Speaking of Psychology - Lefties, righties and mixed-handers: The psychology of brain asymmetry, with Sebastian Ocklenburg, PhD

Episode Date: October 1, 2025

Left-handers make up about 10.6 percent of the world’s population. Sebastian Ocklenburg, PhD, talks about why handedness and other brain asymmetries exist; the genetic and environmental factors that influence handedness; whether there’s any truth to the stereotype that lefties are more creative; why “pawedness” is common in cats, dogs, and other non human animals; and why left-handed people are underrepresented in brain research. Learn more about your ad choices. Visit megaphone.fm/adchoices

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GoogleFi Wireless is not subject to data traffic deprioritization during times of high network usage. We live in a world designed for right-handed people. If you're a lefty, you know the pain of trying to track down left-handed scissors or a pen that won't smudge as you write. Yet despite being overlooked by the designers of school desks, corkscrews, and can-openers, lefties make up a small but consistent part of the population. Today we're going to talk to a psychologist who studies handedness and other aspects of brain lateralization, about handedness, where it comes from, and what it can tell us about how the brain is organized. So why does handedness exist in the first place? Why is it more common to be right-handed than left-handed?

How and when does handiness develop? Is it genetically determined due to early life experiences or maybe a combination of the two? Is there any truth to the stereotype that lefties are more creative than other people? Are neurodivergent people more likely to be left-hand? Does handedness or pawedness exist in other animals? And why are left-handed people often underrepresented in brain research? Welcome to Speaking of Psychology, the flagship podcast of the American Psychological Association that examines the links between psychological science and everyday life. I'm Kim Mills.

My guest today is Dr. Sebastian Acklenberg, a professor of research methods in psychology at MSH Medical School in Hamburg, Germany. His research focuses on hemispheric asymmetries in the brain's language and motor systems, including handiveness. He has looked at how asymmetries develop, including genetic and epigenetic factors, and the neurophysiology of brain asymmetries in the prevalence of handedness in humans and limb preferences in other animals. Dr. Aucklandberg is the author of many peer-reviewed journal articles, as well as the book, The Lateralized Brain, The Neuroscience and Evolution

and evolution of hemispheric asymmetries. He also writes a blog for psychology today called The Asymmetric Brain, the psychology and neuroscience of left and right. Dr. Acklenberg, thank you for joining me today. Thank you, Kim. I'm very happy to be here. Well, let's start by talking about how common is left-handedness. What percentage of the population is left-handed?

So the largest meter analyzes worldwide have showed that the best estimate is 10.6% of left-handed people that varies a little bit, depending on location. So there are some countries where the number is a little bit lower. There are some countries where it goes a little bit higher up to like 18, 19%, but there's no country in the world where there are more than 20% left-tender. So they're always a minority. Why might some countries have higher percentages? What's in play?

I think that has something to do with mostly cultural factors. So you may remember that a couple decades ago in the U.S., there was something called handedness retraining. So in schools, left-handed kids were encouraged to use the right hand for writing. We don't do this anymore in most countries, but there's some countries where there are, sort of negative stereotypes against left-handed people. And those countries typically have somewhat lower numbers of left-handers because, for example, teacher encouraged left-handed kids to ride with the right-hand.

While countries that never had this relearning, for example, some Scandinavian countries, they tend to have like a little bit higher rates because in those countries, also in the older generation, there are more people that are classified as left-handed in objective tests. And that raises the question of what determines handedness. Is it genetic? Is it related to prenatal or early life experiences or is it something else? We know today handiness is a quite complicated thing. So back in the day people thought there is a handedness gene and that determines handiness.

And we know genetic factors for certain play a role. So, for example, if you look at parents and their handiveness, it predicts the handiveness of children. So two left-handed parents have a higher chance of having a left-handed child compared to two right-handed parents. But like molecular genetic techniques showed that the actual amount of hand-diveness that can be explained by these genetic factors is around a quarter. So 25% of hand at this basically is genetic. And we know it is not one gene. It's about 48 different genes that play a role according to large peer-reviewed studies. So it's a very multifaceted phenotype.

