Humans are highly social animals. Rather than remaining among our family or herd from birth to death, we venture out. We spend our days mixing with great numbers of unfamiliar members of our species. To do so safely, scientists believe we have evolved a rough screening process....To decide, your eyes sweep over the person’s face, retrieving only parts, mainly just his nose and eyes. Your brain will then try to assemble those pieces into a configuration that you know something about. When the pieces you supply match nothing in the gallery of known facial expressions, when you encounter a person whose nose, mouth or eyes are distorted in a way you have never encountered before, you instinctively lock on. Your gaze remains riveted, and your brain stays tuned for further information.