Smiles—and frowns—are so contagious that they can jump from person to person in a fraction of a second, according to researchers studying the human brain's amazing ability to read expressions. In a paper published in the journal Trends in Cognitive Sciences, researchers say that when we see an expression on another face it activates what is called "sensorimotor simulation," in which we automatically mimic the other's expression, helping us understand and react to the other person's emotion. This reaction takes place in the space of a few hundred milliseconds and happens completely unconsciously, researchers say. The facial muscles don't always move, but the relevant parts of the brain are still activated.
The muscle reaction helps the brain access memories of similar emotional states, the researchers say. Medical News Today notes that in the distant past, our ability to read a stranger's expression in a split-second could have meant the difference between life and death. "It's kind of like an extreme version of putting yourself in someone else's shoes," lead researcher Adrienne Wood, a social psychologist at the University of Wisconsin, tells the Christian Science Monitor. "You're putting yourself in someone else's facial expression." The researchers say that a better understanding of how we "can infer complex mental states from even the slightest facial expression" could help develop new treatments for people with social disorders. (Other researchers say they know why babies smile.)