Smiling is a universal way to show happiness. But not all smiles are happy. In reality, we smile less for happiness than for social reasons that have nothing to do with happiness.

That said, few things are more ingratiating and calming as another person's genuinely warm smile. But, maybe it's because a genuine smile is such a great thing that we're always looking for the false one.

But we shouldn't assume that a smile that reflects something other than unadulterated joy is always a bad thing. Smiling has an evolutionary function, helping to ensure our survival after birth. Babies first smile while still in the womb and deliberately smile at us shortly thereafter less because they're thrilled to have us as parents and more to keep us happy with them. There's a reason for this. Smiling has high social benefits: those who smile are considered more social, more accessible, more helpful, and more attractive.

But, what happens when you can't smile? The absence of a smile is life-changing, yet until we lose it, we take it for granted. There are many illnesses that make it difficult to smile including Parkinson's Disease, Bell's Palsy, and Moebius Syndrome, a particularly devastating illness that afflicts babies. Today, we talk to Jonathan Kalb, a professor of Theatre at Hunter College who spent three years recovering his smile after developing what he thought was a temporary bout of Bell's Palsy. He wrote this thoughtful essay on his experience for The New Yorker.

Beyond the inability to smile, what happens you just don't want to smile? The social customs for smiling vary between countries, with many countries feeling we Americans simply smile too much. Partly, it depends on whether you're a woman. As a result, women may smile more, even when they don't want to.

GUESTS:

• Jonathan Kalb: Professor of Theatre at Hunter College CUNY and the author of multiple books on theatre

• Marianne LaFrance: Professor of Psychology and Women’s Gender and Sexuality Studies at Yale University and the author of Why Smile: The Science Behind Facial Expressions

• Margaret Livingstone: Professor of Neurobiology at Harvard University, and author of Vision and Art: The Biology of Seeing

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Colin McEnroe and Chion Wolf contributed to this show, which originally aired on March 31, 2015.

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