From the time we pick up a chunky crayon and start scribbling as children, it begins to become clear whether we're right- or left-handed. But what makes one hand dominate? And why are left-handers in the minority?

Curious Cases

To find out more:

To find out more, Adam Rutherford and I decided to investigate the science and history behind human handedness for the BBC Radio 4 series The Curious Cases of Rutherford & Fry.

It soon became clear that there was more to the question than we thought: for example, I had never realised that our body is lopsided in other ways too. Take your eyes, for instance. You can tell whether you are right or left-eyed by trying the following test:

Hold a thumb out at arm's length in front of you. First, look at it with both eyes, then try covering each eye in turn. Your strongest eye is the one which gives the nearest picture to stereo vision.

Similarly, you can test your ears: which ear would you naturally use on the telephone? Or to listen, clandestinely, against a wall?

Overall 40% of us are left-eared and 30% are left-eyed  

It's funny to spot these strange asymmetries in action – I often find myself holding the phone with my left hand and pressing it, rather awkwardly, against my right ear, whilst scribbling down notes with my right hand. If ease was the biggest consideration, this odd arrangement certainly doesn't deliver. It's all about playing to our natural strengths.

Overall 40% of us are left-eared, 30% are left-eyed and 20% are left-footed.

But when it comes to handedness, only 10% of people are lefties.

Why could this be? Why are left-handers in the minority?

In times gone by, left-handedness was drummed out of errant schoolchildren, and oddly negative connotations still linger in our language. The word 'left' comes from the Anglo-Saxon word 'lyft', meaning 'weak'. And the opposite in Latin is 'dexter' which is associated with skill and righteousness.

The word 'left' comes from the Anglo-Saxon word 'lyft', meaning 'weak'  

So what determines whether we are right- or left-handed? From an evolutionary standpoint, specialising with one hand makes sense. Chimpanzees tend to choose a favourite hand for different tasks.

Take termite fishing. After selecting the perfect stick, the chimp pokes it into the termite mound, their sense of touch providing a host of information about how deep, wide and full of tasty termites their house may be. Then they'll gently pull the stick out to reveal their prey, the termites' jaws clamping down hard on the foreign invader. Unbeknown to them, they are about to get chomped by a hungry chimp. By specialising with one hand, chimps become more dexterous, and more termites bite the dust.

But when primatologists study chimpanzees in the wild, their patterns of handedness look very different to ours. For each task around 50% are right-handed, and 50% left. So where in our evolutionary tree does this 1 in 10 ratio emerge?

An important clue comes from Neanderthals' teeth. Neanderthals, it turns out, were clever, but clumsy. Our ancestors used their teeth to anchor slabs of meat, whilst they held a knife in their dominant hand to carve it up. Now and again, they would scratch their teeth. The distinctive pattern of grooves in their front incisors reveals which hand must have been holding the food, and which was grasping the knife. Incredibly, when you compare the number of left- and right-handed Neanderthals, this same ratio of 1 in 10 left-handers that we see today pops out.

We know that left- and right-handedness has a genetic origin. However, geneticists are still trying to pinpoint which bits of DNA are involved, and there may well be up to 40 different genes at play. As things stand, the answer to what determines left or right handedness and why lefties are in the minority remains a resounding "don't know". 

But does being left-handed have any impact on people's lives, beyond finding right-handed scissors, zips and fountain pens a little bit annoying?

Left-handers are much more variable in the way that their brains are organised  

There's been a long running debate about how being left-handed affects your brain. The right side of the brain controls the left hand, and vice versa. And so being left-handed can have knock-on effects on the way the brain is arranged.

"Left-handers are much more variable in the way that their brains are organised," explains psychologist Chris McManus, from University College London, author of the book Right Hand, Left Hand.

"My personal hunch is that left-handers are both more talented, and suffer deficits. If you are left-handed you might find yourself with a slightly unusual way your brain is organised and suddenly that gives you skills that other people don't have."

However, not everyone agrees. Dorothy Bishop is Professor of Developmental Neuropsychology at the University of Oxford and she has a personal interest. "I myself am left-handed and I always wondered why I was different from other people.

"There's been all sorts of claims over the years linking left-handedness with disabilities like dyslexia and autism. On the other hand, there have been positive attributes – it's claimed that architects and musicians are more likely to be left-handed."

A lot of associations between disabilities and handedness are the result of selective reporting bias  

But after looking into the data, Bishop is not convinced. A lot of these associations, she says, are the result of what's called selective reporting bias. Scientists add a question about handedness into their study on, for example, creativity, and become excited if they find a positive association, but don't report the instances when no connections are found.

It's true, she says, that when you look at rare conditions, like Down Syndrome, epilepsy and cerebral palsy, the ratio of left- to right-handers is more like 50:50 rather than 1:10.

But, Bishop says, left-handedness may be symptomatic, rather than causal.

"It's not the left-handedness itself that's creating problems," she explains, "it's more that it can be a symptom of some underlying condition. But in most people it doesn't have any significance at all for intellectual cognitive development."

The debate rages on, and there is still so much we need to discover about the left-handed brain. Part of the problem is that when neuroscientists look at various aspects of behaviour, MRI studies are only done on right-handed people, in order to try and minimise the variation between different participants. Only specific studies on left-handedness will invite lefties to take part.

Since I'm currently seven months pregnant, it's fascinating to think that my baby has already determined whether she is right- or left-handed. We know this because Peter Hepper, from Queen's University in Belfast, has done some wonderful ultrasound studies looking at babies' movements inside the womb.

He found that nine out of 10 foetuses preferred sucking their right thumb, mirroring the familiar pattern we see in the general population. And when he followed those children up many years later, the babies who were sucking their right thumb in the womb became right-handed, and the ones who preferred their left, stuck with that.

So, even though my baby is already favouring one hand over the other, I won't be in on the secret until she decides to pick up those chunky crayons and start scribbling.

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