Guillaume Duchenne, the famous French neurologist of the 19th century, conducted many experiments to study facial expressions. Part of his research involved determining how certain expressions are produced – such as stimulating different muscles with electricity to see what muscle produced what expression. During this research, Duchenne identified that a smile could be divided into two distinct groups.
The first – what he called the “Duchenne smile” – involves a muscle called zygomatic major, which raises the corners of the mouth, and also orbicularis oculi, which raises the cheeks and wrinkles the corners of the eyes. The second (“non-Duchenne smile”) type of smile involves less muscles; more specifically it only uses the zygomatic major muscle.
To better visualise this, think of what a natural, genuine smile looks like – a wide grin on the mouth, lifting of the cheeks and slanting of the eyes. This is a Duchenne smile, as opposed to the forced, non-Duchenne smile you see often in photographs. Duchenne concluded that a Duchenne smile only showed when the person was genuinely experiencing a positive emotion. Non-Duchenne smiles were more associated with polite social behaviour when people were pretending to have a good time.
The easiest way to distinguish the two is to look at the eyes, for a real smile is when the eyes are smiling as well. This is a useful physiological trick to remember when you want to figure out whether someone is smiling because they are genuinely happy, or because they are just trying to be polite. Also, knowing how to smile with your eyes to fake a “genuine” smile can be a handy social skill.