When Hatfield realized she was onto something, she teamed up with colleagues to pinpoint the process. They discovered that, from infancy, all of us imitate facial expressions, postures, and voices of the people around us. Those expressions trigger certain emotions—the same ones experienced by the person we mimic. But the process happens so fast, we're completely unaware of it. Mimicry is a basic biological mechanism that may confer an evolutionary advantage, says Peter Totterdell, PhD, senior research fellow at the University of Sheffield in England. It helps you understand what another person is feeling and thinking—even when she's trying to hide it.
The science of emotional contagion goes back to 400 B.C., when Hippocrates, the founder of medicine, observed that some women seemed to transfer "hysteria" to one another. By the 1700s, researchers began to discover that people mirror the smiles and frowns they see on someone else's face. In the late 1800s, German psychologist Theodor Lipps took the idea a step further, suggesting that this unconscious imitation was the root of empathy. But it's only within the past several decades that scientists have begun to understand the dynamics behind such exchanges, finding that "emotional contagion" affects all human relationships, from marriage to business to professional sports.
Paul Ekman, PhD, a professor emeritus of psychology at the University of California at San Francisco, supplied one piece of the puzzle when he discovered that facial expressions for seven emotions—anger, fear, sadness, disgust, surprise, contempt, and happiness—are the same around the world. One of the first signs of sadness (and perhaps what Hatfield saw on the face of her depressed patient) is a lift at the inner corners of our eyebrows. A hint of anger is a tiny thinning of the lips. The signs are subtle, Ekman says, "but once you learn to spot them, you can see them as they occur."