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-   -   Why are the majority of people right-handed? (https://boards.straightdope.com/sdmb/showthread.php?t=786934)

Charlie Wayne 03-13-2016 04:04 AM

Why are the majority of people right-handed?
 
As I understand, about ten percent of the population is left-handed. I would expect the correct explanation to include a reason why the number is close to ten percent.

I have my own theory. When I was a teenager, I visited a chiropracter and he told me that my left leg was one-quarter inch longer than my right leg. Anyone can tell if one leg is longer than the other by sitting on the floor with your back to the wall and extending both legs straight ahead. It's immediately evident whether one leg is longer and if so, how much.

My theory is that when one leg is significantly (key word here is "significantly") longer than the other, it establishes a different kind of gait when people walk than it would if both legs were about the same length.

I say "significant" is important because my guess is there is a number (let's say it one-quarter inch) and if the left leg is longer than the right by one-quarter inch or more, that is enough to cause the propensity towards left-handedness. But if it's not longer than that number, the leg is not long enough to cause left-handedness.

I suppose one way to try and prove this theory is to study people who were born without the use of their legs. My guess is almost none of them would be left-handed because there is a strong pressure from parents and teachers to force children to use their right hands.

I have no idea whether this theory is correct. Probably not. But I'd be interested to ask what other people think. Does anyone else have a theory?

engineer_comp_geek 03-13-2016 05:28 AM

Coincidentally, I was just reading about this the other day.

Scientists have recently (with the past couple of years) discovered a group of genes that play a large role in handedness. Some of these genes are also responsible for our body's asymmetry. So you're kinda on the right track, except that a more asymmetric body related to limb size is probably more of a result of those same genes that cause left handedness rather than actually being a cause of left handedness.

Some of those genes are also related to things like Situs Inversus, which is when your organs are flipped from normal (your heart is on the right instead of the left, and you liver is on the left instead of the right, etc). Lefties also tend to have a slightly larger Corpus Callosum, which is the thick bundle of nerves that connects the left and right hemispheres of your brain. This is again caused by that same group of genes.

RivkahChaya 03-13-2016 06:44 AM

The reason that most people are right-handed, is that among those genes is a specific one that dictates that a person will be right-handed, absent something like being born without having a right hand, or having trauma to the left motor cortex, as happens in some birth accidents. 80% of the population has this gene. The 20% who do not, have even odds of being either right or left handed. This is seen in identical twins, 85% of whom are both right-handed, 10% of whom are right/left pairs, and only 5% of whom are left/left sets.

No one knows what causes right or left handedness in people without the gene, but people who have been looking at the phenomenon have noted that left-handers who write with a hook tend to have learning problems or motor skills problems, while people who write in a way the mirrors the way a right-handed person writes rarely have any problems, and the hook style is rarer. There is a theory that people who write with a hook may have the right-hand gene, but some problem that prevented them from being right-handed, and also contributed to their learning or motor skills problems.

My husband and son are lefties, and I have several cousins who are lefties. I write right-handed, but in the military, I fired my rifle left-handed, because my left eye was dominant. It wasn't uncommon. We had 60 women in my platoon, and 10 were lefties. Half the lefties fired right-handed, while 10 of the righties fired left-handed.

Richard Pearse 03-13-2016 06:50 AM

Isn't the "hook" just a method of preventing ink smudges?

Chronos 03-13-2016 08:07 AM

Why have a dominant hand at all? Because it's more useful to have one hand that you're really good at than two hands that you're sort of good at, which you'd have if you practiced with both equally.

Why do most humans have the same dominant hand? At a guess, it's because we're tool-users, and it's advantageous for everyone to be able to use the same tools.

Why is it not all of us? Being the opposite handedness from normal can be an advantage in direct physical competition, because you'll react differently than your opponent expects. This used to be an advantage in swordfighting, and is still an advantage in baseball pitching. Apparently, a ratio of about 9 to 1 is the sweet spot where these two benefits balance out.

Why is the common dominant hand specifically the right one? That's just happenstance. With a slightly different evolutionary path, it could have been the left just as easily.

