Hey, Welcome to Science Stuff, a production of iHeartRadio More. Hey, cham And today we're tackling the question what makes someone left handed? Is it something in your genes or is it random? And why do we involved to have a preference for one hand in the first place. We're going to talk to two scientists who have spent decades studying this phenomenon and who are going to step us through what it is, what causes it, and how to tell
if you're actually left handed. It turns out you could have been wrong about it your whole life, So get ready to use both sides of your brain as we try to get a firm grasp on the right way to think about your left hand.
Enjoy.
Hey everyone. Today we're talking to two experts, each of whom have spent over twenty years studying left handedness. The first one is Professor Elka Kraus, and she has a very unique job when it comes to handed people. We'll talk about what that job is, but first I wanted to know what is the actual scientific definition of left handedness. As it turns out there is none, at least there's no definition scientists can agree on it. So here's my
conversation with doctor Elka Krauss. Well, thank you doctor Kraus for joining us.
Thank you.
Can you please tell us who you are and what you do.
I'm a professor for occupational therapy in Germany at University for Applied Sciences in Berlin, and one of my research topics is handedness. I've been studying it and researching it since the nineties.
Amazing. Are you right handed or left handed?
I'm right handed.
Maybe to start us off, can you tell us what is handedness and how does it form in people?
Yeah, so handedness is not just the hand that you write with us. Actually it's got two parts. It's the hand preference how often you use a hand, And it's the hand skill or proficiency, which is relative, so which hand is better than the other hand. They say that about between ten and twelve percent of humans are left handed and the rest is right handed. I see, But I just have to say that we have to look at that critically because defining handedness is very difficult. It's
very complex. There's no standard definition of handedness. The way it's measured is very different. So some people only look at hand preference and the classification. When is a left hand or left handed is also not a standard. So these are the official numbers of ten to twelve percent left handedness, but I personally believe it could be a lot higher.
Why is it difficult to measure handedness?
Well, people have different ideas of what handedness is.
I see, it's hard to measure because there's no consensus on how to measure it. Yes, so doctor Kraus is not your typical scientist. Her interest in what left handedness is comes from the very unique job that she has, which is to help little kids figure out if they are left handed or not. Well.
The thing is, my context is that I work with children who are often on the verge, are going to school and who don't know which hand to write with, so they come to occupation as therapy to be assessed to see what hand should they write with. I see the problem is that if children learn to write with their non dominant hand, we call them switch handers. We know that they can have all sorts of problems because
of that. They are consequences, and this makes handed there's not only a fascinating topic for me, but it actually is a very important one because occupation syrrapis in particular are co responsible for deciding which hand to write with.
Okay, you might be thinking, here, it's not that hard to figure out which hand you should use to write with. You just give a kid a crayon or a pencil and see which hand they use. But as we'll talk about in a minute, for a lot of kids, it's not that simple, and the problem is that making the wrong choice can have pretty serious and life altering consequences. Okay, you mentioned that there are consequences for kids who are forced to use the hand that they're not handed with.
So, as you might have heard, one hundred years ago, people were forced to be right handed, so left hand is. At school, they were taught to use the right hand, and if they didn't, that left hand was tied behind their back or they were actually hit on the hand. It was a very strong bias towards right handed, and as we speak, you could hear that language is also right biased, and so you are right, and it's got a bit of a negative thing in all sorts of languages.
It goes back to a religious thing as well. One believe that the left side the devil was looking over your shoulder, so if you wrote with your left hand, the devil would be able to infuse these evil ideas into you. So you were doing the person a big favor by forcing them to write with the right hand. That caused a lot of painful a lot of left handers.
Some then started to go back to write with their left hand at some point and they found that certain problems that they had concentration, memory, retention, also find motor coordination or gross motor coordination, you know, whatever they did actually became better, which just suddenly they could think free. So there's not that much good research on it, like control studies and whatever, but we have many sort of
qualitative data on it. There's been some study on switch handers looking at the quality of life, and they did find that switch left handers feel that they have a lesser quality of life. They are motor proficiency problems, concentration learning problems, so they might actually have spelling difficulties, they might not remember things. They might be speech problems like stuttering going on, and and some moti visceral things like headaches, stomach,
egche all of that. Because somehow writing, especially writing with a non dominant hand, seems to trigger all difficulties. So you can imagine if you had to do this, you would have to concentrate on so many other things to try and sort of get a reasonable result. It will never be as.
