Humans are programmed to see faces just about everywhere: in clouds, coffee stains, Martian rocks, etc. However, it’s one thing to see a face, and another to believe that that face belongs to a living human being. How do we tell the difference? According to Christine Looser and Thalia Wheatley of Darmouth College, it’s mostly in the eyes.
The two cognitive scientists took pictures of doll faces and matched them with similar looking human faces. They then used software to morph back and forth between the doll and human faces, looking for the tipping point where the face appeared to be fully human. This point consistently occurred about two thirds of the way toward human on the doll-human continuum. The eyes were the most important clue for registering a picture as belonging to a human.
You can see a couple of the morphing examples below.