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

Left-handers make up about 10.6% 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.

Sebastian Ocklenburg, PhD, is a professor of research methods and psychology at MSH Medical School in Hamburg, Germany. His research focuses on hemispheric asymmetries in the brain's language and motor systems, including handedness. He is author of the book The Lateralized Brain: The Neuroscience and Evolution of Hemispheric Asymmetries and writes a blog for Psychology Today called The Asymmetric Brain: The Psychology and Neuroscience of Left and Right.

Kim Mills: 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 handedness 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-handed? And does handedness—or pawedness—exist in other animals? 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 Ocklenburg, a professor of research methods and psychology at MSH Medical School in Hamburg, Germany. His research focuses on hemispheric asymmetries in the brain’s language and motor systems, including handedness. He has looked at how asymmetries develop, including genetic and epigenetic factors, the neurophysiology of brain asymmetries, and the prevalence of handedness in humans and limb preferences in other animals. Dr. Ocklenburg is the author of many peer-reviewed journal articles as well as the book The Lateralized Brain: The Neuroscience 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. Ocklenburg, thank you for joining me today.

Sebastian Ocklenburg, PhD: Thank you, Kim. I’m very happy to be here.

Mills: Well, let’s start by talking about how common is left-handedness. What percentage of the population is left-handed?

Ocklenburg: So the largest meta-analyses worldwide have shown 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’s 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-handers. So they’re always a minority.

Mills: Why might some countries have higher percentages—what’s in play?

Ocklenburg: I think this 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. 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 still 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 teachers encouraged left-handed kids to right with the right hand, while countries that never had this relearning—for example some Scandinavian countries—they tend to have a little bit higher rates because in those countries also in the older generation there more people that are classified as in objective tests.

Mills: 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?

Ocklenburg: We know today that handedness is a quite complicated thing. So back in the day people thought there is a handedness gene and that determines handedness. And we know genetic factors for certain play a role. For example, if you look at parents and their handedness, it predicts the handedness 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 a molecular genetic techniques showed that the actual amount of handedness that can be explained by these genetic factors is around a quarter. So 25% of handedness 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 this 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 complicated to do with the hands, it’s more complicated than throwing a baseball, things like that, and people have a dominance for that. So left-handers, for these people the right motor cortex is dominant for motor behavior and for right-handers the left.

So this is the one part. So genes relevant for brain development are relevant for handedness. In addition to that, there are also other factors like environmental factors that seem to play a role for handedness. One thing that’s quite interesting is that large-scale studies 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 handedness, being breastfeed plays a role, being in a twin birth plays a role, and several other factors.

Mills: How early in life does handedness emerge? I mean, can you tell whether a little baby is going to be right-handed or left-handed?

Ocklenburg: I can with a pretty high chance. So it does not emerge at exactly the same time point for everyone, but there’s quite fascinating research on the question. So people use very high high-tech ultrasound recordings to look at unborn babies in a mother’s womb and looked at which thumb they like to suck or which arm they like to interact with. And now you can see even before birth, humans have a strong preference for one side. And if you didn’t just wait a couple of years and then test these babies after they’re been born and after they went to primary school and you test them at around age 6 or so, when you start to learn writing, you can predict the preferred hand for writing with a more than 96% chance from the hand that is preferred in the ultrasound for thumb sucking or movement. So we can tell from that research that for a large amount of people, handedness is already pretty much determined at birth or even before.

Mills: Why is it more common for people to be right-handed than left-handed? Do we even know?

Ocklenburg: That is a good question. And we don’t know. There is no published psychological research that ever answered this question why it is specific to right side that’s more common than the left in humans conclusively. There’s a couple of ideas and what’s quite interesting is when we for example look at animals, we did large studies on pawedness and cats and dogs, you typically find that left-handedness or left-pawedness, in that case it’s much more common than in humans. So while animals also show handedness this very skewed distribution with the 10.6% left-handers and 89.4% right-handers 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 handedness. And one idea why that is is that obviously human beings learn very complicated motor behaviors from other people. 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 the kid uses for tying the shoe themselves. The same goes for any sort of work-related thing, any sort of production with manual activity. 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.

Mills: I’m just wondering though, it would seem to me that ambidexterity would be something that evolution would’ve favored, and yet it doesn’t seem to be common. Why aren’t more people ambidextrous?

Ocklenburg: 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 that very much if you for example, have a family member that suffers a stroke, right? Somebody has a stroke in the left hemisphere, they often have severe language issues and then can’t talk anymore. And that 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 are doing on everyday basis as humans are quite complicated and they need 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 are equally dexterous with both hands—because the ambidexterity, like true ambidexterity measured with scientifically accurate methods, is something that is extremely rare.

