians and Europeans, rated the smiling male faces in men as more attractive than neutral faces only in long-term relationship, but not in short-term relationship. Additionally, Okubo et al. (2015) reported that females rated the neutral faces as more masculine, and mature than smiling ones. On the other hand, the smiling faces were rated higher on trustworthiness than neutral ones.
Another interesting study by Ketelaar et al. (2012) showed that the degree of smiling in football players of different sizes influenced females’ perception of pro-social traits of the man (friendliness, helpfulness, cooperativeness, attractiveness, etc.) From Dunchene smiles described above, Mehu, Little, & Dunbar (2007) also reported that male dunchene smiles were perceived by other males as most generous signals than male smiles perceived by females, female smiles perceived by males, and female smiles perceived by females group.
To conclude it appears to me that neutral face (no smiling) is associated with dominant trait (serve as an attractive signal for short-term mate) while the smiling face is associated with warmth and trustworthiness (good characteristics for long-term mate).
Also look at this video and see if you can spot duchenne and non-duchenne smiles: https://www.youtube.com/watch?t=16&v=7SqlilB1w3g
Carre ́, J. M., & McCormick, C. M. (2008). In your face: Facial metrics predict aggressive behaviour in the laboratory and in varsity and professional hockey players. Proceedings of the Royal Society B: Biological Sciences, 275, 2651–2656.
Darwin, C. (1872/1998). The expression of the emotions in man and animals. Oxford: Oxford University Press.
Duchenne de Boulogne, G. B. (1862). The mechanism of human facial expression (R. A. Cuthbertson, Trans). New York: Cambridge University Press.
Fletcher, G. J. O., Simpson, J. A., Thomas, G., & Giles, L. (1999). Ideals in intimate relationships. Journal of Personality and Social Psychology, 76, 72–89.
Frank, M. G., Ekman, P., Friesen, W. V. (1993). Behavioral markers and recognisability of the smile enjoyment. Journal of Personality and social Psychology, 64(1), 89-93.
Ketelaar, T., Koenig, B. L., Gambacorta, D., Dolgov, I., Hor, D., Zarzosa, J., & … Wells, L. (2012). Smiles as signals of lower status in football players and fashion models: Evidence that smiles are associated with lower dominance and lower prestige. Evolutionary Psychology, 10(3), 371-397.
Mehu, M., Little, A. C., & Dunbar, R I. M. (2007). Duchenne smiles and the perception of generosity and sociability in faces. Journal of Evolutionary Psychology, 5(1-4), 136-146.
Okubo, M., Ishikawa, K., Kobayashi, A., Laeng, B., & Tommasi, L. (2015). Cool guys and warm husbands: The effect of smiling on male facial attractiveness for short- and long-term relationships. Evolutionary Psychology, 1, 1-8.
Thornhill, R., & Gangestad, S. W. (2006). Facial sexual dimorphism, developmental stability, and susceptibility to disease in men and women. Evolution and Human Behavior, 27, 131–144.
Windhager, S., Schaefer, K., & Fink, B. (2011). Geometric morpho- metrics of male facial shape in relation to physical strength and perceived attractiveness, dominance, and masculinity. American Journal of Human Biology, 23, 805–814.