Why are people left-handed?

We put this question to Naked Scientist Chris Smith...

Chris - Well, 90% of the population are right-handed and 10% therefore, left-handed. There is some evidence that this runs in families. If you look at families that have left handers in them, you can model this and show that there is some evidence that there is a genetic influence. But researchers have tried very hard using some really powerful genetic techniques and they have not yet found any evidence of a gene that causes handedness. So, we think if anything, it's probably a cluster of genes that work in a certain way, rather than an individual gene that either makes you left-handed or not. It probably biases the likelihood of you becoming left-handed rather than absolutely determining left-handedness. What actually is left-handedness or right-handedness equally? Well, if you look at the brain of a human being, you'll see that it's asymmetrical. The left hand side of the brain is actually differently developed than the right hand side. In fact, we know that language function maps onto the left hand side of the brain in the majority of the population.

That makes your hemisphere dominant on that side and because the left hand side of the brain controls the right hand side of the body and vice versa, you end up with your right hand being your dominant hand because it's being controlled by your dominant hemisphere. Why it should've happened like that? We don't know, but it's almost certainly something to do with the evolution of language. And really interestingly, if you look back in history, even cave people from 50,000 to 100,000 years ago, researchers at the University of Montpellier in France have done this, you can show that they almost certainly were equally handed in the same way we are. The evidence for their study was, they went to a primary school and they gave school children a blowpipe and said to them, "Blow some paint onto a wall using your hand as a stencil and you can get the picture." You hold the blow pipe in one hand, you blow hard, use your other hand up against the wall as a stencil. What do you get? Well, if you do this with the school children, you find about 90% of the hands that you get on the wall are left hands. Why? Because you're holding the blow pipe with your dominant right hand because it's easier to control it and get the picture you want. If you look at real cave paintings where cavemen have done exactly the same thing, you'll see the same ratios. So, we're pretty happy that cave people who are living tens of thousands of years ago also had this brain asymmetry, also had this handedness, and this is probably going hand in hand - excuse the pun - with the evolution of language.