Smile

June 9, 2026

AI Summary

5 min read

The Science and History of Smiling

The human smile is far more complex than a simple expression of happiness. It involves specific facial muscles, carries deep evolutionary roots, and has a surprisingly contentious cultural history. In this episode of Stuff You Should Know, Josh and Chuck explore the mechanics, evolution, cultural variations, and psychological effects of smiling—revealing that this seemingly universal gesture is anything but simple.

The Muscles Behind the Smile

Smiling is a muscular event. The face contains a high concentration of fast-twitch myosin fibers, which is why humans can produce such a wide range of expressions. The key muscle involved in smiling is the zygomaticus major, which pulls the corners of the mouth upward and outward. Researchers have developed a Facial Action Coding System (FACS) to catalog these movements. Every smile involves "action unit 12" (AU12)—the contraction of both zygomaticus major muscles. A genuine, joyful smile also involves "action unit 6" (AU6), which engages the orbicularis oculi muscles around the eyes.

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What you'll learn

1 (02:02) **Smiling Muscles and the Zygomaticus Major** - Josh and Chuck introduce the topic of smiling, starting with the muscles involved, including fast-twitch myosin fibers and the zygomaticus major.
2 (07:41) **The Facial Action Coding System (FACS)** - The hosts explain the FACS, which codes facial muscle movements, and introduce Action Unit 12 (AU12) for smiling and AU6 for the eyes.
3 (10:02) **Duchenne Smiles and the Eyes** - The hosts discuss Guillaume Duchenne's work on genuine vs. fake smiles, focusing on the role of the eyes (AU6).
4 (13:23) **Evolutionary and Social Functions of Smiling** - The hosts explore the evolutionary roots of smiling, including submission signals in primates and social bonding in humans.
5 (17:15) **Innate vs. Imitative Smiling** - The hosts discuss studies on blind infants and athletes, showing that smiling is innate, not just imitative.
6 (21:52) **Cultural Differences in Smiling** - The hosts discuss how smiling is perceived differently across cultures, including Paul Ekman's work on universal facial expressions.
7 (28:22) **History of Smiling in Western Culture** - The hosts trace the history of smiling in art and photography, from serious portraits to the modern smile.

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Show Notes

They say that smiling requires fewer muscles than frowning, so why not smile? That doesn’t seem to be true, or at least unproven, and there is lots more about smiling that will keep you glued to your earbuds in this fun episode.

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