I’ve presented this question before, but don’t recall a satisfactory answer: Why are most humans right-handed? Is it a nature vs. nurture thing? Natural selection?
- Depends what you mean by “Why.” If you’re looking for a “purpose,” there’s no purpose in evolution.
- It’s most certainly “nature.”
- Natural selection would imply there’s some sort of survival advantage to being right-handed. Is there?
- It’s probably just a random mutation, like many other traits. Not enough of an evolutionary advantage to kill off lefties before they have a chance to reproduce.
Here’s an answer to a question on Scientific American’s website titled “Why are more people right-handed?”
It’s not the most satisfying answer, though.
Genetic influence, with an overlay of environmental factors.
Mostly genes.
Cite? I had thought that twin studies had shown genetic factors are not the predominant factor.
This type of question pisses me off (not at you). Almost everything about early human differentiation including gay vs. straight is biologically determined. Genes get way too much air time for this type of thing. The reason that I am a male is because the sex hormones made me that way, genetics are only indirectly in control by the gonads producing the right sex hormones at the right time. If they didn’t do that successfully then I would appear to be a female which isn’t nearly as rare as you might think.
The same thing is true for left-handedness. Differentiation occurs well upstream from the genetic component yet that seems to have never made its way into the popular press. Genes <> biologically determined which is a huge factor that most people are not aware of.
Shagnasty - Grad school in sexual differentiation
Most people think of “Environmental” as being the same as “Nurture” which would be a parental influence. That is false. The enviromental piece can easily occur in the womb and often does.
Well, yes. I’m sorry if it seemed I was suggesting “nurture” as a factor. I’m a lefty myself, and so far as I can tell it’s entirely biological. That doesn’t necessarily make it genetic, though, hence my request for a cite that it’s “mostly genes” when I had thought the thinking was that there was a genetic component but that something like hormone levels during gestation play the more significant role. I can’t for the life of me remember where I read that, though.
There are really three questions here:
1: Why do individual humans have handedness? The answer to this seems to be that if one uses the same appendage frequently, one develops greater dexterity in that appendage, and it seems that having very good dexterity in one appendage is generally more useful than having mediocre dexterity in both. I say “appendage” here, because handedness or something equivalent to it is found in most species with multiple appendages: Cats will prefer one paw over the other, elephants will prefer one tusk, etc.
2: Why do most humans have the same handedness? One reason, of course, is that we’re related to other humans. But humans have a rather stronger bias than most, and one reason is probably that we use tools: A tool designed for a person of one handedness will work best for others of that same handedness. So a tribe of people who all have the same handedness will have an advantage over a tribe with an even split, since they can all use each others’ tools.
3: Why is the particular handedness that humans mostly have the right? This one is probably just down to random chance. If you flip a coin, you don’t ask why it happened to come up tails.
(by CP ):
Genetic influence, with an overlay of environmental factors.
Mostly genes.
You guys and your cite crapola…how many times do I have to mention I am the cite? 
It’s like writing a little research paper every time…
On the twin front: the idea that twins are absolute carbon copies of the same substrate is vastly oversimplified. Another topic. But even monozygotic twins don’t share the exact same mitochodrial DNA. And of course no two environments acting on anyone’s DNA are identical, so the environment always has some influence on how genes are expressed.
Anyway, for a cite regarding the first candidate gene (underlining mine) :
http://www.nature.com/mp/journal/v12/n12/abs/4002053a.html
“Left–right asymmetrical brain function underlies much of human cognition, behavior and emotion. Abnormalities of cerebral asymmetry are associated with schizophrenia and other neuropsychiatric disorders. The molecular, developmental and evolutionary origins of human brain asymmetry are unknown. We found significant association of a haplotype upstream of the gene LRRTM1 (Leucine-rich repeat transmembrane neuronal 1) with a quantitative measure of human handedness in a set of dyslexic siblings, when the haplotype was inherited paternally (P=0.00002). While we were unable to find this effect in an epidemiological set of twin-based sibships, we did find that the same haplotype is overtransmitted paternally to individuals with schizophrenia/schizoaffective disorder in a study of 1002 affected families (P=0.0014). We then found direct confirmatory evidence that LRRTM1 is an imprinted gene in humans that shows a variable pattern of maternal downregulation. We also showed that LRRTM1 is expressed during the development of specific forebrain structures, and thus could influence neuronal differentiation and connectivity. This is the first potential genetic influence on human handedness to be identified, and the first putative genetic effect on variability in human brain asymmetry. LRRTM1 is a candidate gene for involvement in several common neurodevelopmental disorders, and may have played a role in human cognitive and behavioral evolution.”
