Trustworthiness is always a tough call but a new study suggests our brains make that decision on a stranger almost instantly, just by sizing up their face.

A related study done earlier had determined that there are certain facial features that most people tend to perceive as trustworthy, such as a u-shaped mouth and larger, baby-like eyes. However, this new study suggests researchers are now claiming that they have discovered at exactly what point we make the decision on trustworthiness, and it's almost instantly.

"Our findings suggest that the brain automatically responds to a face's trustworthiness before it is even consciously perceived," explained Dr. Jonathan Freeman, an assistant professor of psychology at New York University and the study's senior author. "The results are consistent with an extensive body of research suggesting that we form spontaneous judgments of other people that can be largely outside awareness."

The study specifically focused on the region of the brain called the amygdala, the part known to play a key role in our decision-making and emotional behavior. The subjects chosen for the research were shown faces of both real and computer-generated faces while their brains were being scanned to reveal the activity of the amygdala as they viewed each face

The researchers had also shown the set of faces to a separate group of people to have them rated for how trustworthy or untrustworthy they appeared to form a base level for set of subjects being tested with the brain scans.

A key to the experiment was in how quickly the scanned subjects had to view the images as they flashed across the screen for only a few milliseconds. This way they could not consciously view each of the faces.

The brain scans revealed that that each subject's amygdala reacted to the images, actually tracking how untrustworthy or trustworthy a face appeared and to what degree, despite the fact they only viewed the image for a split second.

"The findings are surprising because they show that face evaluation can be done without perceptual awareness," Freeman added. "The social cues for trustworthiness are considerably more subtle and complex than a simple, fearful expression on a clearly emotional face. It suggests that the amygdala's processing of social cues outside awareness may be more extensive than we thought."

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