the Duchenne smile is among the most influential of human expressions.

It's a genuine smile that involves the muscles around the eyes.  

From Healthline,

The human smile is a powerful thing. To elevate mood, inspire empathy, or calm a fast-beating heart, you don’t need a gleaming row of perfect pearly whites. One YouTube baby-laugh video is enough to demonstrate that even an entirely toothless  smile is a wonder-worker.

Researchers who study the effects of human smiles know that the Duchenne smile is among the most influential of human expressions.

A Duchenne smile is the one that reaches your eyes, making the corners wrinkle up with crow’s feet. It’s the smile most of us recognize as the most authentic expression of happiness.

Non-Duchenne smiles shouldn’t necessarily be considered “fake,” however. A more accurate way of describing them might be “polite.”

Polite smiles can communicate social pleasantness, and they can even signal discreet psychological distanceTrusted Source, which may be an appropriate response in many situations.

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