Question

According to ekman's research on facial expressions when people try to hide their feelings with an inauthentic expression, they use different groups of muscles than they do for authentic emotions. there is little support for darwin's evolutionary rationale that certain human facial expressions are innate. when trying to feign sadness, only about 60 percent of people manage to get the eyebrows, eyelids, and forehead wrinkle exactly right. authentic smiles last ten seconds or more whereas false smiles last only about two seconds.

Answer 1

The answer is: When people try to hide their feelings with an inauthentic expression, they use different groups of muscles than they do for authentic emotions.
The movement of these different group of muscles tend to happen really fast (it would be less than 1 second) and could be identified by observing a small twitches in the eyebrows, nose, or the lips area.