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A smile is a facial expression formed by flexing the muscles near both ends of the mouth and by flexing muscles throughout the mouth. Some smiles include contraction of the muscles at the corner of the eyes (also known as 'Duchenne' smiling). Among humans, it is an expression denoting pleasure, sociability, happiness, or amusement. This is not to be confused with a similar but usually involuntary expression of anxiety known as a grimace. Smiling is something that is understood by everyone, regardless of culture, race, or religion; it is internationally known. Cross-cultural studies have shown that smiling is a means of communication throughout the world, but there are large differences between different cultures. A smile can also be spontaneous or artificial.
Primalogist Signe Preuschoft traces the smile back over 30 million years of evolution to a "fear grin" stemming from monkeys and apes who often used barely clenched teeth to portray to predators that they were harmless. The smile may have evolved differently among species and especially among humans. Apart from Biology as an academic discipline that interprets the smile, those who study kinesics and psychology such as Freitas-Magalhaes view the smile as an affect display that can communicate feelings such as love, happiness, pride, contempt, and embarrassment.
A smile seems to have a favorable influence upon others and makes one likable and more approachable. In the social context, smiling and laughter have different functions in the order of sequence in social situations:
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.
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. Japanese people may smile when they are confused or angry. In other parts of Asia, people may smile when they are embarrassed. 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. Whereas smiling freely is far more prevalent in Europe, some see upon it as disagreement. In Southeast Asian cultures, a smile is frequently used to cover emotional pain or embarrassment.
Cheek dimples are visible indentations of the epidermis, caused by underlying flesh, which form on some people's cheeks, especially when they smile. Dimples are genetically inherited and are a dominant trait. A rarer form is the single dimple, which occurs on one side of the face only. Anatomically, dimples may be caused by variations in the structure of the facial muscle known as zygomaticus major. Specifically, the presence of a double or bifid zygomaticus major muscle may explain the formation of cheek dimples. This bifid variation of the muscle originates as a single structure from the zygomatic bone. As it travels anteriorly, it then divides with a superior bundle that inserts in the typical position above the corner of the mouth. An inferior bundle inserts below the corner of the mouth.
While conducting research on the physiology of facial expressions in the mid-19th century, French neurologist Guillaume Duchenne identified 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). A non-Duchenne smile involves only the zygomatic major muscle. “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.”
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 airline Pan American World Airways which went out of business in 1991, whose flight attendants would always flash every jet-setter the same perfunctory smile. Botox was not introduced for cosmetic use until 2002.
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.
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