Good right right. It'd be like living in a constant state of frustration and kind of constant state of feeling like there's obstacles. You're thinking, yeah, oh wow, I had never thought about how hard that must be. Absolutely, and that seems like a very big responsibility to kind of direct a kid to use a writer or left Yes, I know you might think it can't be that hard to tell if a kid is right or left handed, but here's the thing. We live in a right handed world.
Doors are made to be used with your right hand, Toys, drinking cups, everything around us is mostly designed for right handers. And as a kid, you might see both your parents use their right hand to do things, and you might think that's the hand I should use. And so there are a lot of kids for whom it's not obvious whether they're right handed or left handed and which hand they should use to write with.
If you're in a right handed world where everything is right hand orientated, then we cannot be sure that we just say let's let him write with the right hand, or let her write with the right hand because we might be switching somebody, usually a left handed So you can see that my point of departure is not just looking at handedness as a phenomenon, but it's actually identifying those children who might be left handers and preventing them
from switching, or identifying switched handers and maybe reverting them back to their dominant hand.
I see. Now, as doctor Krauss mentioned, there is no single definition of what left handedness is, but there are tends you can take that measure your handedness along several dimensions.
So in my work, I've actually defined six dimensions off handedness.
What are these six dimensions that you measure left handedness on?
So, as I said the first two, the most important ones would be that you have preference and skill. You would think that would be the same thing, but it's not always that your preferred hand is also your better hand.
Really, what, yes.
Are you right handed or left handed?
May I ask I'm right handed? Or wait? If you had to guess, would you be able to guess?
No? But this is the thing. Right handed don't understand this necessary. They think that left handedness must be the opposite of right handedness. So anyway, Those are the two first dimensions. The second two are direction and then very importantly the extent of handedness. So you have hands, as I call them, that of verse very strongly handed. The right hander is that do everything right, left handed that do everything left. But that is actually the minority of people.
Most people are moderately. The extent of hand is moderate. And then you have quite a big group in left handers that are very slightly left handed. They're not strongly left handed, that do a lot of things with the right hand really, and people don't understand that, and they
called the mixed handers, but they're actually left handed. So there are some researchers that found that this group of left handers, they will always write with their left hand, but they might do more than half the things with the right hand.
WHOA.
We see this when we actually do differentiated assessments. So again there is this complex play or symphony of these different dimensions. Actually then determine which hand you use.
WHOA. It's a lot more complicated than maye people think. Absolutely, it seems like you have these dimensions because some people might score high and based on one dimension, you might say, oh, this person's right handed, or left handed, but on another dimension it might be different. Yes, I see, And so what that tells you is that it's sort of a complicated thing and it's sort of on a spectrum. Is that kind of a big takeaway is that it's not just a right or left. There's sort of a lot
of handedness in between. That's right.
I think handedness is a continuum. So if you have extreme left, extreme right and somewhere you know, if every person is somewhere in between.
Wow, But I guess we like binary things, so we might draw a line where it's right or left, but really it's a richer picture. Yes, people can have a preference for right left more or less, but it may not necessarily correlate to which hand is better at doing things.
Yeah.
Wow, I had to realize it's so complicated.
Nor did I honestly when I started.
Really wow, I have no idea all right. So there is no single definition of what left handedness is, and in fact, it seems to be on a spectrum. Some people are more left or right handed than others, And it's entirely possible that you could be a hidden left handed person and you only think you're right handed because that's what you were taught as a little kid. But here's the question, why are we handed in the first place?
Why do we tend to have a preference, however strong it may be, to use one hand and not the other. Is it programmed in our genes? And why did we evolve this tendency to prefer one hand. When we come back, we're going to try our hand at answering these questions. So stay with us. We'll be right back. Hey, welcome back. We're talking about left handedness, and so far are we've talked about how hard it is to actually define it. Now we're going to talk about what actually causes it.
