Two kinds of human smile exist across all cultures: the truthful "Duchenne smile," and the fake "social smile." Now evolutionary psychologists believe the social smile evolved to prevent biting and screaming.

A recent study by San Francisco State psychologist David Matsumoto published today in the Journal of Personality and Social Psychology offers evidence that facial expressions are innate rather than learned. He examined over 4,000 photographs of blind and sighted athletes who had just lost important competitions. Though the blind athletes could not have learned their facial expressions from seeing other people's, they nevertheless produced very similar unhappy expressions when they first heard the news that they'd lost. Later, they moved almost the same facial muscles to produce social smiles during award ceremonies.

Matsumoto explained that a genuine Duchenne smile causes the cheek muscles to move, and also makes the eyes narrow (some describe this as "twinkling eyes"). A social smile affects only the mouth muscles, and usually the lips remain closed in a social smile.

Said Matsumoto:

The statistical correlation between the facial expressions of sighted and blind individuals was almost perfect. This suggests something genetically resident within us is the source of facial expressions of emotion. Losers pushed their lower lip up as if to control the emotion on their face and many produced social smiles. Individuals blind from birth could not have learned to control their emotions in this way through visual learning so there must be another mechanism. It could be that our emotions, and the systems to regulate them, are vestiges of our evolutionary ancestry. It's possible that in response to negative emotions, humans have developed a system that closes the mouth so that they are prevented from yelling, biting or throwing insults.

So we're smiling instead of biting each other? Evolution works!

SOURCE: San Francisco State University

Photos by Bob Willingham.