Peter Morris 03-13-2016 08:17 AM

If handedness is genetic, how come pairs of identical twins often have one left hander and one right hander?

Blake 03-13-2016 08:24 AM

Quote:

Originally Posted by Peter Morris (Post 19176054)
If handedness is genetic, how come pairs of identical twins often have one left hander and one right hander?

See post #3.

Hari Seldon 03-13-2016 09:40 AM

In my left-handed grandson, his propensity to eat his cheerios with his left hand was noted well before he was walking, so I doubt if leg length has anything to do with it. Both his parents are right handed.

My daughter is thoroughly right handed and always has been, but writes with the "hook".

Someone I know lost his right arm (to cancer) and it took him about six months until it felt natural to write and do other things with his left hand.

rsat3acr 03-13-2016 10:07 AM

Well clearly there are fewer genetically superior people, therefore less lefthanders. :-)

watchwolf49 03-13-2016 10:52 AM

I wonder if the ratios are the same in the Northern Hemisphere as the Southern?

John Mace 03-13-2016 12:15 PM

Quote:

Originally Posted by Chronos (Post 19176046)
Why have a dominant hand at all? Because it's more useful to have one hand that you're really good at than two hands that you're sort of good at, which you'd have if you practiced with both equally.

Why do most humans have the same dominant hand? At a guess, it's because we're tool-users, and it's advantageous for everyone to be able to use the same tools.

Chimps are also more commonly right handed. They also use and make tools, but they don't share tools, if that's what you are getting at.

Gorsnak 03-13-2016 12:57 PM

I also have a difficult time seeing what stone-age tools would be handed to any significant extent. Even today, the vast majority of tools aren't handed, scissors being the most common counterexample.

Spears, axes, hammers, scrapers. All completely agnostic about which hand you operate them with.

thelurkinghorror 03-13-2016 02:30 PM

OP needs to read up on how correlation does not equal causation, and if his theory is true it is most likely genetics causing both handedness and leg length.
Quote:

Originally Posted by RivkahChaya (Post 19175980)
My husband and son are lefties, and I have several cousins who are lefties. I write right-handed, but in the military, I fired my rifle left-handed, because my left eye was dominant. It wasn't uncommon. We had 60 women in my platoon, and 10 were lefties. Half the lefties fired right-handed, while 10 of the righties fired left-handed.

Do they provide left-handed rifles in the military or do you just deal with brass ejecting in front of your face? Being cross-dominant is obnoxious sometimes.
Quote:

Originally Posted by watchwolf49 (Post 19176268)
I wonder if the ratios are the same in the Northern Hemisphere as the Southern?

When the antipodean nuns try to force you to write right-handed, they swing the rulers in counter-clockwise windup instead of clockwise.

obbn 03-13-2016 03:04 PM

Quote:

Originally Posted by Richard Pearse (Post 19175985)
Isn't the "hook" just a method of preventing ink smudges?

A a member of the left handed Club I can confirm that the hook was indeed to prevent smudges. That and so you could see what you were writing.

spamforbrains 03-13-2016 03:14 PM

nobody knows:

http://www.bbc.com/future/story/2014...s-right-handed

http://www.scientificamerican.com/ar...-people-right/

KarlGauss 03-13-2016 04:15 PM

Another factor that may influence handedness is breastfeeding, with breastfed infants being less likely to be left-handed (alas, the full text of the article is not free).

The brief excerpt below from the above-cited paper discusses possible mechanisms to account for the relationship between breastfeeding and handedness:
Quote:

Originally Posted by Denny
Possible connections between breastfeeding and laterality have been noted by other researchers. Numerous studies have investigated lateralisation in the tendency of individuals to hold infants and objects representing infants (such as dolls), finding that in general the left hand is used preferentially. This seems to be particularly true for women (for example, Van der Meer & Husby, 2006). Harris, Cárdenas, Michael, Spradlin, and Almerigi (2009) summarise much of the evidence and the possible explanations. One is that this left bias makes it easier for an infant to hear (and be soothed by) the mother's heartbeat. A second theory, the attention-emotional arousal hypothesis, is that babies cradled on the left are better able to perceive emotional signals or cues from the mother since parts of the right hemisphere are dominant in the perception of emotion and for wakefulness and arousal. While these results do not directly relate to breastfeeding, it seems very plausible that this bias may influence how mothers breastfeed.

