Fake smiles lack "full expressions", like
crinkling around the eyes and
symmetry of lips. Forced smiles are less likely to be symmetrical. In my high school and college graduation
studio photos, my smiles are slightly
askew, since I didn't feel like there was anything to be
happy about from being told to turn my head this way, look over here, move my shoulder back, then "SMILE!" Grrrrr. In my family photos my smiles are often more
symmetrical since someone usually makes me
laugh.
Real smiles last for one to five seconds, staying on the person's face from the beginning to the peak to fading.
Fake smiles can stay glued on a person's face for 20 seconds.
Smiling to express happiness is not just superficial. Children who are born blind still smile when they are happy...unless they don't want to.