“…no sooner had smiling become reasonably well established as a reliable signal of trustworthiness than it also became adaptive to be able to counterfeit smiles. Smiles that are under deliberate control are known to use a different set of neural circuitry than spontaneous smiles…Not everyone can fake smiles successfully-- indeed, politicians are predominantly drawn from among those human beings who can. Many people (myself included) are effectively barred from political careers by their inability to produce convincing smiles to order for the camera. But enough people can do so to suggest that the evolution of smile mimicry has proceeded quite far in the human species.

But almost nobody can fake laughter convincingly. Laboratory studies show that many  people are unable to discriminate reliably between spontaneous smiles and those produced by good actors. But virtually everyone can tell the difference between spontaneous laughter and the deliberate laugher of even very talented actors. And deliberate laughter provokes much less positive affect in those that hear it than does spontaneous laughter…The fact that smiling was losing its reliability, because so many people could fake it, made it valuable to have another, better signal of positive affect. And the possibility that laughter evolved later would explain why the ability to fake convincingly has not yet had time to evolve.”

- Paul Seabright, The Company of Strangers, 2004

Close Calls:
http://www.dailymotion.com/relevance/search/la+chance+compilation/video/x7cmf3_chance_fun