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. Among humans, it is an expression denoting pleasure, sociability, happiness, or amusement, but can also be an involuntary expression of anxiety, in which case it is known as a grimace. Smiling is something that is understood by everyone despite 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.
Many biologists think the smile originated as a sign of fear. 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.
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.
Service with a Smile”—This has always been at the core of American businesses since the beginning of the 1900s. Research continually proves that this is true; smiling really does increase attractiveness and likability between humans. In fact, smiling correlates with greater trust, greater financial earnings, and increased interpersonal cooperation. In a time of increased stress due to cutbacks, high debt, and increasing family issues, employees are often required to work with a distressed public. However, a smile tends to convey respect, patience, empathy, hospitality and compassion. For example, when an employee smiles at a stressed customer, and exhibits excellent listening skills, most of the time, there is a report of total satisfaction. Research also reports that people receive more help when they smile. Even the smile of a stranger produces more “Good Samaritan” effects on the receiver. When you smile, even memory retrieval of your name is enhanced as is shown in neuroscience research.
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 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 women, and that facial expressions such as pride or even shame might be more effective.
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. Yet many Americans smile freely at strangers in public places (although this is less common in big cities). Some Russians believe that Americans smile in the wrong places; some Americans believe that Russians don't smile enough. 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.
A smile does indeed have great power and great social rewards. However, it has been proven that an authentic smile  is much more effective than a counterfeit smile. A smile is an outward sign of perceived self-confidence and internal satisfaction. It seems to have a favorable influence upon others and makes one likeable and more approachable.
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, whose flight attendants would always flash every jet-setter the same perfunctory smile.
Happiness is most often the motivating cause of a smile. However, there are many exceptions, especially 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. For chimpanzees, it can also be a sign of fear. The study of smiles is a part of gelotology, psychology, and linguistics, comprising various theories of affect, humor, and laughter. Smiling can also be interpreted as nervousness in an animal—humans also smile as a result of nervousness, embarrassment and even frustration. In one study, created to investigate the correspondence between perceived meanings of smiles and their morphological and dynamic characteristics, it was found that “perceived embarrassed/nervous smiles had greater amplitude, longer duration…related to those perceived as polite.” Work by John Gottman has shown that smiling and other such expressions of positive emotions are important to shaping relationships with others; researchers could predict the quality of marriages many years into the future based on the number of such interactions.