And those genes have, interestingly enough, nothing to do with the hands. They're mostly relevant for brain development. And that's also something people notice, right? So if you just look at the hands of a left-hander or right-hander, you cannot tell whether the person is left or right-handed. That is because handedness is not caused by the hands. It's caused by the brain. It's specifically caused by the motor cortex of the brain,

so the area that controls the movements. And we know that the right motor cortex is controlling the left half of the body and vice versa. and handiness in principle represents something what we would call a hemispheric dominance. It's a hemispheric dominance for fine motor behavior, things like writing, things like wood carving, drawing. So everything that's like complicated to do with their hands, it's more complicated than like throwing a baseball or things like that. And people have a dominance for that. So left handers, for these people, the right motor cortex is dominant from my first. fine motor behavior and for right-handlers the left.

So this is the one part. So genes relevant for brain development are relevant for handliness. In addition to that, there are also other factors, like environmental factors that seem to play a role for handiveness. One thing that's quite interesting is that large-scale studies could show that men are more often left-handed than women, which suggests there is a role of sex hormones like testosterone. And there are also a couple of prenatal factors that have been associated with left-hand and, for example, being breastfeed, plays a role, being in the twin birth, plays a role,

and several other factors. And also here, hormonal effects are discussed. How early in life does handiness emerge? I mean, can you? you tell whether a little baby is going to be right-handed or left-handed? I can with a pretty high chance. So it does not emerge at exactly the same time point for everyone, but it's quite fascinating research on their question. So people used very high-tech ultrasound recordings to look at unborn babies in a mother's room and look at which thumb they like to suck or which arm they like to interact with. And now you can see, even because Before birth, humans have a strong preference for one side.

And if you then just wait a couple of years and then test these babies after they're born and after they went to primary school, and you test them at like around age six or so when you start to learn riding, you can predict the preferred hand for riding with a more than 96% chance from the hand that is preferred in the ultrasound for like. like thumb-thucking or movement. So we can tell from that research that for a large amount of people, hand-edness is already pretty much determined at birth or even before. Why is it more common for people to be right-handed than left-handed?

Do we even know? That is a good question and we don't know. There is no published psychological research that ever answered this question. why it is a specific to right side that's more common than the left in humans conclusively. There is a couple of ideas and what's quite interesting is when we, for example, look at animals, we did large meter studies on partners and cats and dogs. You typically find that left-handedness or left partners in that case, it's much more common than in humans. So while animals also show handiveness, this very screwed distribution,

with the 10.6 left-handers and 89.4% right-handers and humans seems to be pretty unique. And there have been several psychological theories why it could be the case that we have this very high preponderance of right-handlers. And one idea why that is is that obviously human beings learn very complicated motor behaviors from other people, right? So, example, a dad might explain his kid how to tie their shoes. That's, of course, easier if the dad uses the same hand for explaining than the kid uses for tying the shoe themselves. The same goes for any sort of work-related thing, any sort of like production with manual activities. So there seems to be a benefit from having the same handedness as other people because it makes this motor learning easier, which is one of the ideas why there are so many right-handers in use.

I'm just wondering, though, it would seem to me that ambidexterity would be something that evolution would have favored, and yet it doesn't seem to be common. Why aren't more people ambidextrous? So I think in general, in order to answer that question, we have to think a little bit about why do humans have these brain asymmetries in the first place. So we know there are a lot of cognition, a lot of emotion, a lot of other things that makes us human, are asymmetrically represented in the brain. Most common example would of course be language. So the language networks are in the vast majority of people in the left hemisphere of the brain. And most of the time we will not notice this, but we notice it very much if we, for example, have a family member

that suffers a stroke, right? Somebody had a stroke in the left hemisphere. they often have severe language issues and then can't talk anymore. And it doesn't happen after a stroke in the right hemisphere. Now, you may argue, wouldn't it be better if both hemispheres would be able to control language? The problem with that is a lot of the things we're doing on everyday basis as humans are quite complicated and they lead a lot of neural tissue to be performed. So if we had the language represented equally in both sides of the brain, we would likely need larger brains. And that, of course, would have a lot of disadvantages.

