Smiling probably has an
evolutionary basis in the way that primates show
submission. When
Chimpanzees and
Bonobos wish to show threat, they expose their teeth with the lips curled up and teeth apart, ready to
bite. However, when they are being submissive or playful the teeth stay together and the lips are partially opened, the primate form of a smile. While the reasons people smile today aren't nearly so clear cut, this still points to the smile's function in social relationships.
There is a difference -- both physiological and psychological -- between a smile of true happiness and a faked social smile. A real smile (also known as a Duchenne smile after the man who discovered it) is a strong contraction of the zygomaticus muscles, which pull the corners of the mouth upwards. It usually also involves the orbicularis oculi muscles surrounding the eye, which cause visible crows-feet at the eye's edges. When early neurologist Guillaume Duchenne discovered this difference in 1862, he remarked that it's lack "unmasks the false friend." In comparison, faking a smile requires activation of mostly the face's risorius muscles, which pull the lips horizontally apart. The zygomaticus muscles are only used a little bit, to pull the corners of the mouth up slightly.
Interestingly, the pathway between emotional state and facial expression appears to go both ways. In a recent study, a non-verbal communication psychologist named Paul Ekman has discovered that forcing a zygomatic smile after one has learned how to do so causes activity in the left pre-frontal cortex, which is the home of the "pleasure center" (see also Emotion and regional brain activity). This relationship may be the reason that telemarketers are told to smile because it can be "heard" over the phone -- when the telemarketer smiles, his/her brain is activated in a perceptibly more pleasant way.
From a developmental standpoint, the hardware used to smile (and frown, for that matter) develops fairly early. The nerves and muscles that control our mouth develop from the first pharyngeal arch, and the ones controlling our eyelids and brow from the second. Upon unpleasant stimuli -- bitter taste, sharp pain -- these muscles constrict, forming the squint-eyed, wrinkle-nosed, frowning, "yuck face." Pleasant stimuli do just the opposite, relaxing the muscles and thus dilating all of the facial openings, giving a wide-eyed, happy smile. These involuntary expressions can be traced back to our ancestors the fish, which closed gills and eyes when presented with noxious stimuli, and opened them while exploring something that might be food. Babies begin to smile in response to faces and other social stimuli at between eight and twelve weeks. By five months, an observer can differentiate between Duchenne smiles for the infant's parents and "faked" smiles for strangers.