“Emotion is contagious. In a way, this is perfectly intuitive. All of us have had our spirits picked up by being around somebody in a good mood. If you think about this closely, though, it’s quite a radical notion. We normally think of the expressions on our face as the reflection of an inner state. I feel happy, so I smile. I feel sad, so I frown. Emotion goes inside-out. Emotional contagion, though, suggests that the opposite is also true. If I can make you smile, I can make you happy. If I can make you frown, I can make you sad. Emotion, in this sense, goes outside-in.”