In a separate set of findings Engstrom, Meier, Jegier, Motykowski, and Zuleger (2007) present evidence that the right breast produces about 20% more milk than the left, and discuss a number of other studies that have also found this bias. Since the mothers in that study were pump dependent throughout lactation this eliminates the influence of maternal or infant behaviour as an explanation. How this might relate to an effect of breastfeeding on handedness is far from clear.


Charlie Wayne 03-13-2016 05:07 PM

Quote:

Originally Posted by watchwolf49 (Post 19176268)
I wonder if the ratios are the same in the Northern Hemisphere as the Southern?

Do you know if people flush their toilets with their left or right hands in the Southern Hemisphere?

(Just joking)

:)

John Mace 03-13-2016 08:18 PM

Quote:

Originally Posted by spamforbrains (Post 19176830)

Just so you know, you're second cite is 20 years old.

spamforbrains 03-13-2016 08:28 PM

Quote:

Just so you know, you're second cite is 20 years old.
yeah I know, but the newer one says the same thing. The older one is well-written and easy to read.

Try2B Comprehensive 03-14-2016 02:01 AM

Ultimately I bet it comes down to molecular chirality. Somehow.

Surreal 03-14-2016 11:07 AM

Not being right-handed seems to be a risk factor for a huge array of disorders:

http://ccf.fiu.edu/research/publicat...owski_2013.pdf

Quote:

An examination of the literature (more than 46,000 articles—PubMed search, June 2012) shows that “non-right-handedness” is associated with a host of psychological traits (e.g., autism, Attention Deficit-Hyperactivity Disorder (ADHD) developmental coordination disorder, schizophrenia and bipolar disorders, Down’s syndrome, dyslexia, learning disabilities, stuttering, and sexual partner preferences) and medical conditions (e.g., congenital adrenal hyperplasia, arthritis and ulcers, recovery from brain damage, stature, and early menarche).

Broomstick 03-14-2016 11:41 AM

Quote:

Originally Posted by Try2B Comprehensive (Post 19178127)
Ultimately I bet it comes down to molecular chirality. Somehow.

Well, then you'd have to explain why most parrots are lefties (really, they are).

Apparently having a dominant hand is advantageous in some manner, and it may be that which side was normally dominant was a matter of chance in a distant ancestor, and that handedness has been passed down to us.

Charlie Wayne 03-14-2016 02:15 PM

Quote:

Originally Posted by Broomstick (Post 19178907)
Well, then you'd have to explain why most parrots are lefties (really, they are).

Apparently having a dominant hand is advantageous in some manner, and it may be that which side was normally dominant was a matter of chance in a distant ancestor, and that handedness has been passed down to us.

Hello Broomstick, :)

How can you tell that parrots are left-handed? Do you mean that they always stand on one foot and eat by picking up food with the other foot? I'm not sure which foot you would call "dominant".

But that's very interesting. I had a parrot for ten years and I never noticed that she always ate food I gave her by holding it with the same foot.

She hated the store-bought bird food and would make a huge ruckus unless I fed her by hand. She always insisted on eating only the same food that I ate. But I never minded. I loved to feed her by hand. Her favorite food was pizza. Believe it or not.

Broomstick 03-14-2016 03:33 PM

Quote:

Originally Posted by Charlie Wayne (Post 19179347)
How can you tell that parrots are left-handed? Do you mean that they always stand on one foot and eat by picking up food with the other foot? I'm not sure which foot you would call "dominant".

Not always but according to my (admittedly not exhaustive) research most parrots prefer picking up/holding/manipulating things with one foot over the other. My conures have both definitely favored their left foot for this. The three cockatiels I've had don't seem to pick up/hold/etc. much of anything and if they had a preference I've never been able to figure it out. The lovebirds were less inclined to use their feet as hands than the conures but all three seemed to favor the left when they did such things.

