Although Charles Darwin, in his book on expression, published this photograph and described Duchenne's finding, it was ignored until very recently. In 1982, Paul Ekman resurrected Duchenne's distinction to explain why people often smile when they are not happy. As Duchenne had noted, Ekman found that people who smile when they are not feeling enjoyment do not show activity of the muscle around the eyes, just the lip muscle. Many studies now support Duchenne's distinction between these two types of smiling — what scientists now call, in honour of Duchenne, Duchenne smiles, or D-smiles for short (zygomatic major outer part, and orbicularis oculi) and non D-smiles (zygomatic major only). For example, 5-month-old infants show D-smiles when approached by their mother, non D-smile when approached by a stranger. In adults the D-smile is accompanied by the pattern of brain activity found with enjoyment, but that brain activity pattern is not found when the non D-smile is shown. Happily married couples show D-smiles when they meet at the end of the day, while unhappily married couples show non D-smiles.
It is not always easy to distinguish these two types of smiles. If the lips are pulled only slightly or moderately by the zygomatic major muscle, it is easy to see whether the eye muscle is involved, for it will produce crow's feet wrinkles and bagging of the skin under the eyes. Those signs are absent in a slight to moderate non D-smile. However, when the smile is very broad, the lip pulling itself will produce those changes in the face and it is necessary to look elsewhere. Only in the D-Smile will the eyebrows move down ever so slightly.
Instead of signalling genuine enjoyment, non D-smiles serve many different social functions. They may indicate agreement, they show a person is willing to go along with something, even something unpleasant (grin and bear it), and they may also be used to send a false message of enjoyment when none is felt. Research has shown that most people do not notice the difference between D-smiles and non D-smiles, and it is hard not to reciprocate a smile, even a non-D smile.
There is as yet no clear answer to the question, ‘Why do the lip corners go up rather than down with enjoyment?’ Research has found that the smile is a very powerful signal that stands out from all the negative emotional expressions. If the expression is broad, it can be accurately recognized (although not, of course, the distinction between D- and non D-smiles), at about 100 m — the maximum distance of a javelin throw.
See also face; facial expression.
While people often think of smiling as only an indication that a child might find something funny, it is actually one of the most important forms of social communication. Smiling appears within the first few weeks of life as a response to a human voice and becomes a full-fledged social smile at about three months of age. As a social behavior it encourages parents to interact with developing infants and thereby helps ensure the infant will be cared for as well as socialized into the culture, which the parents represent. In other words, it promotes bonding. Child development specialists have studied smiling and have found it to be a complex behavior that is integral to a child's healthy development. For example, the more infants smile, the more time their mothers spend with them. Children who do not smile early and often are not just unhappy. Rather, there is some other issue at hand that needs professional attention.
Bailey, Kimberly. "What's in a Smile?" [web site]. Available from http://bipolar.about.com/health/bipolar/library/weekly/aa000802a.htm?rnk=r8&terms=smiling; INTERNET.
Farris, Marinelam R. "Smiling of Male and Female Infants to Mother vs. Stranger at Two and Three Months of Age." Psychological Reports 87 (2000):723-728.