A smile

A smile is a facial expression formed primarily by flexing the muscles at the sides of the mouth.[1] Some smiles include a contraction of the muscles at the corner of the eyes, an action known as a “Duchenne smile”. Smiles performed without the eye contraction can be perceived as “fake”.

Among humans, smiling is an expression denoting pleasure, sociability, happiness, or amusement. It is distinct from a similar but usually involuntary expression of anxiety known as a grimace. Although cross-cultural studies have shown that smiling is a means of communication throughout the world,[2] there are large differences between different cultures, with some using smiles to convey confusion or embarrassment.

A smile seems to have a favorable influence upon others and makes one likable and more approachable.[3] In the social context, smiling and laughter have different functions in the order of sequence in social situations:

  • Smiling is not a pre-laughing device and is a common pattern for paving the way to laughter;
  • Smiling can be used as a response to laughter in the previous turn.[4]

Smiling is a signaling system that evolved from a need to communicate information of many different forms. One of these is advertisement of sexual interest. Female smiles are appealing to heterosexual males, increasing physical attractiveness and enhancing sex appeal. However, recent research indicates a man’s smile may or may not be most effective in attracting heterosexual women, and that facial expressions such as pride or even shame might be more effective. The researchers ignored the role of smiles in other sexual preferences.[5]

Cultural differences

A Smiling boy from Bangladesh.

While smiling is perceived as a positive emotion most of the time, there are many cultures that perceive smiling as a negative expression and consider it unwelcoming. Too much smiling can be viewed as a sign of shallowness or dishonesty.[6] In other parts of Asia, people may smile when they are embarrassed or in emotional pain. Some people may smile at others to indicate a friendly greeting. A smile may be reserved for close friends and family members. Many people in the former Soviet Union area consider smiling at strangers in public to be unusual and even suspicious behavior.

While conducting research on the physiology of facial expressions in the mid-19th century, French neurologist Guillaume Duchenneidentified two distinct types of smiles. A Duchenne smile involves contraction of both the zygomatic major muscle (which raises the corners of the mouth) and the orbicularis oculi muscle (which raises the cheeks and forms crow’s feet around the eyes).[9]

A non-Duchenne smile involves only the zygomatic major muscle.[10] “Research with adults initially indicated that joy was indexed by generic smiling, any smiling involving the raising of the lip corners by the zygomatic major…. More recent research suggests that smiling in which the muscle around the eye contracts, raising the cheeks high (Duchenne smiling), is uniquely associated with positive emotion.”[11]

The Pan Am smile, also known as the “Botox smile”, is the name given to a fake smile, in which only the zygomatic major muscle is voluntarily contracted to show politeness. It is named after the now defunct airline Pan American World Airways, whose flight attendants would always flash every passenger the same perfunctory smile.[12] Botox was introduced for cosmetic use in 2002.[13] Chronic use of Botox injections to deal with eye wrinkle can result in the paralysis of the small muscles around the eyes, preventing the appearance of a Duchenne smile.

In animals

In animals, the exposure of teeth, which may bear a resemblance to a smile and imply happiness, often conveys other signals. The baring of teeth is often used as a threat or warning display—known as a snarl—or a sign of submission. For chimpanzees, it can also be a sign of fear. However, not all animal displays of teeth convey negative acts or emotions. For example, Barbary macaques demonstrate an open mouth display as a sign of playfulness which likely has similar roots and purposes as the human smile.[14]

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