"I'd have to say never, " he said, smiling with his response.

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Results show that high delayers are more successful at not pressing the button in response to a smiling face.

NEWYORKER: Don��t!

Marco Iacoboni, a lab director at the UCLA Brain Mapping Center, says when people see a smile, so-called mirror neurons fire in their brain and evoke a similar neural response as if they were smiling themselves.

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These findings would not have surprised Charles Darwin, who in addition to theorizing on evolution in The Origin of the Species, also developed the Facial Feedback Response Theory, which suggests that the act of smiling actually makes us feel better (rather than smiling being merely a result of feeling good).

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