Face transplants are in the news quite often these days, most prominently Connie Culp and James Maki. The majority of us can dole out a lot of empathy for the unfortunate situations these people were put in, but at the same time, you just want to sit and stare. There’s a twisted fascination with their physical appearances, and although you might feel a twinge of guilt for doing so, you can blame part of your reaction on millions of years of human evolution.

To ensure the long-term survival of our species, we’re genetically predisposed to be attracted to symmetrical faces. The idea is that normal, healthy development free of disfiguring diseases or genetic mutations produces a symmetrical face. We unconsciously see symmetry as a marker of genetic quality. Our reaction to a face that is disfigured, however, also has links with short-term survival.

To do so safely, scientists believe we have evolved a rough screening process. When someone unfamiliar approaches you in the aisle of a grocery store, a glance at his face and its expression helps your brain to sort that person into one of two broad categories: safe or potentially unsafe. The amygdala (the brain area associated with judgment) depends upon the emotion conveyed by the person’s facial features to make that crucial call. Is he happy? Angry? Irritated?

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.

“When a face is distorted, we have no pattern to match that,” Rosenberg said. “All primates show this [staring] at something very different, something they have not evolved to see. They need to investigate further. ‘Are they one of us or not?’ In other species, when an animal looks very different, they get rejected.”

And so, we stare. (An averted gaze is triggered in some people. This too can be overridden only with great difficulty.)