When out and about in public, we may come across hundreds — maybe even thousands — of people, especially those who live in a major metropolitan area. But how many do we actually notice? And for the few faces that we do pick out, why do we see some faces and disregard others?

A study in the journal Nature Human Behaviour suggests that the unconscious mind may be looking for two characteristics in all those faces: who looks threatening and who might be trusted. When we make the unconscious choice to pick out certain faces in large crowds, the study says, we’re essentially picking out the faces of individuals who could cause potential harm and those who could give us a certain degree of comfort and make us feel safe.

Friend or foe?

Researchers at The Hebrew University of Jerusalem (HUJI), along with others, conducted six experiments with 174 participants for the study. The scientists presented the participants with 300 sets of fast-changing images. In one eye, the participants were shown images of human faces. In the other eye, they saw images of various geometric shapes. When the participants recognized the image of a human face, they were asked to press a computer key.

The images of human faces weren’t consciously recognized immediately due to the high level of stimulus. It took the participants at least a few seconds to process that they were seeing the image of a human face. But the types of faces that were most quickly recognized and processed were faces that gave the impression of power and dominance — faces that we typically deem threatening.

"Walking around the world our unconscious minds are faced with a tremendous task: decide which stimuli 'deserve' conscious noticing and which do not," Ran Hassin, the James Marshall Chair in Social Psychology at HUJI, tells Science Daily. "The mental algorithm we discovered deeply prioritizes dominance and potential threat. We literally saw the speed with which these images broke through the unconscious mind and registered on a conscious level with each key press."

Though the unconscious process of facial recognition is mostly concentrated on the awareness of danger, the researchers found that we're also looking for those who make us feel secure.

"These processes are dynamic and often based on personal motivation. Hypothetically, if you're looking for a romantic partner, your brain will 'see' people differently than if you're already in a relationship," Hassin says. "Unconsciously, your brain will 'prioritize' faces of potential partners and deemphasize other faces. Likewise, the same might be true for other motivations, such as avoiding danger. Your eyes might pick out certain 'menacing' faces from a crowd and avoid them."

How the findings can help

Hassin says the research could have some useful applications for the future. "It might be possible to train and untrain people from perceiving certain facial dimensions as threatening. This could be helpful for those suffering from [post-traumatic stress disorder] or depression," he says. "Likewise, we could train people with autism to be more sensitive to social cues."

Whatever the uses, Hassin and his team have shown us that, whether we realize it or not, when we gaze out into the masses, we might be looking for more than just another face in the crowd.