Unlike other primates, the whites of human eyes contrast sharply with our colored irises and dark pupils. One theory suggests that our eyes evolved this way specifically to make it easier to figure out the direction of another person's gaze. If this theory is correct, you would expect humans to pay more attention to eye orientation than other primates do.

To find out, researchers at the Max Planck Institute for Evolutionary Anthropology in Leipzig compared the behavior of adult chimps, gorillas, bonobos and human children.

A person stood in front of the ape or child and looked up and to the sides, either moving only his head, only his eyes, or both his head and eyes. Apes and children both looked where they thought the experimenter was looking. But the apes paid the most attention to the motion of the head, whereas the children paid the most attention to the motion of the eyes.

But why would humans evolve easy-to-read eyes? For other primates, camouflaged eye movements clearly serve a purpose. “For nonhuman primates, you don't want anybody to see what you're looking at. You want to eat it, or mate it, or chase it,” states Brian Hare, a biological anthropologist who co-authored the paper. Instead Hare says we evolved to make it easy for everyone to see where we were looking. The advantages of cooperation through mutual gaze must have been so great that they outweighed the advantage of a poker face.