Is it hereditary or random? Or are there other reasons why you choose to use one hand over the other. To help answer these questions, I reached out to another scientist who spent decades studying this phenomenon. Here's my conversation with doctor Sebastian Oglenburg. Well, thank you so much for joining us at you're Okenburg.
Thank you so much for inviting me. I'm a germ professor of a research method in psychology at medical school in Hamburg and Northern Germany, and I spent the last twenty years doing research on left handedness and vainness symmetries. So I'm really interested in understanding why people are left tended and what it means to be left handed.
Amazing that's perfect for us today.
So left tendedness is, of course, on the surface, it's a preference in behavior, like people who are left tended like to use their left hand to do things like writing or drawing or using scissors. But handedness has nothing to do with the hands themselves, despite the name. If you ever looked at the hands the left hand and right hand, you would not be able to distinguish just by looking at the hand whose left handed, whose right handed right?
So they're looking pretty identical.
So handedness is something that has not much to do with the hands, but a lot to do with the brain. So what handedness is is a form of so called hemispheric asymmetries, meaning that our brain has two halves, and these two halves both do amazing things, but they do a little bit different things, and handedness is one form of these symmetries.
Can you give us other examples of brain asymmetries.
So these brain asymmetries are quite common, and there's a lot of things in a brain that are asymmetric. The most well known example language, so we know that for most people, language to speaking to other people and understanding what they're saying is controlled largely, not exclusively, by the
left side of the brain. So for example, if you ever had a relative who suffered a stroke to the left side of the brain, they have severe problems with speaking in the weeks and months after they had the stroke. People who have a right sided stroke typically do not have this language problem. Other things that are quite come
are the processing of faces. So for me, like to recognize your face, this is something that is mostly done by my right hemisphere, so it's a specific area called the fusiform face area that is located on the right
side in most people, and so on. So there are many other things like visospatial attention, recognizing bodies of other people, mathematical operations, so a lot of things that are complicated and hard to do are organized in asymmetric networks, and handedness is one form of these the symmetry so it is a dominance of the motocortex, so the brain areas that are controlling our movements for things like writing, drawing, and so on. What we would call fine motor movements.
So left handers have a right hemispheric motor dominance body sphinx and right handers a left hemispheric dominance because the left side of the brain is controlling the right side of the body and vice versa.
WHOA, what does it mean dominant? It means it's better at things, or it it just kind of asserts control of things. What does dominant mean?
It means several things.
So for the one part, it means if you like decide consciously or subconsciously, like you want to draw a picture, then generally you would decide for your dominant hands. So on one part of it, it is a preference that you're having as a person. But this preference also goes along with differences in skill. So even if you would like draw with your nondamental left hand, you would not be able as much as you try to make the same quality of drawing, so there is a difference in skill.
There's also neurobiological differences. So typically in a right hander, so the part of the motor cortex is representing the hand would be larger and have differential connections. So it's something that also develops over the course of your lifetime because your networks are not like completely fixed at birth. By you know, using your dominant hand more over your lifetime, you also strengthen connections to other networks in your brain.
So it's the mixture of preference, skill, ill you're adda to me, and folastic training processes over the coursing f life time.
Wow, that is very complicated. So handedness is a property of your brain, not your actual hands, and it has to do with the fact that your brain, for a lot of things, is a symmetrical It has two sides, and one side just happens to be more dominant for how you plan to move your body. For right handed people, it's the left side of your brain, and for left
handed people it's the right side of the brain. Of course, Now the question is what causes one side of your brain to be more dominant and why is it that ninety percent of the time it ends up making people right handed versus ten percent of the time it makes them left handed. What causes left handedness? Like what causes for a right handed person, I need that left brain preference and skill and eventual training.
So we know this is odso not as simple as people thought. So it is a trade that has substantial genetic contributions but also non genetic contributions. So there's a lot of exciting, large scale cooperative research that has shown that there's about forty eight locations in the genome that are relevant for handedness.
