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In physiology, a smile is a facial expression formed by flexing the muscles most notably near both ends of the mouth. The smile can be also around the eyes. Among humans, it's customarily an expression of pleasure, happiness, or amusement, but can also be an involuntary expression of anxiety, in which case it can be known as a grimace. There is much evidence that smiling is a normal reaction to certain stimuli and occurs regardless of culture. However, happiness is most often the cause of a smile. Among animals, the exposure of teeth, which may bear a resemblance to a smile, is often used as a threat or warning display - known as a snarl - or a sign of submission. In chimpanzees, it can be a sign of fear.

Contents 1 Types of smiles 2 Historical background 3 Examples 4 References 5 See also 6 Images 7 External links

Researchers have identified two main varieties of smiles:

• The "Duchenne smile", after the researcher Guillaume Duchenne, is the most studied, and involves the movement of both the zygomaticus major muscle near the mouth and the orbicularis oculi muscle near the eyes. An example of the smile is shown in the girl's smile in the middle of the page. It is believed that the Duchenne smile is only produced as an involuntary response to genuine emotion, and is therefore what one could call the "genuine" smile. Due to the involvement of the muscle near the eyes, it is sometimes said that one can tell whether or not a smile is "real" by whether or not it "reaches the eyes".
• The "Pan American smile", on the other hand, is the voluntary smile involving only the zygomaticus major muscle to show politeness; for example, by a flight attendant on the former airline of the same name. Considered "insincere", this type of smile has also been called the "Professional Smile" by David Foster Wallace in his comedic short story "A Supposedly Fun Thing I'll Never Do Again".

Many biologists think the smile started as a sign of fear. Primalogist Signe Preuschoft traces the smile back over 30 million years of evolution to the "fear grin." Monkeys and apes used barely clenched teeth to portray to predators that they were harmless. Biologists believe the smile has evolved differently among species and especially among humans.

Biology is not the only academic discipline that interprets the smile. Those who study kinesics view the smile as an affect display. It can communicate feelings such as: love, happiness, pride, contempt, and embarrassment.

Researchers have come to the conclusion that smiles are innate. For example, blind people smile without learning it through observation. However, the use of smiles varies from culture to culture. For example, Middle Easterners think Americans are insincere because they smile so often.^{[citation needed]} Another example is that the Japanese smile less because of societal pressures to hide emotions.^{[citation needed]}

• Miller, Professor George A., et. al. Overview for "smile." Retrieved 12 December 2003 from this page.

• Conniff, R. (2007). What's behind a smile? Smithsonian Magazine, 38,46-53.

• Ottenheimer, H.J. (2006). The anthropology of language: An introduction to linguistic anthropology. Belmont, CA: Thomson Wadsworh.

• Scanner shows unborn babies smile

• Laughter
• Emotion
• Emoticon
• Facial Action Coding System
• Frown
• Facial expression
• Smiley
• Social Psychology

Girl with closed mouth smile.

Girl with open mouth smile.

Politicians and leaders are often shown smiling as this is considered a sign of confidence

Artwork on this ball is a common abstract representation of a smiling face.

• See examples of genuine, false, and contempt smiles
• Spot The Fake Smile on BBC Science & Nature
• Expressions of Positive Emotion in Women's College Yearbook Pictures and Their Relationship to Personality and Life Outcomes Across Adulthood.