Australasian Science: Australia's authority on science since 1938

An Honest Face

By Tim Hannan

The brain decides whether an unfamiliar face is trustworthy, even before it is consciously perceived.

It is said that you never get a second chance to make a first impression, but a recent study suggests we might not even get a first chance. It seems that the human brain is biased to making snap judgements on the trustworthiness of someone’s face, even before it is consciously perceived.

Psychologists have long studied the processes involved in recognising and evaluating other people’s faces. From an evolutionary perspective, distinguishing a friend from a stranger would have carried significant evolutionary significance, as would the ability to form a sound judgement about a stranger on the basis of their appearance.

Previous research has shown that an individual’s overall reaction to unfamiliar faces involves a judgement of two separate factors: dominance and trustworthiness. It is also established that people tend to agree about the trustworthiness of unfamiliar faces, allowing for the contribution of cultural differences.

Specific characteristics of human faces appear to be especially responsible for these judgements. Across several studies, faces with high inner eyebrows and prominent cheekbones are associated with a higher degree of trustworthiness.

The authors of a study published recently in the Journal of Neuroscience explored the neural processes involved in judgements of trustworthiness. They noted that people evaluate an unfamiliar face remarkably quickly – exposure to an image of a face for as little as 50 ms is sufficient for participants to form views about the unfamiliar person’s trustworthiness.

One part of the brain that is centrally involved in evaluating trustworthiness is the amygdala, an almond-shaped region in the medial temporal lobe that is responsible for social and emotional behaviour. Specific regions of the amygdala respond differently when a person is viewing unfamiliar faces that are considered either trustworthy or untrustworthy.

A team from New York University and Dartmouth College decided to try to determine how quickly this process actually occurs, and whether the judgement occurs consciously or unconsciously. While participants were in a brain scanner, the researchers presented pictures of unfamiliar faces that had been previously rated by others as either trustworthy or untrustworthy. The images of the faces were presented for only 30 ms, which is below the threshold for conscious perception of a visual stimulus.

The test image was immediately followed by a neutral image of a face rated by others as neither trustworthy nor untrustworthy. This procedure, which is termed “backward masking”, was employed to ensure that participants had less than 30 ms to “see” the face by disrupting any possibility of prolonged analysis.

The remarkable finding was that the amygdala was clearly sensitive to facial trustworthiness, despite the faces being presented so briefly that they were not consciously perceived. That is, the patterns of activation in the amygdala of those who could not consciously perceive the subliminally presented faces were similar to those in previous studies who could perceive the faces. The researchers concluded that this demonstrates that the human brain makes important judgments about people even before we are aware that we are seeing them.

This study adds to the ever-increasing body of research demonstrating that our brains are constantly engaged in a broad array of perceptions, judgements and decisions of which we are largely unaware. In this case, the brain is prompting the formation of specific impressions of other people’s trustworthiness through the briefest glances at their faces, prior to and beyond our conscious control.

This judgement based on facial features may well have been of significant survival value throughout human history, helping us sort friend from foe or distinguish those whom we should trust from those of whom we should avoid.

On the other hand, subliminal judgements about a person’s nature, qualities or intentions based on facial characteristics alone may have subserved a predisposition to fear or aggression on encountering strangers, and contributed to humanity’s history of tribal disputes and racial hatred.

Tim Hannan is an Associate Professor of Clinical Psychology at Charles Sturt University, and the Past President of the Australian Psychological Society.