You said, so it's forty eight.
That the play roles quite interesting that the familial handedness has a strong effect on the child's handedness.
With two left handed parents would.
Get a child, the child as a much higher chance with also being left handed than the child of two right handed parents, And the same goes if there's just like one right handed one left handed parent, and the child would be intermediate.
I see.
Do we have a sense of what percentage left handedness is due to genetics?
So we know from in studies and from other family studies it's probably around twenty five percent, which would mean there are seventy five percent of factors that are not directly explained by it. Wow, But it's also not generally very well understood whether remaining a seventy.
Five percent come from I see.
So they seem to be like sort of individual biological processes in a person that happens that might also be due to genetics, but are not caught very well, and these like twin family designed. So there's an ongoing discussion about this percentage because it seems to be quite low, so it might be higher.
In fact, I see it might be higher than twenty five percent genetic, but not necessarily meaning the cost could be more genetic, but we don't know. Yes, yes, left handedness is not completely genetic. If both of your parents are left handed, then you do have a higher chance also being left handed, but it's not guaranteed. According to our experts, there's no one gene that makes you left handed.
Scientists have found forty eight gens that seem to be involved, and they only predict about twenty five percent of left handedness. The rest could be things in your environment.
There are also a couple of environmental, non genetic factors that play a role. For example, gender actually does so men are more likely to be left handed than women. There's also a couple other things, for example, people who have been like rest fed and for a higher chance of being right handed.
Ah, so breastfeeding has something to do with it.
Yeah, it's a very curious finding. So it seems to be a rather strong effect, which I don't think is well understood on a neurobiological level, but it seems to be there's an early effect of some sexophmoons on hand that's development of brainer symmetries.
So the basic picture here is that handedness. Whether you prefer to use your right or left hand, but whether one hand is more skilled than the other, is all in your brain, and a lot can happen to your brain. Starting from the time you're growing inside your mother's womb as a fetus, your body basically has to start taking sides. For example, at some point yourselves have to decide which side of your body to grow the heart in. Yes, it's not a given that your heart will be on
the left side of your chest. About one in twelve thousand babies conceived half dexterocardia, or the heart on the right side of the chest. There have even been cases of people finding out late in life that they have it. In twenty twenty three, who was reported in a medical journal that a perfectly healthy man one hundred and two years old went in for cataract surgery and during the checkup was stunned to find out his heart was on
his right side. So you might want to double check that your heart is in the right place or the left place. I mean, but that's basically what happens with left handedness. According to doctor Okelenburg, when your fetus yourselves essentially start to pick which side of your brain will have the more dominant motor cortex. You can even start to see it and which hand the fetus uses.
More So, if you look at altsound recordings of unborn fetuses, you could see that they already have some sort of preference. For example, they interact with their left or right hump a little bit more than by the other one. And if you then wait for the baby to be born and test them during school, then these like prenatal preferences forehand use predict the writing handedness with more than ninety five percent of curiosity.
Now, scientists aren't really sure why your body picks one side or the other. It seems to be a combination of genes and hormones, and as we talked about before, this can also change after you're born, depending on what you're tagged as a kid. But here's the next big question, why even pick aside at all? What's the point of being handed? Why couldn't we all just be Ambidexter's or just as good with the left hand as the right hand.
Why did humans evolve to have a hand preference? And are we the only animal in nature the ad has it. When we come back, I'll hand it over to our experts to answer these questions. So don't lose your grip on this podcast. We'll be right back. Hey, we'll come back. We're talking about left handedness with two scientists, and so far we've learned that the handedness sort of lies on a spectrum. Most people are strongly right hand and that a few people are strongly left handed, and some people
are somewhere in between. We also learned scientists aren't sure what causes handedness. There are about forty eight genes associated with it, but how those genes affect which of your mortar cortex areas turns out to be the dominant one is still a mystery. And apparently these genes only account for about twenty five percent of whether you're right or left handed. The other seventy five percent could be environmental
factors or hidden deeper in your genetic code. But now we're going to tackle an even bigger question, which is why are we handed in the first place, I mean, why pick aside, why aren't we all ambi dextrics. As it turns out, humans are not the only animal that takes up paw. You've written about the idea that there's handedness or potness in animals.
