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Can you trust a smile?

A sign with a flaming head

How can you tell if someone looks trustworthy? Could it have anything to do with their facial expression? Some new research suggests that people are less likely to trust someone if their smile looks fake. Of course, that seems like common sense – you’d never think to yourself ‘wow, what a phoney’ and then decide to trust someone anyway. But we’re talking about very subtle clues here. The kind of thing that might only produce a bit of a gut feeling, or you might never be conscious of at all.

To do this experiment, researchers at Cardiff University told volunteers to pick someone to play a trust game with. The scientists told the volunteers to make their choice based on a short video of each person smiling – but they didn’t know the scientists could control certain aspects of each smile, and could make some smiles look more genuine than others.

Things get very subtle: it turns out that genuine smiles take longer to build up to, and come back from, than fake smiles. For a real smile, your facial muscles begin to move, eventually forming the grin we’re all familiar with, after which your face gradually returns to normal. When people try to fake a smile, their face tends to snap into the grin almost immediately and hold it there for an unnaturally long time. Suddenly the smile disappears and their face goes back to normal. In short, fakers concentrate just on making the ‘smiley face’, but the fake emotion snaps on and off like a switch. A real smile washes over the face like a wave.

The researchers used computer graphics to control the timing of each stage of a smile. In some videos, the smile built up and diminished the way real smiles tend to; in others it snapped into and out of existence in a more fake way. Amazingly, the differences amounted to less than a second for many of the stages. Could the volunteers tell the difference with such a difficult clue? It seems they could. The volunteers were less likely to pick the people with less authentic smiles. They didn’t always spot the phoney, and some fake smilers still ended up being trusted enough to play the game. Still, it suggests that we as humans have evolved some pretty sophisticated ways of catching cheaters if all it can take to give you away is a smile that’s too sudden. It’s a lesson to trust your gut feelings: those feelings are often a sign that your brain is adding together a lot of tiny but important clues.

A man dressed up as a detective

Robot street smarts?

In the future, artificial intelligence might have to learn this trick too. Imagine a robot interacting with a human – the robot would need to be able to pick up the clues if the human was trying to trick them! We wouldn’t want robots falling for confidence tricksters, now would we?