In comparative studies of true smiles (with emotion of happiness) and lie smiles (without emotion of happiness), it was found that:

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The main difference between true smiles and lie smiles is found in the use of facial muscles around the eyes. True smiles involve muscle contractions around the eyes, while lie smiles do not. This difference is commonly referred to as "Duchenne's smile," named after French neurologist Guillaume Duchenne, who first identified the phenomenon. There was no indication that there was no difference between the two types of smiles or that children are better at producing lie smiles than adults. There is also no evidence to suggest that the main difference is found in the use of facial muscles around the mouth.

In Comparative Studies of True Smiles (With Emotion of Happiness)