Yeah, so in general, I've worked a lot on this.
I think back in the day, people always thought did only humans show handedness? Because all humans right, But of course you cannot have a look at the writing patterns of a cat because they rarely use pets.
But at some point people.
Started to look at poor preferences or flipper preferences or whatever in animals, using experimental designs that would allow them to actually measure normal behavior.
Of the animal.
And what you find, Dan is that most animal species actually show left and right handedness. So asn enormous amount of studies on that. It's just just like more than one hundred different species it's been investigated, and I think the general pattern is that a lot of species show sort of like individual preferences.
So if you.
Look at a cat, most cats would have rather strong preference.
For the right side or the left side.
So what you could do, for example, if you have a cat at home, you want who knows the cat left port the right port. You could do something it's called the food reaching task. So you take like the little inlet from a toilet paper roll, close up one side with a little bit of like a cling wrap foil or whatever, and then you put like some like cat food tokens into this toilet paper roll and just have a look whether the cat is reaching into it
with the left or the right paw. So if you use these kind of things, you typically find that individual cat has portaners. But in generally for a lot of animal species, left portners is more common than in humans.
So we had this large meta analysis in twenty twenty about human handedness showing it's ten point six a percent of people are left handed, but if you look at cats or dogs, it's more like thirty to forty percent of animals being left pot So it seems to be animals do have handedness, but for a lot of animals species, left handedness left partners is much more common than in humans, so it goes more into like an equal distribution. There's
also research with animals in the laboratory. So for example, people go to cat or dog hotels and ask the owners whether they can test their animals there while the owner's own vocation do a little bit of like interactions with them and then find out whether they're left or right port and be for example, also have some like citizen science projects where we ask people to record their animals with a smartphone while they are doing.
Basically the scientific work.
So there is a really big amount of research on animal hotness and handedness and generally shows humans are not unique or special in that.
They have handedness. Most animal species have.
It too, I see, but we're sort of unique in how much more prevalient right handedness is.
Yeah, that's not completely unique.
So there's also some parrot species to have about similar distributions of actually left.
Clawdness really the way there are parents that skew the other way. They're like ninety percent left handed, left clawed.
Yeah, left footed, claud whatever. There's a lot of research coming from Australia on that. So if you look at birds, a lot of birds don't do a lot of food handling with their claws, So parrots actually are species that do handle food and they also do other like a little bit more complicated tasks.
Yes, it turns out humans are not the only animals with handedness or a side preference. Lots of animals have it, even down to flatworms which don't even have a brain. But that still doesn't answer the question of why this preference exists. Well, according to doctor Auglenburg, you might all have to do with efficient See.
Why does this happen?
I think this is sort of like a way of making our brains more efficient. Your brain is about two percent or so, depending a little bit of my over body weight, but it takes about twenty percent of all
the calories I'm taking in to keeping it working. And so if you know the brain is the most energy hungry organ in the body, then it makes a lot of sense that you specialize that you have like networks that are only on one side, because if you double them, like for example, kidney or something, you would like increase the energy amount the organism needs without having like a
direct gain in the situation. And the other thing is also if you do not take some like measures to make the brain efficient and as small as possible, at some point, umes would have been like really large heads, which would have all sorts of problems, for example, doing birth, doing like everyday life. You don't want to have a head like four times the size.
Right, if we hadn't specialized the hemispheres, you mean.
Yeah, So if you hadn't blessed the hemispheres and you would double everything in the brain, you would have had a much larger head. Right, So this would not be good for a number of reasons.
All right, that makes sense. We didn't involve to be ambidexrous because for the most part, we don't really need two dexterus hands. It's more efficient to invest brain resources into making one of your hands more skilled than the other, because that's probably good enough for most tasks like using a tool or picking apart some food. But that still doesn't explain why humans are sort of unique having one kind of hand inness be more common than the other.