For example, when it comes to birth, so you would be needed to be born even earlier as they are now and so on and so on. So there seems to be an evolutionary force towards making brains efficient. So it's a division of labor that makes optimal use of the neurons we have in our head. I think this is the main reason why so many things are asymmetrically organized and why it's not generally common that people have language on two sides or equally dexterous with both hands because the ambidexterity, like true ambidexterity, measured with scientifically accurate methods,

is something that is extremely rare. So in Heddonus research, we do distinguish a little bit between what we call mixed-handed and what we call ambidexter. So a lot of people would use the right hand for pretty much everything they do. And some people use the left hand for pretty much everything they do. And there's some that are in between, right? So you may know some people that, for example, right with the left, and they draw with the left, but then they throw baseball with the right and use a broom with the right and so.

These people, however, they're not ambidextrous. They're mixed-handed, right? They can do certain activities better with the left hand and certain activities better with the right hand. For someone to qualify as truly ambidextrous, their person would be equally as accurate and equally as fast with both the left hand and the right hand. And this is, for example, for writing, extremely rare. There's not a lot of really big studies on ambidexterity, but we would estimate it's about 0.1% of the population. So roughly 1,000 people. And it often has a lot to do with people training this actively, right? So there are some people who want to be ambidextures.

For example, it's something like tennis players sometimes do. they actively train the non-dominant hand to be like more surprising in a game and so on and so on. But what we often find is that a lot of people who think there are ambidextures, if you actually test them and you do like a couple of standard psychological hand skill and hand preference tasks, they do not turn out to be ambidextrous. They turn out to be mixed standard. We're going to take a short break now. When we return, I'll talk with Dr.

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And what does asymmetry look like across the animal kingdom? Yes, so people thought back in the day that handedness something that only be shown by humans. they could not have been more wrong. So in general, we could show in a large-scale study where we compared findings from studies looking at partners and other forms of limp preferences and animals, that it is the rule, not the exception, that animals show some sort of preference.

Whether that be pardness and cats or dogs or flipperedness and turtles, there is a wide variety of these asymmetries. And it can show, obviously, on a behavioral level, but it can also show another thing. For example, the most primitive organism has been shown to show symmetries is actually warm that doesn't even have limbs, say elegance. And here it could be shown that neurons

that were relevant for olfactions, so the worm smelling different chemicals, substances were asymmetrically organized. So we know in general that this seems to be a really core evolutionary principle of brain or nervous system organization. What's a little bit interesting in regard to humans is what I already said, that humans seem to be a little bit unique in the way that our symmetries are very strong, that we have a very strong handiveness, very strong preference for the right hand,

but also that we have a very pronounced language asymmetry and we have many other forms of the symmetries. That may be the case because we're doing quite complicated things with language and water behavior that other animals do not do in that sense. So it's a lot of neural tissue we need for that. So there's a strong pressure to make this as efficient as possible. So we are strong asymmetries. But in general, you will.

find symmetries in pretty much all animals people have looked at. And it's quite interesting. You can also, if you're interested as the listener of that podcast, if you want to look into that in your pet, you could, for example, do something like a food reaching task. So if you have some sort of like paper roll and you put a little bit of cat food or duck food into that, and then you can look which paw the animal would use to get the food out, food out. And then you can do that maybe like five or six times. And if you notice your animal is always using the same paw, you can be pretty sure your cat is left paw or right paw.

And obviously, animals also have a lot of other preferences. So for example, you can look at things like their sleeping side. So if the cat curls up to the left or curled up to the right side, if you put them into their little basket and you're sleeping, and many other things. So it is something that's not only affecting our lives with humans a lot, but also those of our pets and many other animals. Now, do the brains of people who are left-handed and right-handed look different?

Are they different structurally? Yeah, they're different structurally in terms of the brain areas that are directly relevant to handiveness. So they are their motocort, he says. they're not differently structured in a lot of other different brain areas. So these ideas that maybe left-handers have, I don't know, higher or lower intelligence, that's not the case. So it is something that affects the motor system.

