When the next President of the United States is inaugurated in January, either Barack Obama or John McCain will be asked to raise their right hand before swearing the oath of office. But that is the wrong hand for both men.

They are left-handed, as were four of the last six presidents: Bill Clinton, George H.W. Bush, Ronald Reagan and Gerald Ford. The current President Bush and Jimmy Carter are the only right-handers to have occupied the Oval Office since 1974, an era of lefty dominance that is to be extended by at least another four years.

Nor is this November's battle between a left-handed Democratic and Republican nominee unusual in recent history. There were similar all-southpaw contests in 1992, when Mr Clinton beat the first President Bush, and in 1996 when he won a second term against Bob Dole. Al Gore, who won the popular vote in 2000 but lost the election to Mr Bush, is left-handed, as is John Edwards, the Democratic vice-presidential nominee four years ago. So too is Michael Bloomberg, the Mayor of New York, who has flirted with a White House bid this time and is sometimes mentioned as a possible running-mate for either Mr Obama or Mr McCain.

A “Lefties for Obama” group with its own website is urging voters to “Make Obama Number 8 in '08”. Academics are scratching their heads - with whichever hand they want - for an explanation.

Just over one in ten of the US population are left-handed and, traditionally, they have been regarded with suspicion by right-handers. Witches were said to greet the Devil and perform black magic with the left hand. The Latin word for left is “sinister”; in French it is “gauche”, which also means improper; while in English, of course, “right” also means “correct”.

Jimmy Efird, at the Centre for Biometrics at Ohio State University, put the probability of left-handers having such success in recent presidential elections at less than one in 10,000. Jan van Strien, an expert on biological psychology at Erasmus University Rotterdam, said that a link between personality and left-handedness has not been established. He added, though: “It's possible that left-handed politicians are more strong-willed because they've chosen not to conform to a right-handed world. Left-handers may have something special, but it can be for better or for worse: many suffer from personality disorders and other issues.”

A higher percentage of mathematicians, scientists and artists are left-handed, perhaps because they are more likely to use both hemispheres of their brains to visualise problems. Bilateral brain function could also help them to develop the social skills needed to be successful in politics. Before 1974, there were three left-handed US presidents: James Garfield, Herbert Hoover and Harry Truman.

James deKay, author of The Natural Superiority of the Left-Hander, says that they have attributes that would make them good presidential material. “Logic belongs to right-handers,” says deKay. “Intuitive thinking belongs to left-handers. Left-handers tend to work alone, are good at handing off assignments and don't micromanage.”

In elections, it is sometimes asked, whose finger do you want on the nuclear button? After January, whoever wins, it will be a finger on the left hand - as usual.

Tom Baldwin is left-handed.

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