Run Gutman explains the power of smiling:
by Zoë Pollock
Experimental psychologist Marianne LaFrance highlights the key element in a sincere smile:
It's a muscle, called the orbicularis oculi, that encircles the eye socket. Most people don't pay very close attention to and it's very hard to deliberately adopt. So when people genuinely smile, in a true burst of positive emotion, not only to the corners of the mouth, controlled by the zygomaticus major, but this muscle around the eye also contracts. This causes the crows feet wrinkles that fan out from the outer corners of the eyes and its also responsible for folds in the upper eyelid. Most people can't do that deliberately. … [O]f course your garden variety psychopath or Machiavellian personality tends to be better at it.
The Dish 