Muttered asides apart, the Prince of Wales’s awkward grin did little to fool experts. His expression was “perhaps the most forced and unconvincing grin I have ever seen”, Professor Richard Wiseman of the University of Hertfordshire, a psychologist who is conducting an experiment to test the ability to tell a fake smile from the real thing, said.

The Prince’s face is a textbook example of how not to feign warmth and happiness, said Professor Wiseman, who introduces his study today at the Edinburgh Science Festival.

While his mouth is curled into a wide, almost clown-like beam, the rest of his face remains so impassive that it betrays his true emotions far more effectively than a heavy frown.

The biggest giveaway is the absence of crow’s feet around his eyes, the hardest feature of a genuine smile to fake. The only ones visible are age-related wrinkles. “He has bared his teeth and the expression is entirely generated in the lower part of the face,” Professor Wiseman said. “Cover up the lower part of the face and the upper part could be almost any expression.

“This is sending out two signals. The first is very aggressive, in the same way that animals bare their teeth when they feel threatened. But then this is combined with a very forced grin to make the baring socially acceptable. It is as if he is saying, ‘I am angry but cannot show that, so I will pretend it is a smile’. It is clearly a false smile generated by someone who feels genuine anger.”

The younger princes also have the demeanour of people trying to look happier and more comfortable than they are, Professor Wiseman said. “Harry’s thin lips and narrow eyes suggests someone who is focused and concerned. William is looking away. (His) lowering of the inner part of the eyebrow and pursed lips give off a far more aggressive impression. This is a wonderful photograph. It shows the different ways in which people try to cope with a difficult situation.”

Many of the ministers arriving at No 10 for the Cabinet meeting yesterday also sported false smiles, Professor Wiseman said. Gordon Brown’s face is “relatively immobile — a face that would not signal any extreme emotion even if it were being felt. The lips are held together and the eyebrows are low.

“On the plus side this gives the impression of being trustworthy and in control. On the negative side it can be seen as rather cold and uncaring.”

Patricia Hewitt, the Trade and Industry Secretary, and Tessa Jowell, the Culture Secretary, look more genuine but neither shows a true smile, Professor Wiseman said. “Hewitt’s is a classic expression of surprise. The mouth has dropped open rather than being pulled up at the sides, as would be the case with a genuine smile, the eyebrows have been pulled up and away from the eyes and the eyes themselves are wide open. People sometimes confuse this expression with that of smiling but the signals are quite different.

“Jowell’s is a fake — but well practised and professional — smile. Clearly the mouth has been pulled up at the sides to produce a broad grin. There is crinkling around the sides of the eyes (a sign of a genuine smile) but, more importantly, the eyebrows have been pulled down, creating a crinkling around the top of the nose. This crinkling is more associated with a fake than genuine smile and the whole picture looks rather posed. But it certainly would give a positive impression unless you knew the tell-tale signs.”

The most real political smile belongs to the Cabinet’s newest member, Ruth Kelly, the Education Secretary. “This is probably the best example of a genuine smile and I suspect it may have been in response to something that she found funny or pleasant. The mouth is smiling but, more importantly, so are the eyes. Try covering the mouth and just look at the eyes. They look positive and happy because of the tightening of the muscles around the eye, pulling the skin between the eyes and the eyebrow down and crinkling around the edge of the eyes.”

Professor Wiseman’s experiment, in which anyone can participate, involves viewing pictures of smiling people on a website and attempting to judge which are genuine.

The goal is to determine whether certain people are much better or worse at reading emotion from the faces of others and to find out which parts of the face most accurately convey a person’s true state of mind. Anyone wishing to take part in Professor Wiseman’s experiment can view the images at www.sciencefestival.co.uk

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