There's an often-quoted statistic that right-handed people live on average nine years longer than left-handed people. As the daughter of two left-handed parents, and the sister of a left-handed brother, this, to me, is a rather worrying idea. But is there any truth in it?

The finding was advanced in two articles in the late 1980s and early 1990s by American psychologists Diane Halpern and Stanley Coren - both published in prestigious scientific journals, Nature and the New England Journal of Medicine.

But what could the explanation for the premature death of left-handers possibly be?

Something to do with the fact that tools are not designed for them?

"Knives are very awkward," says Claire Allen from Hampshire in southern England.

If this were true it would be like smoking 120 cigarettes a day Chris McManus, University College London

"They are designed for right-handers - if you use it as a left-hander it cuts on the slant all the time, whereas for a right-hander it cuts straight."

I know that my mother would be bereft without her special left-handed sewing scissors.

But surely nothing like this could be responsible for cutting life short by almost a decade?

"It's not at all plausible," says Chris McManus, professor of psychology and medical education at University College London and the author of Right Hand, Left Hand.

"If this were true it would be the largest single predictor we had of life expectancy - it would be like smoking 120 cigarettes a day plus doing a number of other dangerous things simultaneously. It really is highly implausible that an epidemiologist wouldn't have spotted it previously."

If it's so implausible why was it published in respected academic journals and why has the myth endured?

Image caption Left-handers were once mistakenly regarded as cack-handed

Because, according Chris McManus, the researchers made a "very subtle error".

The studies were conducted in Southern California, where lists are published of everyone who has died.

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Halpern and Coren took a list of the people who had recently died and contacted their families, asking whether or not their relative had been right- or left-handed.

Looking at 2,000 cases, they saw that the average age at death of the left-handers was about nine years younger than of the right-handers. On that basis, they concluded that left-handers died earlier.

At first glance, that seems persuasive. What did the researchers do wrong?

"Their mistake was that they only looked at the dead," Chris McManus explains.

The point is that left-handers are more common now than they used to be, so - at least at the time the research was published - left-handers were on average younger than right-handers.

"The natural rate of left-handedness is around 10% or 11%, but the rate was pushed down artificially during the Victorian period," says Chris McManus.

"You can see it going down from about 1800 onwards through to about 1900."

Not only would left-handed people have been encouraged not to be during this period, life was also pretty difficult for them and they quickly became very conspicuous.

"They went to work in factories using machines designed for right-handers - and the left-handers looked awkward," says McManus.

"And then compulsory schooling came along and they were obliged to sit in classrooms and try and write with their right hand using an ink pen and they made a mess. The result of all of this was that left-handers became stigmatised - regarded as cack-handed, stupid."

Image caption Obama is the third left-handed US president since ambidextrous Ronald Reagan

So, some of the people who had died on those Californian lists may well have been born left-handed, but spent most of their lives acting and identifying as right-handers. Their families would have described them as such, when the researchers came knocking.

This would have skewed their results.

To see why, imagine an exaggerated scenario where there were no left handers at all born before 1973 - 40 years ago.

If we now look at death records for 2013 and ask who, among the dead, was left-handed we would see that all of them died at or before the age of 40. That would be much younger than the average of age at death of right-handers.

Nothing like this exaggerated scenario ever occurred in reality - but the number of people identifying as left-handed did grow dramatically during the 20th Century.

So the idea that left-handers die nine years earlier than right-handers is a myth.

What about non-fatal injuries? Should we be worried about people like Claire Allen struggling in the kitchen with knives designed for the right-handed? Or my own mother for that matter, if she were forced to use right-handed scissors?

"There is some evidence that left-handers are more likely to have minor accidents," says Chris McManus.

"But it's pretty insubstantial and I doubt if it's affecting the mortality rate very much at all."

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