We’ve known for a while that having certain facial features will lead people to judge you as either trustworthy or untrustworthy. And in fact our brains make those complex judgments much faster than we could have ever imagined. Now scientists have worked out how we do it.

It’s all about physiognomy

Back in 1772, Swiss poet Johann Kasper Lavater popularised an ancient Greek practice in his Essays on Physiognomy. Put simply, physiognomy is the assessment of a person’s character from their face or other external features. For example, Lavater suggested “the nearer the eyebrows are to the eyes, the more earnest, deep and firm the character”.

Sound completely ridiculous? Maybe, but over the last few decades, research has shown we absolutely do judge people according to how they look. Scientists have even modeled the specific physical characteristics responsible for our first impressions.

Would you trust this face?

We all agree on who looks trustworthy

When it comes to trustworthiness, faces with high cheekbones, high inner eyebrows and smiles are consistently ranked as the most trustworthy.

In a study published earlier this month, researchers presented ten volunteers with 300 computer-generated faces. The ‘trustworthiness cues’ of the faces had been manipulated while keeping other facial features constant.

Exactly as predicted, there was remarkable agreement among the volunteers as to which of these faces were trustworthy. This was also true for non-manipulated pictures of strangers’ faces.

But can your brain judge the trustworthiness of a face you aren’t even aware you’ve seen? Yes, indeed it can.

We know who we trust even if we don’t ‘see’ their face

The researchers then showed a new group of volunteers the trustworthy and untrustworthy images for a split second, a mere fleeting glimpse. The pictures were flashed up and removed again so quickly that although people’s eyes were able to see the images, their brains weren’t able to actually register they had seen them.

To be absolutely sure the volunteers hadn’t consciously perceived the faces, the researchers also used a nifty technique called ‘backward masking’.

Backward masking means showing an irrelevant image so quickly after the picture of interest (in this case a face), that the study volunteers were prevented from consciously seeing the faces. This technique is known to prevent the brain from processing an image the eyes have seen.

What’s happening in the brain?

Of course the researchers wanted to know what was happening in these peoples’ brains. So at the same time as all this was going on, the brain activity of the volunteers was closely monitored and recorded.

The small almond-shaped region of the brain known as the amygdala was the area of interest. The amygdala is responsible for decision-making, memory and certain emotions. We know the amygdala is also in charge of sending out signals to enact the famous fight or flight response.

And what did the brain scans show? That even though the study participants hadn’t consciously seen the faces, the amygdala had ‘decided’ which faces were trustworthy.

Different parts of the amygdala ‘lit up’ in the scans in response to the trustworthy and untrustworthy faces. This shows that our brains are able to work out if someone is trustworthy even if we aren’t actually conscious we’ve seen their face.

So even if we believe we haven’t seen anything, our brains clearly show that at an unconscious level, we have! It’s hard to get your head around (excuse the pun).

Less than the blink of an eye

What’s more, it took just 33 milliseconds (a tenth of the time it takes to blink) to make this judgment. The fact our brains can judge something as potentially subtle and complex as trustworthiness without any conscious thought is pretty extraordinary in my book.

Our findings suggest that the brain automatically responds to a face’s trustworthiness before it is even consciously perceived.Jonathan Freeman, New York University

This research suggests we have evolved to be instantly tuned into people we perceive as untrustworthy. Perhaps our brains have evolved to make these judgments fast enough to allow us to respond appropriately – and either approach someone or hot-tail it out of there. Probably a very useful skill in days gone by when tribal conflicts were commonplace.

In fact, other research has shown we are incredibly fast at making judgments about other traits too: attractiveness, likeability, competence and aggressiveness.

What remains to be decided is whether there is any link between how trustworthy a person looks and how trustworthy he or she actually is. That might be harder to test.

Links and stuff

• Press release from New York University
• Report in The Guardian
• Original research paper in The Journal of Neuroscience