The following correction was printed in the Guardian's Corrections and clarifications column, Friday October 31 2008

Our calculation of the chances of four out of five US presidents being left-handed was slightly awry in the article below. We gave the probability as 0.00009, given that one in 10 of the population is left-handed. That is the probability of any particular sequence of one right-handed and four left-handed presidents. The odds of that occurring in any order would be 0.00045

Amid the many conspiracy theories swirling around the US presidential race - Barack Hussein Obama's Islamist takeover, say, or John "Bush" McCain's plans to steal the election - a truly sinister confluence of events has largely gone unnoticed.

Some see the devil's hand at play in an election that, whatever the outcome, will see America make a a fundamental shift, not from right to left, but from right-handed to left-handed.

Both Obama and McCain are sinistral - lefties to you and me - in contrast to the present incumbent of the Oval office. One of them will be the fourth left-handed president out of the five past holders of the world's most powerful office, a fact that has intrigued neurologists and confounded probability theorists.

Obama or McCain will become the 44th US president, and within that distinguished company will be the eighth known to be left-handed. The victor will become the sixth lefty out of 12 presidents since the end of the second world war, stretching back to Harry Truman.

The other postwar lefties-in-chief are Gerald Ford, the ambidextrous Ronald Reagan, George Bush Sr and Bill Clinton. Since 1974 the only right-handed presidents have been Jimmy Carter and the outgoing Bush.

Roughly one in 10 of the population is left-handed, so to have four out of five recent incumbents in the top job drawn from that group is striking: the probability is 0.00009.

Some statisticians would say it is just a coincidence that over time will be evened out. Certainly, the prevalence has been lower among the 12 postwar British prime ministers; only two of them, Churchill and Callaghan, were left-handed. But Daniel Geschwind, a professor of human genetics at UCLA, thinks it is beyond coincidence.

"Six out of the past 12 presidents is statistically significant, and probably means something," he said.

Membership of the rarefied group also extends across the political divide and into US public life. Apart from Obama and McCain, it includes the TV mogul Oprah Winfrey, the Nobel peace prize laureate Al Gore, New York mayor Michael Bloomberg and Homer Simpson's neighbour Ned Flanders.

Geschwind points out that there are established differences in brain patterns. The language function lies in the left hemisphere of the brain for 98% of right-handed people, but is on the left for only about 60% of left-handers. About 10% of lefties have their language function on the right hemisphere - far more than their right-handed brethren - and about 30% straddle both sides.

How these factors play out in practice is only thinly understood, but it may help to explain why there are a disproportionate number of left-handed maths professors at MIT, for instance, as well as artists from Michelangelo to Paul Klee, not to mention Winfrey, Gore and Bloomberg.

Left-handed US presidents go back to James Garfield, who, legend has it, could write in Latin with one hand and simultaneously write the same sentence in Greek with the other.

The rollcall has not always been a happy one. Garfield was assassinated in 1881 (though the assumption that left-handed people are prone to an early grave has been debunked). Herbert Hoover was the second confirmed left- handed US president, and we know all too well what happened under him.

Fidel Castro is also a member of the club, as is Osama bin Laden.

Over the centuries, southpaws, as they have been known since the early days of baseball - have had a rough ride. Etymology tells the story. "Sinister" comes from the Latin word for left, later taking on the connotation of evil or unlucky.

According to the Left-Handers Club, a support group based in Surrey, the Devil is almost always portrayed as left-handed. The basics of life, from desks to scissors and computer mouses, are geared to the right-handed majority. Which is why Ned Flanders in the TV series The Simpsons set up a shop called the Leftorium.

Kristy Ainslie, a biomedical engineer in San Francisco, knows all about these inconveniences from personal experience. Eight months ago she set up a Facebook group, Lefties for Obama, partly as a satire on the way Obama has been depicted as someone outside the mainstream.

She thinks there may be something to the presidential cluster of lefties: "When you are left-handed you have your own unique perspective and have to be willing to stand out from the crowd. That's not bad training for a president."

• Ed Pilkington is right-handed