Association for Psychological Science: “A smile begins in our sensory corridors. The earcollects a whispered word. The eyes spot an old friend on the station platform. The hand feels the pressure of another hand. This emotional data funnels to the brain, exciting the left anterior temporal region in particular, then smolders to the surface of the face, where two muscles, standing at attention, are roused into action: The zygomatic major, which resides in the cheek, tugs the lips upward, and the orbicularis oculi, which encircles the eye socket, squeezes the outside corners into the shape of a crow’s foot. The entire event is short — typically lasting from two-thirds of a second to four seconds — and those who witness it often respond by mirroring the action, and smiling back…”