Your instinct not to trust some people is an evolutionary response to keep you safe. Flood G./Flickr, CC BY-NC

Trust is unconsciously determined, thanks to the amygdala: study

The part of the brain responsible for the fight-or-flight response also plays a key role in unconsciously processing a face’s trustworthiness – in a matter of milliseconds.

A study published today in The Journal of Neuroscience shows the amygdala, a brain structure typically associated with primal responses such as fear, can also subconsciously process information about a human face and determine its trustworthiness in a fraction of a second.

Researchers from New York University used magnetic resonance imaging (MRI) to monitor subjects’ amygdala activity while they were shown a series of real and computer generated faces with slight changes in “trustworthy features”, such as higher inner eyebrows and more pronounced cheekbones.

The faces were presented for only milliseconds at a time – enough for the amygdala to react, but not enough for the subjects to consciously make a decision on trustworthiness.

Directly after the first picture, a second picture with a neutral or trustworthy face was shown for a longer period of time to prevent the signal of the first image being consciously processed.

Faces with low, neutral and high ‘trustworthiness’. Journal of Neuroscience, CC BY

Some regions inside the amygdala showed activity in reaction only to an untrustworthy face. Other regions showed activity in response to any face but the strongest reactions were to an untrustworthy face.

“The amygdala is strongly associated with the processing of threat cues,” Ricky van der Zwan, Associate Professor of Psychology at Southern Cross University, said. “Part of the reason that it is so quick is because when you’re confronted by a threat you want to be able to act quickly, so this research shows that the amygdala is processing untrustworthiness as if it were a threat.

"The amygdala works to keep us safe. It wants to give us the best chance of surviving a bit longer, in this case, by staying away from the untrustworthy people.”

The amygdala’s position in the brain. Life Sciences Database/Wikimedia Commons, CC BY-NC-SA

Skye McDonald, Professor of Clinical Neuropsychology at UNSW, said it’s possible untrustworthy-looking people spark a sense of unease, but we can eventually override that initial distrust.

“More elaborate processing which is conscious, and involves lots of cognitive processes such as memory, reasoning, problem solving, will also take place and may override that first impression,” she said.

“Conversely, initially ‘trustworthy’ faces may eventually come to be associated with a distinct sense of distrust due to the behaviour of that person.”

Romina Polarmo, Associate Professor of Psychology at the University of Western Australia, said the amygdala is connected to almost every other part of the brain, so it could be that other regions are providing feedback into this area for facial processing.

“This study looked to see whether one facial processing area of the brain showed activation at the same time as the amygdala, but the study also didn’t see any activation in that region when they showed the pictures for a long period of time – when the brain had time to consciously process the faces,” she said.

Associate Professor van der Zwan added: “It’s all about the first impression. In 33 milliseconds we decide whether or not we can trust someone. Something that someone unconsciously decides in an instant could take a long time to overcome.”