Informal browsing turns this up as a frequent observation, that parrots tend to be left-dominant. It's not as strong as in humans, but it does show up statistically.

Quote:

But that's very interesting. I had a parrot for ten years and I never noticed that she always ate food I gave her by holding it with the same foot.
Not all parrots have a dominant foot, and even those that do tend not to be as preferential as humans.

I should probably point out that avian brains are structured very differently than mammalian brains, and that might well account for some of the differences here. Which is why I speculate that handedness might have have an advantage, but which hand is preferred may be due to random chance in an ancestor. Humans might have had that ancestor as a righty, birds like parrots might have had a lefty in that position.

Most mammals (again, informal research) don't show a preference for one hand over another. The great apes (chimps, gorillas, orangutans) show some tendency towards a dominant hand, but which hand seems a bit random and there are plenty who don't show handedness. Chimps and gorillas might be slightly more inclined to be right handed, and orangutans slightly more likely left handed but the evidence is weak at best from what I've seen.

It might be (note all the qualifications indicating speculation here - it's all over what I've found on the subject) that preferring one hand over the other makes you a better toolmaker/user, in which case the strong tendency to favor one hand makes a lot of sense in humans, but doesn't account for being right handed so frequently as opposed to left. It might account for handedness in parrot species that frequently grasp/manipulate objects, in that specializing with one leg for grasping a perch/supporting the body and the other for finer manipulations also makes sense, for similar reasons as humans, but again which hand is preferred is not accounted for.

Crows also show a preference in such matters - let's get more formal and call it "lateralization" which gets away from "hands" and towards "whatever the critter manipulate with or favors". New Caledonian crows are also tool users and makers and also show a lateralization preference - with most being righties! At least when they're making tools. Here's a bit more on handedness in animals

So... apparently lateralization appears in species that make/use tools and might have something to do with brain organization. Which side is favored seems a bit of a crap shoot, but the more a species makes and uses tools the more likely the majority will favor one side or another.

Bottom line: it appears to be connected to tool making and use, and since humans are so specialized in that role it makes some sense we'd have strong lateralization.

md2000 03-14-2016 05:32 PM

Quote:

Originally Posted by Chronos (Post 19176046)
Why have a dominant hand at all? Because it's more useful to have one hand that you're really good at than two hands that you're sort of good at, which you'd have if you practiced with both equally.

Why do most humans have the same dominant hand? At a guess, it's because we're tool-users, and it's advantageous for everyone to be able to use the same tools.

Why is it not all of us? Being the opposite handedness from normal can be an advantage in direct physical competition, because you'll react differently than your opponent expects. This used to be an advantage in swordfighting, and is still an advantage in baseball pitching. Apparently, a ratio of about 9 to 1 is the sweet spot where these two benefits balance out.

Why is the common dominant hand specifically the right one? That's just happenstance. With a slightly different evolutionary path, it could have been the left just as easily.

That's probably the likely explanation. We learn a lot of tasks (motor skills) by repetitive training until it becomes instinctive. By using the same hand every time, we learn the task faster and get better at it, until it just "feels right".

One explanation I recall reading (obviously someone being somewhat sarcastic) was that left-handedness was the brain development compensating for oxygen starvation and brain damage during birth. :)

thelurkinghorror 03-14-2016 05:35 PM

Quote:

Originally Posted by Broomstick (Post 19179577)
Crows also show a preference in such matters - let's get more formal and call it "lateralization" which gets away from "hands" and towards "whatever the critter manipulate with or favors". New Caledonian crows are also tool users and makers and also show a lateralization preference - with most being righties! At least when they're making tools. Here's a bit more on handedness in animals

Nature? NatGeo!? Okay that's it, turn in your Dope card (you got one, right?). Here's a credible cite. :p

Fear the Turtle 03-14-2016 05:44 PM

Anyone want to hazard a guess as to why I'm right handed but left footed? From my earliest memories, it always felt more natural to kick a ball with my left foot.

