A fake smile actually uses different muscles than a real smile. Since a real smile is involuntary and a fake one is deliberate, different portions of the brain control the muscles used for each. The muscle responsible for drawing the corners of the mouth outwards, zygomaticus major, is active in both types, but additional muscles are involved in real smiles. When a person genuinely smiles, the orbicularis oculi and the pars orbitalis contract in addition to zygomaticus major, causing the cheeks to raise and the skin around the eyes to crease. A genuine smile is also known as a Duchenne smile, as French neurologist Guillaume Duchenne discovered the distinction between the muscles used in fake and real smiles in the 19th century.
One of the best ways to spot a fake smile is to look at the end of the eyebrows, which dip slightly when a smile is real. Also, in a real smile, the fold of skin between the eyebrow and the eyelid moves downward. False smiles are less likely to be symmetrical than a real smile, as voluntary control of the zygomaticus major is not always perfect. A genuine smile lasts for up to five seconds, while a fake can last much longer. Though a false smile can be very convincing and can even cause the eyes to scrunch up a bit, with practice, many people can distinguish a genuine Duchenne smile from a deliberate one.
Smiles expressing happiness are not just superficial; people in every culture smile the same way, and even people born blind smile involuntarily when they are happy. Many biologists think that the smile originated from an expression of fear, like a grimace, as some primates bare slightly clenched teeth to indicate that they are harmless to potential predators. Being able to spot a fake smile may also be an evolutionary adaptation, as determining whether others are really cooperating and bonding with you or not can confer an evolutionary advantage.