THEY say “tomayto” and we say “tomarto”. And now a study has established that the Americans and British also have different smiles.

While we British smile by pulling our lips back and upwards and exposing our lower teeth, Americans are more likely simply to part their lips and stretch the corners of their mouths.

So distinct is the difference that the scientist behind the research was able last week to pick out Britons from Americans from close-cropped pictures of their smiles alone, with an accuracy of more than 90%.

The study by Dacher Keltner, a psychology professor at the University of California in Berkeley, near San Francisco, analysed the 43 facial muscles used by humans to charm, smirk and appease.

He found the British were also more likely to raise their cheeks when they smile, showing the crow’s feet at the corners of the eyes. This produces a more sincere, hard-to-fake smile.

The most common British smile — restrained but dignified — is called the Duchenne smile after Guillaume Duchenne, a 19th century French doctor who analysed facial expressions.

Keltner has nicknamed it the “Prince Charles”, as he believes the Prince of Wales has the typical British smile.

“Charles shows his lower teeth fully using his risorius muscle that runs all the way around the mouth,” said Keltner. “It is a polite, formal expression of pleasure.

“But it’s also very ancient, perhaps going back to the first smiles deployed by our ancestors when they invited other primates to co-operate rather than fight.”

By contrast, Keltner found most Americans had the far less expressive “Pan-Am smile”, named after the defunct airline’s gesture of welcome. This depends only on the zygomaticus major corner-tightening muscle and has also been called the “Botox smile” because, like the cosmetic treatment, it leaves the muscles at the corners of the eyes motionless.

Last week Keltner was able to identify correctly the nationalities of 14 out of 15 smiling mouths shown to him with the rest of the photo obscured. The one he failed to identify correctly was of Venus Williams. the American tennis champion.

Keltner, who will publish his findings in his forthcoming book Understanding Emotion, said the average smile could be detected from 300ft away and lasted only three seconds. He added: “In that time you can reveal much more of yourself than you ever intended. Sometimes you give away big facts.”

Tim Henman’s grin showed he was “genuine, coy and flirtatious”, he said, while David Beckham’s smile and jutting chin indicated he was “determined to win at any cost”. The smirking grin of Chris Tarrant, host of Who Wants to be a Millionaire?, suggested a man who “felt he was getting away with something”.

Despite Tony Blair’s adoption of many American political techniques, Keltner said his smile retained many British characteristics.

“I see a bit of the full risorius in Tony Blair, but the American political elite cannot do it any more, which is a bit sad,” he said. “They have lost that dignity. In blind tests George Bush’s smile emerged as cynical rather than pleasurable.”

The genuineness of a good British smile is all in the eyes — Keltner has found that only 5% of people can fake a smile that uses this muscle.

Infants use the Pan-Am smile when unknown adults enter a room as a gesture of appeasement and, Keltner says, so does the actress Julia Roberts.

“She has a wonderful smile, but it does not often reach her eyes in public. By contrast, Angelina Jolie not only smiles broadly, and twinkles, but also tilts her head a little, which pushes the pleasurable body language into a higher gear. That is a smile which is impossible to resist.”

Other research has shown that women smile more than men in public, but stop smiling in private.

The power behind the smile may also be more potent than anybody has previously realised: Keltner recently released a study of photographs of women in college yearbooks dating back to the 1960s in which he separated the Duchenne smilers from the artfully posed.

Researchers then tracked the women down and found that those who had smiled most happily at college overwhelmingly tended to have had the happiest lives since they had graduated. “It’s a virtuous circle,” Keltner concluded. “Happy smiley people cheer others up around them, which in turn makes them more stable and less prone to depression or divorce than those who faked it in their yearbooks.”

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