From "engines"
The French neurologist Guillaume Duchenne
mapped 100 facial muscles in 1862. In the course of
that work, he had something to say about smiling. He
pointed out that false, or even half-hearted, smiles
involved only muscles of the mouth. But "the sweet
emotions of the soul," he said, activate the pars
lateralis muscle around the eyes.

Since then, physiologists have talked about the
Duchenne marker in a smile. It's a slight crinkling of
crows-feet and a droop in the eyelid toward the
temples -- along with a lift of the cheeks and the
corners of the mouth. You know the sign. You
recognize true delight in a friend's face.