This week, scientists reported they'd figured out the features of smiles that convey different emotions. The muscles that produce these expressions were first studied in a series of startling experiments by French neurologist Guillaume-Benjamin-Amand Duchenne.

The experiment: In the mid-1800s, Duchenne used small probes to electrically stimulate muscles in the faces of six different people and reproduce various expressions. He built his own electroshock apparatus and enlisted a photographer to use the then newly developed camera to capture the fleeting expressions.

What he found: Duchenne described the muscle actions that produce more than 30 distinct emotions, ranging from doubt and stupefaction to joy and terror (with and without pain or torture). He found that a combination of the muscle that pulls the lips up at the corners and the involuntary action of the orbicularis oculi muscle around the eye created genuine smiles of happiness. When a smile is faked, it doesn't engage the eye muscle, he proposed. (Duchenne saw the face as the reflection of the soul so his observation about voluntary v. involuntary action is an important if loaded one.)

The legacy: Charles Darwin wrote about the importance and care of Duchenne's work in the introduction to his book, The Expression of the Emotions in Man and Animals. Genuine smiles were dubbed Duchenne ones and several diseases of muscle tissue —for example, Duchenne muscular dystrophy — bear his name. His claim that the involuntary action of the eye muscle in true smiles can't be faked has persisted, though recent studies suggest people can deliberately produce a Duchenne smile.