As we mentioned before, most other animals have a nearly equal split between right and left handers or powers, but in humans, ninety percent of people are right handed. Why is that well? One idea, according totor Oglnburg, is that having most people need the same handed makes it easier to teach each other.
And one of the ideas that it is just like easier to learn like a complicated thing if you.
Don't have to like also mimor reverse it in your brain.
Right, So if you're right handed, it's easier to learn writing from a right handed person because you can just like copy what.
They're doing compared to a left handed person.
So this ease of training and highly complicated things is something that is one of the hypothesis widest is such a strong right sided preference.
I see and I read that there's something called the fighting hypothesis related to sort of the evolution of handedness. What does that mean?
So there is a hypothesis that tries to link handedness directly to some sort of evolutionary survival fitness advantage. And the idea behind this is that left handers have a sort of like advantage in fighting in combat sports. It's very well known and there's a lot of research paper
showing that. For example, I think like boxing, because a lot of people that do fight they've been trained in like fighting right handed opponents, which means that if you face the left handed opponents and they're attacking from basically the other side, you might have less experience in this sort of combat situation, and then the left tender might have a advantage. And this sort of like shows pretty well if you look at the sports for things like, oh,
you call it like fencing. So the fighting advantage hypothesis is that left handers may stay in the population because they have a survival fitness advantage in fighting, but only as long as they're rare. If there's a lot of left handers there would be sort of like then destroying
the advantage because then they're not very surprising anymore. Right, right, And it's a bit of an ongoing discussion whether you can explain behavior like directly with these fitness advantages, because I mean not sure how many fights you or your ancestors ever got that had direct relevance for your survial. So I would say it's a hypothesis. I would take it with a grain of salt.
Okay, last question, anything else you think people should know about left handedness.
I generally would say, your handedness is something that refers to the motor cortex. And if you look at like the largest meta studies integrating research over the last couple decades as pretty convincing evidence that left handedness and intelligence is not linked, so leatenders and right handers.
Seem to be equally smart and capable.
The same goes also for creativity, there was always this idea left handers some more creative, and it's.
Quite interesting if you look at it.
If you ask left and right handed people whether they believe left handers are more creative than right handers, is a ginormously strong statistical effect. So everybody believes left hand is some more creative than right handers.
And actually parents use this that advice.
Right, so you're really having a left handed child, you tell him become an.
Artist or you become a rock star, whatever.
But if you actually have left handers right handers create art, and then you take like artists who are experts in this field and let them rate the products, there's absolutely no difference. So I think in general you should think of handedness and something that is referring to the motorabilities. It's caused by the mortal cortex and does not have any sort of strong effect on like how smart you are, creative, you're any other like brain abilities, because it's just is
not affecting the brain networks that do these things. Anything related to thinking or this idea there's sort of like a coherent left handed personality.
They're rere spirited, creative thinkers and stuff like that.
That's just very like old ideas that did not hold up very well.
Sorry, left handers.
So, left handers, is there anything else you think people should know about left handedness or handedness in general.
Well, I think it's very important to look at left handedness as a normal form of handedness. There's nothing abnormal about it. They are just different in many ways.
So this is.
Really something that you cannot just change the handedness just by writing with that hand.
I think you're saying that trying to switch someone doesn't just automatically flip the brain. It's like you're really forcing something that the brain is not predisposed to do.
Yes, that's well said. You know that's not anything. So go to an occupation service, have been tested out and make sure that the child writes with the right hand.
No, it was correct, yes, right, not the right hand, the correct hand. That's funny, all right. Well, it sounds like the question of why some people are left handed is still a bit of a mystery. We know a lot, but there's still a lot we don't know. I guess we just need to wait for the right person to figure out what's left. Hey, thanks for joining us. See
you next time. You've been listening to Science Stuff production of iHeartRadio, written and produced by me or HM candidate by Rose Seguda, executive producer Jerry Rowland, an audio engineer and mixer Jacey Pecrom And you can follow me on social media. Just search for PhD Comics and the name of your favorite platform. Be sure to subscribe to sign Stuff on the Arheart Radio app, Apple Podcasts, or wherever you get your podcasts. Please tell your friend friends we'll
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