Now go argue among yourselves; the Pedant is past his nap time. No this particular study is not conclusive, even for that particular gene, but just like the rest of pretty much every trait you’ll ever be able to name, we are our genes. Be patient; it will all get sorted out eventually. Never bet on nurture or environment except to the extent that they are layered upon a genetic underpinning. The genomic writing is on the wall.
Here’s the deal:
There is no chance we are predominately right handed because of random chance, other than the general notion that everything is a given way because of chance (that is to say, nature could have chosen left-handedness over right, or neither–in which case we’d be ambidextrous or evenly bimodally distributed). Since 90% of us are right-handed, there is a biologic reason. The skewed distribution toward one side across nearly all populations tells you it ain’t some sort of original initial start-off right-hander whose disposition is now perpetuated by dads nurturing righties to make sure their kids live more easily in a right-handed world and don’t become schizophrenics.
Now the term “handedness” is a shorthand for “predisposition for dominant motor cortex for coordinated motor skills” or some such notion; itself a simplified term for the idea that most people naturally develop better coordination using their right side. Doesn’t mean some folks can’t be left-handed or ambidextrous due to either genetic variation or training.
No, it’s not simple Mendelian inheritance. But it is genetic, mostly.
I’ve seen this one before: A half-answer. It basically agrees that most people are right-handed, but doesn’t get very far in explaining WHY.
The body is somewhat asymmetrical, the brain especially so. As each hand is controlled by a different side of the brain, then the real question is probably why is the brain asymmetrical? Note that a particular species wide left-right brain pattern may have nothing to do with natural selection and may just be tying in an earlier random choice (just as there is nothing intrinsically better with R-amino acids over S-, but once the evolutionary choice was made we are stuck with them).
So to my mind the only question is why the is brain arranged the way it is. Any argument based on tools is likely to fail as simple rocks, sticks etc tend to be useful for both right and left handers, and animals show similar preferences.
It may be that (i) splitting the brain in separate parts allows best multiprocessing (ii) that it is important to let one part be dominant to give rapid decision making (iii) a side by side (e.g. left-right) model is better for that than a front to back one <insert reason here>.
I guess what I am saying then is that left right handedness may simply be a incidental outcome of the left-right brain arrangement, rather than something special in itself which needs some tool or evolutionary explanation.
But there’s an easy way to disprove the fact that it’s mostly genetic: look at the offspring of two left-handed people. Statistically only 40% of a left-handed couple’s children are left-handed. That’s significantly higher than the general population, true, but if being left-handed was caused by a recessive gene like is sometimes suggested it would make almost all children of two left-handed parents left-handed too.
(by CP: )
“No, it’s not simple Mendelian inheritance. But it is genetic, mostly.”
No, it’s not simple Mendelian inheritance. But it is genetic, mostly.
Complicating matters considerably has to be the prevalence of mixed-handedness, e.g. folks who write and eat right-handed, but throw and bat left-handed.
(bolding mine)
This I don’t understand. I’m left handed in almost everything. I play golf and bat right handed. Both golf and batting involve both hands and although it’s called right handed for me, my left arm and hand are my dominant hand in doing them. I just swing my left arm back handed when I’m doing it.
I started playing golf right handed because my right handed uncle used to take me to play golf when I was 9 or 10 years old and right handed clubs were the only ones I had access to.
Speaking specifically of athletes, I think this has more to do with “nurture” vs nature. In much the same way left-handed drivers have to learn how to drive a manual transmission car with their right hand on the stick-shift.
Young athletes (T-ball, Little League, etc.) are often coached to throw right, bat right, etc, either because they start playing the sport before they know for sure they are left-handed, or because of inadequate/inflexible coaching, or even because of equipment limitations (I can’t find a reverse catchers mitt! or even a left-handed 9-iron! Oh well I’ll just learn these sports with “rightie” equipment).
Just a thought.
I’m bumping this thread back to the top because I still would like Cecil’s thorough opinion on this subject. So far all we have is conjecture.
This is me. Eat, write, pee left-handed. Everything else, right-handed. As a guitar player, I play “right-handed” even though the dexterity is on the left hand (I don’t play finger style). When a left handed person tells me he wants to start playing guitar and where he can find a lefty guitar, I tell him, “why look for a lefty guitar? They cost more, are harder to find, and you can’t play with either hand anyway! Get a right handed one!”
As Chronos said upthread, the tool (guitar) is biased for right-handedness.
Sometimes that is the best that we have. There are still many unsolved mysteries in the universe, especially in the brain sciences, and this is one of them. Cecil won’t be able to answer it (nor can anyone else for that matter). If you want the best answer available, you will have to go to a well-stocked academic library and look at the most current journal articles of this subject and there are many covering many sub-fields. Graduate level neuroscience textbooks sometimes contain theories on this but their publication cycle usually means that they are out of date even by the time the first book rolls off of the press.
I just thought it was because the first clubs were right-handed. Golf clubs that is.