Think about the last time you played with a baby. When you put your hands over your eyes, she did the same thing. "Mirroring" is a natural human behavior. Through imitation, our species is able to learn, communicate, and empathize. Our natural inclination for mimicry means that we often "catch" each other's moods, which is why you might start to feel anxious when your spouse starts stressing about seeing their in-laws for the holidays, or why an easygoing, buoyant new hire can change the energy in an office.
The good news is that positive emotions are more contagious than negative ones. A Yale University School of Management study found that cheerfulness and warmth spread far more quickly throughout an office than irritability and depression. The best way to spread these good feelings? With a big toothy smile, the most contagious gesture of all.
When you smile at others, you literally "infect" them with happiness. Psychologist Fritz Stack discovered this when he had participants in a study hold a pencil with their teeth, which activated the smiling muscles. When they were smiling, participants found cartoons funnier than when they weren't smiling. Translation: It's a lot easier to make your client, your boss, or your spouse more receptive to your ideas if you say it with a smile.