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?

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.

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.

Isn’t the “hook” just a method of preventing ink smudges?

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.

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

See post #3.

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.

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

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

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.

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.

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.

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.

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

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.

nobody knows:

http://www.scientificamerican.com/article/why-are-more-people-right/

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:

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

(Just joking)

:slight_smile:

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.

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