I know that approximately 10% of people are left-handed. But why? I cant think of any environmental selective pressure for this. I would think it would be more or less evenly split.
Also (related), is handedness heritarary? If one or both of your parents are lefties, are you more likely to be?
Does nurture have anything to do with it? If your ADOPTIVE parents are lefties, are you more likely to end up left-handed, regardless of your birth parents handedness?
Many, many animals have “handedness” or “footedness”. In many (most?) species the right/left split seems about equal, except in humans and a few other species. So favoring one hand or paw or limb over the other does seem pretty common, it’s the skew towards so many people being right handed that’s unusual.
(As an aside - parrots seem to be more often left-footeded than right footed. I’ve been owned by 8 parrots over the years and all have been lefties)
It has been speculated that developing consistent handedness in humans is related to greater manual dexterity and/or brain specialization for language or other features. But that’s only speculation and what evidence there is is far from settling any such questions.
Handedness is, to some degree, hereditary. The fact that 90% of humans are right handed would seem to indicate that all on its own.
Some reasons why, in a species that so very much favors right-handedness as the norm, people might be lefthanded:
1 Damage to the right hand/side of body/left side of brain. This is fairly obvious: people born with only a left hand would become lefties by default. Likewise, a stroke that disables the right side of the body can do the same. Damage can occur from any time after conception through old age.
2 It might be that people are either right-handed or born with a 50/50 chance of either choice, so 50% of the population winds up righty and half the remaining (25%) wind up righty for an end result of 75% being right handed. Add in some cultural/environmental factors favoring the right handed and you would well account for most of the 90%
3 Unquestionably environment is a factor. See below.
This one I don’t know.
Then again, we know that societies/cultures that strongly discourage left handedness can “convert” some lefties into pseudo-righties that do a lot of stuff with the “wrong” hand and so appear to be right handed when really it’s just that they’ve trained to use their non-dominant hand.
So it’s possible to odds of being lefty go up somewhat with two left-handed parents providing a left-favoring environment, but who knows?
And we do know that some people continue to use their dominant hand regardless of environment factors discouraging that.
I assume you mean 10% left-handed?
I don’t think “nurture” or “fitting in with the other humans” is a factor. We knew our daughter was right handed as an infant. I had to cut the nails on her left hand about three times as often as in the right, because the nails on her right hand were worn down by use.
Yes, i know kids used to get their hand swatted if they tried to write with their left hand, but that didn’t make them right handed, it just taught them to write with their right hand. I’m mostly ambidextrous, and can write with both hands, but my handwriting is much better with the left hand because i have so much more practice writing that way.
(I was forced to learn to write with my right hand in high school, when i broke my left wrist.)
I’ve heard of a lot of craziness from early, more barbaric times, but forcing a kid to break their own left wrist so they’d write with their right hand… Just, wow… j/k
People are not symmetrical. Asymmetry starts very early in the development of the fetus, and defects in the asymmetry causes medical problems. So, it doesn’t seem surprising to me that there might be a strong biological component to handedness.
I don’t know how this fits with the OP but I think it’s an interesting side note:
I was born with an essential tremor. It runs in the family. As I get older, the worse it gets. At 51, there are now certain things I do with my left hand that I always used to do with my right. Like hold a spoon, shave, or brush my teeth.
Not sure why the tremor is worse in my right hand, but I suspect it has to do with the fact I was born right handed.
Sorry, yes I did mean 10% left handed.
Figured. You might want to ask a mod to edit it, since it’s your opening post 
Serious question: does that play a role in why you choose the chihuahua (which consists of 70% yap, 30% tremble) as your avatar?
Haha, no I didn’t make that connection until just now.
I like it tho’. Just a happy coincidence I guess.
I know anecdata is not data, but in my family (six people,) my mother, brother and sister are all lefties, so that’s 3/6. Fifty percent is way higher than the norm so the main explanation would be that it is genetic, from my mom.
Easiest way to do this is to report your own post, with a request. Any mod can do that, not just the GQ mods. Or PM a mod.
I highly recommend the book “Right Hand, Left Hand: The Origins of Asymmetry in Brains, Bodies, Atoms, and Cultures” by Chris McManus. Basically he explains this as it occurs during development at the molecular level.
It’s at least partly genetic:
Anecdotally, my dog was decidedly left-pawed. It’s not something you are looking for but living with them all the time you just notice it and it becomes obvious they have a handedness (pawedness?) preference.
I have no idea if dogs are predominantly one or the other or evenly split.
It should be pointed out that it not only our HANDS that our lefty or righty. It’s also our feet, eyes, ears, and even parts of our mouths. And they don’t all correlate with our hands. For example, I’m a righty, but when taking pictures or talking on the phone, I’m a lefty.
I would imagine that humans have a stronger bias than most other animals, because we use tools so much. Most tools can be made to be used equally by either hand, but you’ll usually get better results from a tool made for one specific hand, and some tools (such as scissors) are inherently handed. And given that humans share tools, it’s advantageous for most of us to use the same kind of tools.
But that’s relevant only if left-handedness prevents some people from reproducing. Could that be true of scissors?
Interesting point.
My right eye always had better vision (less nearsighted) than my left eye (I’m right-handed).
On the other, er, hand, when I’m carrying stuff, I tend to load up my right arm, leaving my left one free for things like doorknobs. I have no clue whether this is at all related to me breaking my right arm in 1st grade - which led to me initially learning to write with my left hand, and I don’t know if that all contributed to the fact that I have fairly bad handwriting to this day.
My husband, and his father, are both southpaws so I do think there’s something genetic at play. When my son was in preschool, he broke his right arm rather badly - leading to very limited use for several months. This masked his handedness - which was already somewhat in question due to the autism / dexterity issues. We finally had him go for some occupational therapy for a while for a number of issues, and the therapist concurred with the opinion that he was turning out righthanded.
Something I was reminded of the other day: we own a pair of left-handed scissors. Exactly like right-handed ones, except that when you hold them, the blade on top is the one closest to you. And the damn things simply DO NOT WORK when I use them with my right hand. WTF??