It's something that is reflected in the structure of the motor system. But it doesn't go beyond that. And I think that the brain areas that are directly relevant to a function, like the motor cortex, that is allowing me to write, they show changes. Many other brand areas typically don't. Now, a lot of people might not be aware, but we also favor other body parts. So we're not just left-handed or right-handed,

but we might be left-footed or right-footed. But if you're left-handed, are you more likely to be left-footed or left-eyed? Yes, so there is a fascinating number of these behavioral, asymmetries and there's always like something people haven't thought up and then there's a new research paper and I'm like how that's that's really cool so yes people have a favorite foot and yes headness and footedness are related they're not 100% related so we had a large study on that showing that around 65% of right handers are also right footed they're also these sensory

biases so people have a favorite ear to listen to a door or like a favorite eye to look to a microscope and what I find very fascinating is also that people have sight asymmetries in different forms of social touch right so for example if you give somebody a high-five you have a preferred hand to give a high-five people kiss another person they have a strong preference to turn the head to one side. It's quite interesting. If you kiss like people to the left or the right side, for all your life to try the other one. Most participants say that's an extremely weird feeling. People have a favorite side to hug other people on. People have a favorite side to taste food on, so their favorite food taste maybe a little bit different if you chew it on the left, on the right side.

And so on and so on. So these asymmetries in behavior that, of course, reflect the similarities in the brain to some extent. They're very ancient. They're like popping up left and right in everyday life even today. And most people don't really notice it because they don't like even actively think about it. But if you really like, you know, monitor yourself a little bit when you're going through your day and you just actively pay attention to behaviors where you can choose either

the left or the right side, you will notice that. you have a lot of these, like, preferences that really barely change, right? You don't start, like, suddenly hugging somebody on the left side if you're always to the right side. So these are very deeply ingrained behavioral preferences. Now, there's a stereotype that creative, artistic people are more likely to be left-handed. Is that borne out by the research? No, it is not. So if you look at like studies, large scale, highly controlled psychological studies from the last couple of years,

it is pretty clear that if objective tests of creativity are used, there are no significant differences between left and right handers. So there's very creative left handers, but there's also extremely uncreative left handers. And there's very uncreative right handers, but there's also very creative right handers. where there is a large and significant effect is in people's attitudes and beliefs towards the creativity of left-handers. So both left-handed people and right-handed people strongly believe that left-handed people are more creative. And this also affects job choices.

So for example, parents tell their kids more often to maybe go into a creative profession when they're left-hander. So there seems to be some sort of like stereotype effect here based on what, for example, has been communicated in the media about left-handedness in the past decade. That is, however, not backed up by modern, highly controlled psychological science. Now, you've written about how left-handed athletes tend to be more successful at their sports than right-handedness. What's behind that? that's also quite interesting because it seems to be an effect of their rarity.

So it's not like that left-handed athletes per se have some higher ability in sports. For certain types of sports, such as record sports like tennis and combat sports, like for example, boxing, left-tenders have an advantage on that advantage of surprise. So most people, if they train, they train with right-handed people just because they're much more right-handed people. So this is what people are prepared if they go into like a boxing fight or a tennis match. And then when somebody comes up when they're left-handed, they might have played against the left-hand or fought against the left-ender before. But in general, like the probability is much lower. So it's unlikely they have as much training experience as when fighting or playing a right-handed opponent.

And it shows. So there are data that show that left-handed people in these sports are more successful. And that leads to like a certain over-representation. So there are, for example, more left-handers and professional boxers than in the general population. But of course, the advantage would disappear at some point if there are too many left-handers because then they're as common as right-handers. But it doesn't mean they're like generally better at sports. It's just the system basically gives them a development.

advantage due to their rarity. Let's talk for a minute about neurodivergent people. Are they more likely to be left-handed than right-handed? And if so, why? That's a quite interesting area of research because the answer is yes. So there have been large-scale meteor analyses, for example, on people in autism spectrum disorders. And people on the spectrum typically have a high-scale. a higher chance of being left-handed, being mixed-handed than the general population.

And why does this more common in neurodiverse individuals? It's not well understood. So we know there's certain genetic pathways that are overlapping between being on the spectrum and this, but this is just a really, really tiny amount of variants. But the general finding is quite strong. So the effects are quite large. And we're trying to understand why in research. What got you interested in this line of research? And are you a lefty or a righty? I'm right-handed. I'm not left-handed at all. I'm really like 100% right-hander. So I'm generally really interested in how the brain generates behavior. And I find it extremely fascinating that these evolutionary, very old, structural organization principles, like all

brains have two halves, right? If you look at the human brain, it's the most striking visual thing about it, right? This has two halves. Why does it have two halves? Right. So, and understanding this and understanding how this affects psychology and how this affects our everyday lives or everyday behavior is something I always felt incredibly fascinating because these are like very outstructural things but they're still affecting us to the very day. So for example, if you look at people doing selfies for social media, there's a very strong effect that people would put their left cheek first and research has shown that people also find portraits of people that show their left cheek.

