Back in the 19th century, people were keen on all sorts of crazy fucking medical experimentation. One man, Guillaume Duchenne, was obsessed with finding the perfect smile. And his method to find the perfect smile involved using electricity to shock various parts of the face, from people of all ages and walks of life, in order to force them to smile.
Duchenne started zapping all along people’s bodies, until he’d mapped out how current traveled along the human form. After he’d established that map, he isolated muscle groups and concentrated on activating the most complicated and readable of all muscle groups: the facial muscles. Duchenne photographed the results of most of his shocks, and so collected an extensive gallery of gruesome expressions. He would hit one half of the face while leaving the other half relaxed. He’d catalog different groups of muscles, that worked in different expressions, and tried to combine them.
He’s most famous, though, for his work on smiles.
The “Duchenne smile” is a genuine one, one that seems happy and friendly. It’s the result of two different groups of muscles working together. Cheek muscles turn the lips up in a voluntary maneuver that can be faked. But the muscles under your eyes that crinkle the skin to the outer side of each eye are involuntary, and they don’t necessarily start working just because you need to make a good impression at a job interview. Only the two combined really look genuine. When you smile with just your lips, you look like you’re either faking or in pain.