This week, millions of people around the world will no doubt experience rapid mimicry-an involuntary, split-second mirroring of another person's facial expressions-as they exchange smiles over gifts, good meals and holiday traditions. This phenomenon, observed in humans and many other primates, is considered a basic building block of our ability to feel empathy. "When your companion or friend smiles, you don't know why exactly, but you immediately react with the same smile to him or her," says Elisabetta Palagi, an evolutionary biologist at the University of Pisa in Italy. "It’s an extremely important phenomenon, because through this mimicry you can...