The stares of strangers endured by Connie Culp, recent face transplant recipient, might have little to do with cruelty or lack of empathy. These responses are likely a result of neurologic, biologic and evolutionary factors.

Prior to her operation, the center of Culp's face was blank skin traversed by a single raw scar where she once had a nose, upper lip and cheeks. The disfigurement made her the target of something perhaps even less fixable: millions of years of evolutionary uncouth. When she went out in public, people gaped at her. After her operation, her face still looks unusual and the stares continue.

"We stare. Even if you don't want to, even if your better judgment tells you 'I need to be nice to this person. They've obviously suffered a tragedy,' there's something so alien and uncomfortable — it just doesn't look like us," said facial expression expert Erika Rosenberg, who focuses on evolution at UC Davis' Center for the Mind and Brain. "It goes back to a very primal thing."

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.

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. 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.)

It doesn't take much of a facial anomaly to trigger a transfixed response; a normal human face upside down will do it. Or one that is simply unmoving.

In her work with Paul Ekman, who pioneered the widely accepted theory that human emotion conveyed via facial expressions is biological in origin, Rosenberg studied a group of people with a condition that prevents their facial muscles from moving.

"They talk about how difficult it is to interact with people because people can't handle looking at a face that doesn't move," Rosenberg said.

However, a surgery that allows them to lift their tongue has a "transformative effect on their lives," noted Rosenberg. "Just being able to lift the tongue and move their faces enough to create a little smile. It shows you how profoundly important it is to have a face that works."

It could be that the faces don't match emotions we are familiar with, or it could be they don't look like we expect humans to look, says Rosenberg. "Either way, there's something very fundamental about having a normal working face that we need in our society."

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