Why are some people left handed?

We find out about asymmetry in nature: by firstly examining our hands. Do other species have handedness? Is it genetic? And is it true that left handers are bad at language?

Chris Smith spoke to UCL's Chris McManus, author of the award-winning book Right Hand, Left Hand....

Chris - why are some people left handed and some people right handed? Why do humans, and other species, have this asymmetry? And does it matter?

Chris M. -   It matters because we're better at using one than the other, but why is it?  Why are we better at using one?  Well, there are several reasons there.  One is it actually always benefits just to use one or two things more than the other. 

So, there are monkeys in the wild, chimpanzees who are searching for termites or things like that.  They stick a stick into a tree, pull out the termites and they eat them.  Half of them is their right paw and half of them is their left paw.  The ones who try and use both collect less termites than the ones who specialise in using just one.  So, it pays to practice and it's always better to use one rather than the other.

Chris -   But am I right in saying you don't see the majority of chimpanzees in the wild who are right-handed in the same way if I went to anywhere in the world and look at any human.  I'm going to find that same right-handed dominance.

Chris M. -   That's right.  Almost all animal species are a mixture or sort of racemic mixture of half right and half left whereas humans, it's 90% right-handed,10% left-handed.  That's something that needs explaining.  So, something else must be explaining that in addition to being specialised.  It's a sense in which we're all right-handed, it would be much easier because we'd be right-handed because we're the same as everybody else.  It's easier if the whole society does things the same way, just the same ways it pays to all drive on the right same side of the road, but we're not.

Chris -   Ben...

Ben -   Is it true that we're all born with a handedness or does it develop?

Chris M. -   Yes, we're born and in fact, there are studies where you look at ultrasound studies of pregnant women and you can watch the foetus moving around inside the uterus.  Lots of foetuses as early as 4 or 5 months of pregnancy such one thumb and about 90% suck the right thumb and 10% suck the left thumb and you follow them through, and they become right and left-handed.  So, it's there before birth, yes.

Chris -   I was going to say because when my daughter was born, she is left-handed.  She is 7 now, but she was very little and she'd sort of be sitting in a high chair feeding herself with one hand or the other, or when she used - she has this thing about pens, loved pens.  And I used to sort of play devil's advocate and try taking the pen out of one hand and put it in the other hand, in the right hand, to see what would happen and it immediately went back to the left.  So, very early, very, very early, there is this bias, but why?

Chris M. -   There is a bias, but it's actually not as easy to see as you might think.  I get a lot of parents saying, "Are they going to be right-handed?  Are they going to be left-handed?"  And children neonates go through a phase up to about a year, 18 months which is called the chaotic phase where they seem to be trying out both hands.  I think part of this is just trying to understand how hands work.  It was easy in womb.  They were floating around in this sort of space-like fluid environment and they could easily move one thumb to the mouth and so on.  Put them in air with gravity and life gets a bit more complicated.  Plus, they're trying to sit up and do other things.  They're exploring a lot of things, but then it's usually by about a year or two years of age that it becomes clear if they're right or left-handed.  They can move around.

Chris -   What happens if you look in the brain though at someone using their right and left hand who has a right-handed bias or a left-handed bias?  Is there clear differences in the way the brain is working?

Chris M. -   Well, there's clearly very enormous differences.  I'm right-handed and as I move my right hand, the left half of my brain is controlling that.  So, I'm doing things from the left half of my brain.  Now, part of the story there is that not only am I moving my hand with the left half of my brain and also, talking to you with the left half of my brain because language is in the left half of the brain in most people as well.

Chris -   But why should that matter, Chris?  Why should it be important that language is in the same part of my brain as my handedness is?

Chris M. -   We don't know, but we do seem to find that when you get people who get these modules in the two hemispheres of the brain and there are lots of them in different combinations, some of them start to be disadvantaged.  So, presumably there's some sort of optimal biological combination.  I suspect it's to do with how bits of brain communicate with other bits of brain because it's a lot of different areas doing different things at the same time.  Sometimes it has to move across between the hemispheres which is not a very fast process.  Sometimes it has to move backwards and forwards.  So, presumably there are optimal ways of doing that and one is to have the hand controlling your speech, language, so the brain hemisphere controlling your language, the brain hemisphere controlling your hand next to one another.  And that's probably why I'm waving my right hand around in the air as I'm talking to you.

Chris -   You're speculating wildly, but what about the genetics of this?  I said my daughter is left-handed.  My wife and I are right-handed, but my dad was left-handed.  So, is there a gene which causes left-handedness?  Does it run in families?

Chris M. -   Well, it certainly runs in families and if you take two left-handed parents, they're much more likely to have left-handed children than if you have two right-handed parents.  Having said that, they're about three times more likely but still, three quarters of the children of two left-handed parents are right-handed.  So, that makes the genetic slightly strange.

Chris -   Not easy, yeah.

Chris M. -   It's not easy.  It does look as it runs in families.  There's a lot of chance involved and part of the trouble is, we can't see that very easily in small modern families.  Charles Darwin had 10 children.  His wife was left-handed and two of those children were left-handed.  But with 10 children, it becomes a bit more obvious what's going on.

Chris -   What about the power of modern molecular biology and the so-called genome wide association where you can go through enormous numbers of people in the population looking for genetic signposts that sort of co-associate.  They crop up at the same time in certain traits.  What happens if you do that to handedness?

Chris M. -   Well, we tried that and we looked very much.  We published the paper last year on it and we hope very much to find the gene.  We found nothing and most of the point, we had the statistical power to provide it if it were there.  So, it wasn't there.

Chris -   So, there's no one gene that causes that?

Chris M. -   There's no one gene, but what there does seem to be is, it's more than possible that there's a large number of genes.  All of which have small effects.  In that sense, it's no different to height, intelligence, weight, and a thousand other complex traits that we're looking at.  I think probably what's happening is that you have a left-handed child and a left-handed parent and they will have the same gene producing that.  But I have a left-handed mother and a left-handed daughter and it will be a different gene that's made our children left-handed.