That Look -- It's Catching!
Emotions, Like Germs, Are Easily Transmissible. The Trick Is Passing and Receiving the Right Ones
"In a study at Uppsala University in Sweden, researchers exposed people to pictures of happy or angry faces for 30 milliseconds, immediately followed by neutral faces. Even though the participants didn't realize they'd just looked at a happy or angry face, they responded with distinct facial muscle reactions of their own that corresponded to the emotion they'd just seen.
"Those incremental muscle movements then trigger the actual feeling by causing the same neurons to fire in the brain as if you were experiencing the emotion naturally, according to Hatfield..."
Read the whole thing, it's a good article.