Smile

A smile is formed primarily by flexing the muscles at the sides of the mouth. Some smiles include a contraction of the muscles at the corner of the eyes, an action known as a Duchenne smile. 
Among humans, a smile expresses delight, sociability, happiness, joy 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, there are large differences among different cultures, religions and societies, with some using smiles to convey confusion or embarrassment.
Evolutionary background.
Primatologist 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, or to signal submission to more dominant group members. The smile may have evolved differently among species and especially among humans.
Social effects.
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 in many different forms. One of these is an 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 did not explicitly study the role of smiles in other sexual preferences.
As reinforcement and manipulation.
The influence of smiling on others is not necessarily benign. It may take the form of positive reinforcement, possibly for an underhand manipulative and abusive purpose. See also superficial smile.
Cultural differences.
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. In some 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, or even a sign of stupidity.
Systematic large cross-cultural study on social perception of smiling individuals documented that in some cultures a smiling individual may be perceived as less intelligent than the same non-smiling individual (and that cultural uncertainty avoidance may explain these differences). Furthermore, the same study showed that corruption at the societal level may undermine the prosocial perception of smiling—in societies with high corruption indicators, trust toward smiling individuals is reduced.
There can also be gender differences. In the United States and the United Kingdom, women report men telling them to smile. For example, Greg Rickford, a member of the British Parliament told a female journalist to smile rather than answer the question she had asked. Biological anthropologist Helen Fisher states that while this could be either caring or controlling behavior, such behavior is unlikely to be welcome. The subject has been covered in the book "Stop Telling Women to Smile" by Tatyana Fazlalizadeh.
Dimples.
Cheek dimples are visible indentations of the epidermis, caused by underlying flesh, which forms on some people's cheeks, especially when they smile. Dimples are genetically inherited and are a dominant trait. Having bilateral dimples (dimples in both cheeks) is the most common form of cheek dimples. 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. Dimples are analogous and how they form in cheeks varies from person to person. The shape of a person's face can affect the look and form as well: leptoprosopic (long and narrow) faces have long and narrow dimples, and eryprosopic (short and broad) faces have short, circular dimples. People with a mesoprosopic face are more likely to have dimples in their cheeks than any other face shape.
Duchenne smile.
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). The Duchenne smile has been described as "smizing", as in "smiling with the eyes". An exaggerated Duchenne smile is associated with lying.
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 now-defunct airline Pan American World Airways, whose flight attendants would always flash every passenger the same perfunctory smile. Botox was introduced for cosmetic use in 2002. 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 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.