How much you smile, and how wide that smile tends to be, may correlate with your lifespan.
Really.
Here’s researcher Ron Gutman, at a TED conference, discussing what science has discovered about smiling:
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And here’s the closing scene from Woody Allen’s great film, Mighty Aphrodite:
Prometheus Unbound 
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Interesting. I’ve always thought that people running for office do best when they have a great smile so to know that it may help you live longer is a bonus. I’ve read that when sick, watching comedies is good medicine.