Ronald Reagan on a trip to Russia. Pete Souza Which of the foreign diplomats in your country are spies? Which of your spies is a double agent? And which foreign leaders are planning to bomb their neighbours?

All governments strive to answer such questions. American spooks think studying body language may help.

Spooks are seldom as easy to spot as Leslie Nielsen in "Spy Hard" (pictured). A diplomat who doubles as a spy will typically become agitated when he stops doing humdrum work and switches to the secret stuff, says Joe Navarro, a former FBI man.

A skilled counterintelligence agent can detect the resulting changes in body movement, he says. People hiding something important unconsciously modify their dress, facial expressions, gestures and gait. They also change the way they smoke and check their watches, and how close they stand to shop windows. Mr Navarro refuses to be more specific because "we're still using" these tricks.

One success of the FBI's Behavioural Analysis Programme became public in 1999, after agents trained by Mr Navarro noticed the curious non-verbal cues of a Russian diplomat. A probe led to the discovery of a bug at the State Department. Nearly all federal security agencies now study body language, says Mr Navarro.

For example, since 2009 a Pentagon think-tank called the Office of Net Assessment has spent roughly $300,000 a year on interpreting the body language of foreign leaders, says Valerie Henderson, a spokeswoman. The Body Leads Project has subjected Osama bin Laden and a dozen leaders of potentially hostile nations to what it calls "movement-pattern analysis".

It takes a good 20 hours to understand 30 minutes of video, says Martha Davis, a former consultant to an American defence contractor. It is not as simple as seeing that someone is fidgeting and concluding that he is lying. Rather, the trick is to spot statements that deserve extra scrutiny, such as a denial made while shaking one's head a moment longer than usual, she says.

"Tinker Tailor Soldier Spy." "Tinker Tailor Soldier Spy"

Critics caution that movements can be rehearsed to mislead. But involuntary "micro-expressions" — which may last no longer than 1/25th of a second — often reveal concealed emotions, says Paul Ekman of the Paul Ekman Group, a body-language consultancy in California.

His clients include the New York Police Department's counterterrorism division, intelligence agencies and special forces who may need, he says, to know whether the captured survivors of a firefight are telling the truth when interrogated. Each micro-expression's meaning isn't always clear, however. The face of a man about to detonate a suicide-belt resembles that of a man who fears he has left home with the stove on, Mr Ekman says.

Context, therefore, is important. Consider the body movements of Vladimir Putin in late February as Russia conspired to snatch Crimea from Ukraine. The Russian president's posture, eye movements and other non-verbal cues showed that he was feeling less confident than his swaggering speeches suggested, says Karen Bradley of the Laban/Bartenieff Institute of Movement Studies in Brooklyn. Mr Putin's poise improved as it became clear that the West would not stop him.

Other governments are interested in body language, too. In an article in an Israeli magazine in 1979 Amatzia Baram wrote that Saddam Hussein's body language revealed that he hated Israel only somewhat, but hated Iran enough to invade it. Eleven months later Saddam's tanks rolled into Iran. Israeli military intelligence promptly hired Mr Baram, and he is now at the University of Haifa. He also advised the American army after Saddam's capture.

America has spent $900m since 2007 trying to read the body language of passengers in airports. This is a waste of money, says Ms Davis: a few seconds of observation is not enough, and the screeners are hopelessly undertrained.

Done properly, however, and focused on the kind of people most likely to have interesting secrets, body-language analysis offers juicy rewards. Allan Pease, an Australian consultant, says that at least three Russian government agencies pay his firm to decipher the body movements of Western leaders such as Barack Obama, David Cameron, and Angela Merkel.

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