Science Questions

Why are people left-handed?

Tue, 21st Apr 2015

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Question

Michael Higgins asked:

I am left handed and it is my understanding that a much smaller percentage of the population is left handed rather than right handed. I am wondering what role genetics plays in determining if a person is left or right handed and what role, if any, environment might have.

Answer

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

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.

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Hello , am a medical student as i studied it there are two cerebral hemispheres in our brain , one of them contain the "wernick area" which is responsible for languae comprhension and "speech area" that are located in what is called the activated "dominant" hemisphere which is usualley the left hemisphere in majority of people , so people are right handed are because of activated or "dominant" hemisphere is left and left handed because of dominant hemisphere is the right one ;) A-Memo, Sun, 29th Jun 2014

Whilst A-Memo's model makes some kind of sense, it doesn't explain all the statistics.

In my youth, about 10% of the population was lefthanded but since schools stopped forcing kids to write with their right hand, the number seems to have increased to around 20% or more.  If you look at the distribution of lefthanders across professions, it is remarkably biassed: about 25% of mathematics undergraduates of my cohort were lefties, but less than 5% of historians. There are disadvantages to being a lefthanded musician (a leftie orchestral violinist is a danger to his desk partner, leftie guitars are rare, and the difficult twiddly bit for a keyboard instrument is usually written for the right hand) but many lefthanded composers, from CPE Bach to Paul McCartney. Sinistrals in senior management earn signficantly more than dextrals. 

But a 10 - 25% occurrence of a trait is unusual in genetics, and these odd correlations are interesting. My hypothesis is that half the population has a genetic defect in that we cannot use our left hand well, and the other half are born ambidextrous. The dextral defect may indeed be a consequence of the need to devote a lot of the brain to speech, but a 50% occurrence would be consistent with other minor genetic traits.

Given that half the population are obligate dextrals and half have a choice, it seems reasonable to construct a society with a righthanded bias. Given a simple choice, it is likely that more than half of the ambis will choose to be dextral for convenience, hence the gradual increase in lefthandedness towards 25% as society becomes more tolerant or parents and teachers recognise some actual advantages in sinistrality.

I suspect the number of true genetically obligate sinistrals is very small, and may well be associated with language defects, as has been suggested from time to time, but the number of ambis who choose to use the left is gradually increasing and thus makes it difficult to identify obligate sinstrals and examine the genetic hypothesis.    alancalverd, Sun, 29th Jun 2014

As a leftie (without significant language deficits) myself, I've noticed the variability in handedness at different tasks. For example, I play racket sports left handed and striking sports (cricket, golf, baseball, etc) right handed. There seems little doubt that some lefties are more obligate sinistrals than others - I'm fairly ambidextrous, but then I like to practice it.

It's tricky to estimate the genetic contribution to the degrees of handedness and ambidextrousness, beyond mass statistical analyses, because of the developmental minority effect - there is still some degree of implicit pressure to conform to a world socially and culturally tuned to dextrals (from handshakes to can openers), and some degree of behavioural plasticity enables adjustment to such pressures, even when they're not forced.

There may also be a small selective advantage in sinistralism from this underdog effect, in much the same way as an unusual percentage of successful entrepreneurs are dyslexic. For a detailed and entertaining look at this effect, check out Malcolm Gladwell's 'David and Goliath - underdogs, misfits and the art of battling giants'. dlorde, Sun, 29th Jun 2014

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