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?

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.

Read more