Smile

A smile is a facial expression formed primarily by flexing the muscles at the sides of the mouth.

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[2] or to signal submission to more dominant group members.

It may take the form of positive reinforcement, possibly for an underhand manipulative and abusive purpose.

Many people in the former Soviet Union area consider smiling at strangers in public to be unusual and even suspicious behavior,[9] or even a sign of stupidity.

For example, Greg Rickford, a member of the Canadian Parliament told a female journalist to smile rather than answer the question she had asked.

[13] Cheek dimples are formed secondary to a bifid zygomaticus major muscle, whose fascial strands insert into the dermis and cause a dermal tethering effect.

As it travels anteriorly, it then divides with a superior bundle that inserts in the typical position above the corner of the mouth.

[22] While conducting research on the physiology of facial expressions in the mid-19th century, French neurologist Guillaume Duchenne identified two distinct types of smiles.

It is named after the now-defunct airline Pan American World Airways, whose flight attendants would always flash every passenger the same perfunctory smile.

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.