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.