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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.

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