"Forced" or "Polite" smile
Only muscles directly around the mouth are involved.
Many facial muscle groups move involuntarily. The forehead muscles will push down, cheeks and jaw move and the nose may wrinkle. The easiest way to spot a real smile is to look at the eyes. In natural smile, the eyes become 'squished'. Only a small percent of people can voluntarily move these eye muscles to fake a real smile.