Eighteen kinds of smiles
According to Paul Ekman, ‟Smiles are much more complicated than most people realize. There are dozens of smiles, each differing in appearance and in the message expressed. There are many positive emotions signaled by smiling—enjoyment, physical or sensory pleasure, contentment, and amusement, to name just a few. People also smile when they are miserable. These aren't the same as the false smiles used to convince another that positive feelings are felt when they aren't, often masking the expression of a negative emotion. We found that people are easily misled by these false smiles.”
Eighteen kinds of genuine smiles
Ekman has distinguished not less than eighteen different kinds of genuine, non-deceptive smiles and has described the facial expressions and the particular muscles in the face involved differently with these various smiles.
The false smile is intended to convince another person that positive emotion is felt when it isn't. Nothing much may be felt, or negative emotions may be felt that the liar tries to conceal by using the false smile as a mask. It is the only smile that lies. There are a number of clues for distinguishing false smiles from the felt smiles they pretend to be: false smiles are more asymmetrical than felt smiles. The false smile will not be accompanied by the involvement of the muscles around the eyes. The lack of eyebrow involvement is a subtle cue, but a crucial one for distinguishing felt from false smiles when the grin is broad.
From the recent photography book 108 Sourires