Broomstick 03-14-2016 06:24 PM

Quote:

Originally Posted by md2000 (Post 19179944)
One explanation I recall reading (obviously someone being somewhat sarcastic) was that left-handedness was the brain development compensating for oxygen starvation and brain damage during birth. :)

While left-handedness can be a compensation for damage (people who lose their arm or are paralyzed by a stroke can learn to use the non-dominant hand for tasks that used to be done by the now missing or useless hand) it's not always. There are developmental situations where all the internal organs are a mirror image of the usual arrangement, and such people are, not surprisingly, frequently left-handed. That's not damage, that's a reversal of some early signal in the embryo.

If a lefty's brain is arranged more as a righty's that might be a case of damage compensation, but if the lefty's brain is a mirror image of the typical righty's I'd suspect it's a developmental reversal of some sort (not everything has to be reversed. Pre-birth development can be complicated). We can actually make such determinations in a living brain. When my mother (a lefty) had a stroke they did imaging that showed her brain organization was mirror to the typical righty's (which was fortunate, because if it wasn't the stroke would have been far more damaging than it was). The doc's said it was important to know these things when planning for rehab. They also said that you occasionally find a righty with a "mirror" brain, that is, their brain is a mirror image of a typical righty, which means in that case being right-handed might be a case of pre-birth damage compensation in someone who should have been a lefty!

ratatoskK 03-14-2016 07:16 PM

Read Right Hand, Left Hand: The Origins of Asymmetry in Brains, Bodies, Atoms and Cultures.It has all you need to know.....

j666 03-14-2016 09:46 PM

Quote:

Originally Posted by Surreal (Post 19178807)
Not being right-handed seems to be a risk factor for a huge array of disorders:

http://ccf.fiu.edu/research/publicat...owski_2013.pdf

Prefering to have a sexual partner is a disorder?

Quote:

Originally Posted by Fear the Turtle (Post 19179996)
Anyone want to hazard a guess as to why I'm right handed but left footed? From my earliest memories, it always felt more natural to kick a ball with my left foot.

But if you write in the sand with your toe, which foot do you use?

No, really. I catch and throw equally well (which is to say not very) with both hands, iron only with my right hand (even with irons with the cord in the center), sew and saw only with my left, cut with the left but stir with the right, cut in with my left hand and paint with my right, brush my teeth with the left but lather and loofah preferentially with the right ...

But by foot or by hand, I alway write with my left.

Maybe you balance better on the right leg.
[And most of my dogs have been right pawed]

Try2B Comprehensive 03-14-2016 10:41 PM

Some crabs are obviously handed...

guestchaz 03-16-2016 08:30 PM

Quote:

Originally Posted by thelurkinghorror (Post 19176725)
Do they provide left-handed rifles in the military or do you just deal with brass ejecting in front of your face? Being cross-dominant is obnoxious sometimes.

pssst, I know it isn't really germane to the main discussion, but, they will provide you with a deflector or catcher if you bug your platoon sgt and/or supply sgt long enough and fill out the paperwork enough times, at least in the US army anyway.

MacLir 03-17-2016 10:33 AM

Allegedly, when I was a baby I showed a left preference, but was trained to be right-handed to fit in with the majority. Subtly not forcefully, and from the very beginning. Some of the alleged physical indicators (finger length, etc.) support this. :dubious:

Possibly as a result, I tend to have less of a hand preference than most people I've met. I eat "British style" when it has a lot of cutting or if my right arm is restricted, "American style" when it involves a lot of scooping, and never consciously think about which one.

I "mouse" with my left, but use a right-hand mouse - to me mousing is an inherently left-handed activity (leaves your right free for the keyboard, and you can have the main QWERTY keyboard aligned with your monitor more easily.) :rolleyes:

I've noticed I have a tendency to use my left for activities with a higher probability of hand injury - without conscious thought of it.

Basically, things with a high degree of fine motor skills go to my right hand, medium levels have a habitual right tendency but can be done either way, and low skill activities are whichever hand is easiest.

wevets 03-17-2016 04:28 PM

Quote:

Originally Posted by Broomstick (Post 19179577)
So... apparently lateralization appears in species that make/use tools and might have something to do with brain organization. Which side is favored seems a bit of a crap shoot, but the more a species makes and uses tools the more likely the majority will favor one side or another.

