The natural smile, which is an involuntary
action, is known as the zygomatic
smile. The zygomaticus muscles
contract, which causes the lips to curl upwards and 'crow's feet
' to form around the eyes. Zygomatic smiles are a good representation of the mood of a person.
In a false smile, the risorius muscles contract, stretching the lips sideways, but not upwards. There is also no change around the eyes.
False smiles are also asymmetrical. This is because a conscious smile involves both hemispheres of the brain and the dominant side of the brain will create a stronger signal to one side of the face. Although this is only subtle, it is apparent on some level to human viewers. A natural smile is controlled by the lower part of the brian, and therefore both sides of the face are affected equally, giving a symmetrical smile.
False smiles also last longer in duration than natural ones. A real smile will be between two thirds of a second and four seconds in length. Almost all smiles over five seconds in length are forced.