Science Interviews

Interview

Sun, 20th Mar 2005

Left-handedness

Professor Chris McManus, University College London

Part of the show Autism, Intelligence and Left-Handedness

Chris - Now Chris, you wrote a book, 'Right Hand, Left Hand', which won you a whole heap of prizes because it's something people are absolutely fascinated by. Why do these left-handers exist?

Chris M - The simple answer is because of genes. Some of us have one set of genes while others of us have another. It's the same reason that some people have blue eyes or blond hair. And therefore, some people just have their brains the other way round. That's the simple answer. The really difficult question to answer is why did that ever happen? Why did we become mostly right handed in the first place and why did others become left handed? That's a good evolutionary story as there have to be advantages to being right handed and advantages to being left handed. They're difficult questions.

Chris - Lots of people say that sportspeople are better if they are left handed. Is that true?

Chris M - It's true for some sports. It's not true for playing golf for instance. It doesn't mater which hand you use to hit a tiny ball into a hole that's very far away. You're equally good if you're right or left handed. However, left handed tennis players have a strategic advantage. The reason is is that right handers aren't as used to playing left handers as left handers are to playing right handers, and therefore the left handers benefit from it.

Chris - If we were to look back in the past, how do we know if left handedness was equally common then as it is now?

Chris M - It's quite difficult, but behaviour does get recorded. If you look at old paintings or old sculptures, then there's good evidence that about 90% of people were left handed back until about 5000 years ago when people started making paintings. Before that it gets more difficult. We can look at the iceman found up in a glacier in the Alps a few years ago. He died about eight or ten thousand years ago. He was carrying arrows, and you can tell by the way those arrows were wound and the way the feathers were put on that two had been wound by right handers and one had been wound by a left hander. So you can use all sorts of tricks like that. You can look at stone tools which are two million years old, and interestingly when you pick up a stone and hit it with another one, a right hander does it differently to a left hander.

Chris - Were around 10% of cavemen left handed and 90% right handed then?

Chris M - We think so 200 000 years ago but we're not sure two million years ago. That's the big question.

Chris - Now during our piece on the Science Festival, young Rosie said that she'd been over to the archeology department and used a blow pipe to make an imprint of her hand on a wall. There was a wonderful experiment done by some scientists in Montpellier in France looking at this question in cave paintings. Tell us a little bit more about that.

Chris M - Essentially what you find is negative prints of hands on the wall of the cave. What they seem to have done is to fill one hand with charcoal from out of the fire, put the other hand against the wall and then blown the black powder all against the wall. They've then removed their hand and left an outline of their hand. About three quarters of these hands are left hands, and the other quarter are right hands. The really tricky question is what is the ratio of left and right handedness? Most people use their right hand to hold the dust and blow it onto their left hand. Not everybody does that. So they got modern students in Montpellier doing this, and they found that about three quarters of the time they put a left hand up; the other quarter a right hand. Since nowadays about 10% of people are left handed, we can be mostly certain that about 10% of people then were left handed.

Chris - A couple of weeks ago we published a story about work at the University of Limerick about racehorses. They found that certain horses have a preferred direction. The males seemed to prefer going left, and the females seemed to prefer going right. This was true whether they were deviating around an obstacle or rolling over. Do you think it's true that animals have a preferred hand or foot?

Chris M - There are lots of stories about this. The general story is that individual animals are generally a certain handed, footed or paw-edness, but exactly half of them are left handed and half of them are right handed. I would say that you could try this with your cat at home. Get a tin of cat food, leave about half an inch of food in the bottom and watch which paw it uses to get it out. They're pretty well consistent, but about half of them use the right and half of them use the left.

Chris - Tell us about the Muppets and about Titanic.

Chris M - The Muppet story is an interesting one. Muppets are left handed. Jim Henson who created the Muppets was left handed, and so the standard story you find on all of the websites is that this was the left handers revenge. He wanted to create a world where most people were left handed and only a few people were right handed. Needless to say, it's not true. The real story is much more interesting. It's very difficult being a puppeteer. The most difficult thing about being a puppeteer is controlling the head, because you have to get all of the movements of the head right. So puppeteers use the right hand, the most skilled hand, for moving the head. The second most difficult thing is controlling the hands, so they use their left hand to do that. So most Muppets are left handed. It was nothing to do with Jim Henson being left handed at all.

Chris - Now what about the film Titanic, because all the people in it who are waving from the docks are all waving with the wrong hand.

Chris M - What happened with the Titanic is that they didn't have enough money to build a whole boat, so they built half a boat. The problem was that they needed to see people from both sides of the boat. So they filmed them on just one side of the boat but flipped the images over in the camera. They taught all the main characters to do things with their left hand and had the insignias on hats in mirror writing, but unfortunately they couldn't train the five thousand extras on the dockside to wave with their left hand. They waved with their right hand but it looked as though they were waving with their left hand.

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