Many biologists think smiles have roots as a fearful gesture, as a sort of “fear grin.” Today though, human smiles are a universally recognized as submissive gesture and can represent pleasure, amusement, aggression, or anxiety (grimace). Smiles are the way we show others that we are non-threatening and wish to maintain peace.
Chimpanzees, like humans have two common sets of smiles. One is a playful smile and the other is an aggressive smile. Humans, when aggressive, will display with a smile by pushing forward their lower lip and baring their lower teeth. This in turn induces either an appeasement or submissive smile in our counterparts, or aggressive body language possibly leading to conflict.
A friendly smile, on the other hand, occurs by pulling the corners of the mouth upward which may or may not include crow’s feet. Those that lack smiles altogether or who smile infrequently are much more dominant and want to be seen as such.
The grimace happens in situations where one is forced to talk or debate under a stressful situation. You will find that this sly face appears almost universally even though no pleasure is felt. The grimace is a disarming gesture meant to appease others, but it usually comes off as insincere and smug. We will see this face under high stress situations.
Research has shown that our facial muscles tend to mirror what we see in others and that this reaction is subconscious. While it is possible to control our facial muscles by consciously over-ridding this tendency, most others will simply, by nature, imitate what they see in us. This is why it is so important to, both imitate smiles when seen, and to also avoid holding sour faces while making first impressions.