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Author Topic: Why are people left-handed?  (Read 2149 times)

Offline thedoc

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Why are people left-handed?
« on: 23/04/2015 12:03:00 »
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.
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« Last Edit: 23/04/2015 12:03:00 by _system »


 

Offline A-Memo

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Re: Why are people left-handed?
« Reply #1 on: 29/06/2014 05:46:16 »
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 ;)
 

Offline alancalverd

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Re: Why are people left-handed?
« Reply #2 on: 29/06/2014 10:07:51 »
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.   
 

Offline dlorde

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Re: Why are people left-handed?
« Reply #3 on: 29/06/2014 14:59:18 »
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'.
 

Offline thedoc

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Hear the answer to this question on our show
« Reply #4 on: 23/04/2015 12:03:00 »
We discussed this question on our  show







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