cheek in front more emotionally warm or emotionally positive. So people like naturally, without this cognitively reflecting on the psychological science behind this, go into an asymmetric posture to have other people like their pictures more. And that's not something where they're like, oh, I do this now. This is something that's very deeply ingrained and it's based on very old neuroscientific processes. I find that very fascinating. Now, I've read that left-handed people are often underrepresented in psychological and brain research and that they're often screened out of research studies.

Why does that happen? And is that a problem? That happens and it is a problem. So the reason why that happens is that researchers in psychology for a long time thought that left-handers may introduce noise into the data. They make it more complicated to analyze the data. Let me explain why. So, for example, if you do a study on the psychology of language,

and you use an MRI scanner to look at the brain activation of people during a language task. So if you look only at right-handed people, you would see a left-hemispheric activation during language task in about 95 to 100%. of people. So almost everyone. So if you now look at left-handed people, you will find this left-sided activation of the brain and only 75% of left-tenders. Twenty-five of them would show an unusual right-hambesheric activation. This is just normal diversity. This is how human brains are organized. There's nothing like bad or wrong with that. But if you know, would do a psychological experiment,

you would just do a population sampling where 10% of people are left-handed and 90% are right-handed, you would have a few people that would show a reverse pattern of results and their brain activation, the left-handers that have the right-hemispheric activation, than the others.

And if you now do a statistical analysis of data that's very much focused on the average across all participants. What you will find is then the overall effect is a little bit like less clear, is a little bit more noisy because you have some people that are having a reverse effect. But as psychologists, of course, we should embrace the diversity. We should make sure in order to do good science that we include all aspects of the human brain in our study. because what happens if you like systematically exclude a group of people,

what happens maybe if, for example, their like clinical intervention planned on things like that, they might not be optimal for left-handed people, right? They might not be great for left-handed people. There might even be detrimental for left-tinent people. That's the big issue, like brain stimulation techniques, right? So there's psychologists who use stimulation of certain brain areas, as therapeutical tools.

For example, the idea in depression that it could be helpful. And what happens there is that the stimulation in those studies, those clinical studies, is often on one side. But what our research would suggest is that may not be optimal because if people have a switched lateralization of their emotion, that's also more common left-handers, they may not benefit from a treatment that is good for the majority of people. So what needs to be done is in both basic psychological research and clinical psychological research,

especially if it comes to brain stimulation of one half of the brain, is that left handers should be included. And we just need a different way of analyzing the data. We shouldn't just like really mush all people together. but we should more look at like the individual variation and also reflect that in our statistics, right? So you can say like there are some people like 75% show it on the left and the other on the right and be like sure these groups differentially or something like that. But we definitely should not continue to exclude left-handed people in psychology. And just to wrap up, what are you working on now?

what are the big questions you're still trying to answer? What we're working on now is we're trying to understand what happens when people are training their non-dominant hand. So we have studies where people like volunteer to like train their non-domin hand a little bit over time and then we repeatedly look at their brains and try to find out what can be learned, what cannot be learned, and what is the neuroscience behind that. And I think other research areas that are quite interesting are topics like epigenetics, like what are the molecular processes over which these environmental factors are affecting handiveness.

There's a lot of neuroimaging actually going on. So what neurotransmitters and so on are mapping on brain scans and things like that. And I think in general people try to understand much more what this actually happened. on a biological level. Well, Dr. Acklenburg, I want to thank you for joining me. It's been a very interesting conversation. Thanks. Thank you, Kim.

I was happy to be here. You can find previous episodes of Speaking of Psychology on our website at speakingof psychology.org or on Apple, Spotify, YouTube, or wherever you get your podcasts. And if you like what you've heard, please subscribe and leave us a review. If you have comments or ideas for future podcasts, you can email us at speaking of psychology at APA.org. Speaking of psychology is produced by Lee Winerman.

Thank you for listening for the American Psychological Association. I'm Kim Mills.