Bottom line: it appears to be connected to tool making and use, and since humans are so specialized in that role it makes some sense we'd have strong lateralization.


Although there are some exceptions - grey whales are mostly right-[s]handed[/s] [s]-flippered[/s] -mouthed, we know this from imaging their feeding dives and observing the relative wear on the right-versus-left sides of their heads.

Much handedness may indeed be related to tool-use, but other forms of manipulation of the environment may be important to the exceptions.

Surreal 03-17-2016 06:03 PM

Quote:

Originally Posted by j666 (Post 19180604)
Prefering to have a sexual partner is a disorder?

It depends:

http://boards.straightdope.com/sdmb/...d.php?t=491944

j666 03-17-2016 10:03 PM

Quote:

Originally Posted by Surreal (Post 19189049)

I know I don't want to read that thread.

Sleel 03-25-2016 04:38 AM

Previous thread

My reply in that thread:

Quote:

Handedness is one byproduct of brain lateralization. We still don't really know why lateralization became prevalent, but it seems to confer some advantages for separate processing. One reason why lefties are supposed to be more creative is that they tend to have less strongly lateralized brains, so they come up with connections between disparate ideas more easily. High intelligence also seems to be lightly correlated with left-handedness.

There's a pretty good chunk of The Language Instinct and How the Mind Works, both by Steven Pinker, that discuss lateralization, particularly the connection with language. Those are probably the most approachable, yet scientific books I've read on the subject.

Handedness is actually a continuum, with few people being totally right- or left-dominant. Some degree of mixed body dominance is common. You might have someone who is strongly right-handed, but strongly left-eyed and weakly left-footed, like a friend of mine I taught to shoot better. You should shoot based on eye dominance, not hand dominance; target acquisition and aiming are the most important things. It took some work to get him using his left hand, but his accuracy and speed went way up afterward.

While there is no single "left handed" gene, there are some genetic factors; handedness can run in families. But environment seems to be a stronger selector. Stressful conditions at conception and birth have an effect on the prevalence of left-handedness.

http://boards.straightdope.com/sdmb/...ply&p=16490512

Quote:

I would also imagine this effect would become more pronounced when it came to weapons, for combat, for hunting. People who were more proficient would kill off their opponents and have an easier time feeding themselves. Thus, if there was some social order for being trained to use them right-handed, and there's some genetic predisposition toward one or the other, those who would have had a bit more natural skill would start to win out and spread that genetic predisposition, even if it is slight.
Actually, violence selects for more lefties. In other words, the minority preference confers an advantage in combat. Either that, or left handed people are naturally more violent :)

(Full disclosure: I'm left handed. You got a problem with that?!)

From some of my anthropology reading I remember that the Yanomami have something like 20% incidence of left-handedness. "Fierce" men, who go on more raids and are more willing to undergo inter-tribal duels, have more wives than men who show less aggression, and these men are more likely to be left handed. (In duels they beat each other over the head with really long wooden poles, deliberately receiving head wounds to show how tough they are; this is not dissimilar to Heidelberg dueling).
On preview: NOT a reply to the left-handed pedophilia thread. I'm pretty sure I stayed away from that mess.

paulmarkj 03-25-2016 05:33 AM

Quote:

Originally Posted by RivkahChaya (Post 19175980)
...people who have been looking at the phenomenon have noted that left-handers who write with a hook tend to have learning problems or motor skills problems, while people who write in a way the mirrors the way a right-handed person writes rarely have any problems...

That is an old fashioned prejudice. The idea that left hand people are 'awkward' comes from the fact they have to use right handed objects. Many right-handed people don't realise how difficult this is.

Try these:

1) Ask people to write backwards. Left handers are much better then right handers for two reasons: first, they backward direction is (at last!) in their favour. Second, left handers are used to adapting, not copying, hand movements.

2) Give a righty a left-handed corkscrew and watch them struggle!


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