In research performed at the University of Clermont-Ferrand in France, subjects were asked to interpret real vs. fake smiles, while holding a pencil in their mouths to repress the muscles that help us smile.

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Such a smile, commonly referred to as a Duchenne smile, after the 19th century French neurologist who first described it, activates major muscles around the mouth and the eyes.

WSJ: Stress-Busting Smiles

By contrast, a standard social smile, which is sometimes called a Pan Am smile after the polite expression the former airline's stewardesses used to greet passengers, activates only the muscles around the mouth.

WSJ: Stress-Busting Smiles

Typically, someone who is in agreement with you will smile and nod as you speak. (Disagreement shows up in compressed or pursed lips, clenched jaw muscles, or a head turned slightly away, so eye contact becomes sidelong.) But smiles are often used as a polite response and to cover up other emotions.

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