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You kinda know when you feel it, but science says the tell is in the eyes.
A real smile involves contraction of two muscles:
1. Zygomatic major muscle (which raises the corners of the mouth)
2. Orbicularis oculi muscle (which raises the cheeks and makes your eyes crinkle).

Named after the French anatomist, Guillaume Duchenne, the “Duchenne” is the real smile.
So, in a “non-Duchenne” smile, only the zygomatic major muscle contracts. In other words your mouth turns up, but your eyes don’t crinkle.

Can you fake a smile? After Duchenne, several researchers used these ideas to research smiling and believed that it was not possible to “fake” a smile, because up to 80% of people can’t consciously control the muscles around the eyes that make them crinkle.

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Why all the interest in whether a smile is real or fake? Because of the interest in the key to people’s belief-and-buying behavior: trust. People are quicker to trust (i.e., vote for, follow, invest in, idolize) someone who is showing what is believed to be genuine emotions rather than fake or contrived ones.

Questioning Duchenne’s 80% figure, Krumhuber and Manstead (2009) decided to research whether it was true that most people couldn’t create a fake smile that looks real.

They found the opposite of what was previously believed. With a modern video twist.
1. 83% of the people could produce fake smiles that other people thought were real (photos of people pretending to smile)
2. Videos tell a better story.

It’s harder to fake a smile in a video. Not because of the crinkly eyes. In a video, the tell was in:
a. how long they held the smile
b. whether they saw other emotions besides happiness; for example, a flicker of impatience.

The video made it easier to detect a fake smile because it lasted longer and was dynamic, instead of just an instant’s snapshot.

So, it turns out, it’s not saying “cheese” that’s important for a great picture or group video shot.
It’s everyone actually feeling truly happy about being in that moment.

Krumhuber, Eva G., & Manstead, A. (2009).  Emotion, 9(6), 807-820.
Read more: http://www.businessinsider.com/100-things-you-should-know-about-people-part-2#you-can-tell-if-a-smile-is-real-or-fake-more-accurately-with-video-42#ixzz1wjqLfKq7

Latest Update: Jun 13, 2016