If you try to rob a bank in Seattle in the near future - I'm not suggesting you test this out; just take it on trust - you could be in for a surprise. Bank robbers, of course, do everything they can to try to avoid surprises. ("What I love about this job is its unpredictability - you never know what's going to happen!" is one of the things you never hear bank robbers say.) But the surprises they are worried about are things like the sudden appearance of police officers, or quick-witted customers trying to tackle them to the ground. The really surprising thing about the FBI's new Safecatch system, currently in operation in Seattle, is that it involves training bank employees to terrify robbers by smiling at them.

"If you're a legitimate customer, you think, 'This is the friendliest person I've met in my life.' If you're a bad guy, it scares the lights out of you," one bank executive explained. Bank robberies have halved, year-on-year, since the scheme was introduced. Smiling pierces the anonymity thieves cultivate, creating precisely the connection they're desperate to avoid. You didn't think the grinning "greeters" in the doorways of big American shops (and, increasingly, British ones) were really there to make you feel welcome, did you?

Of course, a smile produced in the high-stress context of a bank robbery is going to be a fake one. But that does not necessarily spoil the effect. As part of his research into the bodily signs of lying and deception, outlined in Weekend a couple of weeks ago, the psychologist Richard Wiseman revealed how bad we are at telling real smiles, which involve the eye muscles, from fake ones, which use only the mouth. There's something else researchers keep confirming, though, an utterly strange phenomenon which accords with none of our beliefs about how emotions operate: fake smiling even works on ourselves.

In one landmark study, German students were called into a lab and told they would be helping to test different ways for paraplegic people to hold pens. Some were asked to hold a pen between their teeth - an action that produces an involuntary smile. Others were asked to hold it with their lips, which induces a frown. Soon after, they were shown a cartoon and asked to rate how funny they found it. The teeth-holders were unequivocally more amused.

You can, of course, experience this effect for yourself. Take a few deep breaths and notice your mood. Then pull your lips into an exaggerated smile and hold it for three or four seconds. You should notice an elevation in your mood. Alternatively, perhaps you notice that the person sitting beside you on the bus is starting to look unsettled, and wondering again why it's always them who ends up next to the weird, grinning passenger.

This is the problem with psychology experiments: do them in universities and people give you research funding; do them on public transport and all they give you is funny looks.

oliver.burkeman@theguardian.com