So in handedness research we do distinguish a little bit between what we call mixed handedness and what we call ambidexterity. 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. So you may know some people that for example, write 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 are, they’re not ambidextrous, they’re mixed handed. They can do certain activities better with the left hand and certain activities better with the right hand. But someone who qualifies as truly ambidextrous that 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 one in thousand people. And it often has a lot to do with people training this actively. So there are some people who want to be ambidextrous for example, it’s something like tennis players sometimes do they actively train the non-dominant hand, to be more surprising in the game and so on and so on. But what we often find is that a lot of people who think they’re ambidextrous, if you actually test them and you do 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 handed.

Mills: And we talked a moment ago about handedness or pawedness in animals. Are there only mammals that exhibit this and what does asymmetry look like across the animal kingdom?

Ocklenburg: So people thought back in the day that handedness was something that’s only been shown by humans, and 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 pawedness and other forms of limb preferences in animals that it is the rule, not the exception that animals show some sort of preference, whether that be pawedness in cats or dogs, or flipperedness in turtles. There is a wide variety of these asymmetries and it can show obviously on the behavior level, but it can also show in other things. For example, the most primitive organism that has been shown to show asymmetries is actually a worm that doesn’t even have limbs – C. Elegans. And here it could be shown that neurons that were relevant for all factions, so the worm smelling different chemical 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 asymmetries are very strong, that we have a very strong handedness, very strong preference for the right hand, but also that we have a very pronounced language asymmetry and we have many other forms of asymmetries. That may be the case because we are doing quite complicated things with language and other 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 have strong asymmetries, but in general you will find asymmetries in pretty much all animals people have looked at.

You can also, if you’re interested as the listener of that podcast, if you want to look at into that in your pet, you could for example do something like a food reaching task. So if you have some sort of paper roll and you put a little bit of cat food or dog food into that and then you can look which the animal would use to get the food out, then you can do that maybe like five or six times. And if you notice your animal always using the same paw, you can be pretty sure your cat is left pawed or right pawed. And obviously animals also have a lot of other preferences. For example, you can look at things like their sleeping sides. 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 they’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.

Mills: Now do the brains of people who are left-handed and right-handed look different? Are they different structurally?

Ocklenburg: Yeah, they’re different structurally in terms of the brain areas that are directly relevant to handedness, so their motor core. You see they’re not differently structured in a lot of other different brain areas. So these ideas that may be 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 brain areas typically don’t.

Mills: 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?

Ocklenburg: Yes. So there is a fascinating number of these behavioral asymmetries and there’s always something people haven’t thought of. And then there’s a new research paper and I’m like, oh, that’s really cool. So yes, people have a favorite foot and yes, handedness 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 also have these sensory biases. So people have a favorite ear to listen to or a favorite eye to look to a microscope. What I find very fascinating is also that people have asymmetries in different forms of social touch. So for example, if you give somebody a high five, you have a preferred hand to give a high five. If people kiss another person, they have a strong preference to turn the head to one side.

It’s quite interesting if you kiss people to the left or the right side for all your life to try the other one. Most participants say that 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 tastes may be 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 asymmetries in the brain to some extent that are very ancient. They’re popping up left and right in everyday life even today. And most people don’t really notice it because they don’t even actively think about it. But if you really 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 preferences that really barely change. You don’t start suddenly hugging somebody on the left side if you always do the right side. So these are very deeply ingrained behavioral preferences.

Mills: Now there’s a stereotype that creative artistic people are more likely to be left-handed. Is that borne out by the research?

Ocklenburg: No, it is not. So if you look at 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. 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 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, so there seems to be some sort of stereotype effect here based on what for example has been communicated in the media about left-handedness—that is however not backed up by modern highly controlled psychological science.

Mills: You’ve written about how left-handed athletes tend to be more successful at their sports than right handedness. What’s behind that?

Ocklenburg: 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 racket sports like tennis and combat sports, for example boxing. Lefties have an advantage and that advantage is surprise. So most people, if they train, they train with right handed people just because there are much more right handed people. So this is what people are prepared if they go into a boxing fight or a tennis match. And then when somebody comes up and they might’ve played against left-hand or fought against the left-hander before, but in general 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 that shows. So there are data that show that left-handed people in these sports are more successful and that leads to a certain overrepresentation. So there are for example, more left-handers in professional boxers than in a general population. But of course the advantage would disappear at some point if there are too many left-handers. But it doesn’t mean they’re generally better at sports, it’s just the system basically gives them an advantage due to their rarity.

Mills: Let’s talk for a minute about neurodivergent people. Are they more likely to be left-handed than right-handed? And if so, why?

Ocklenburg: That’s a quite interesting area of research because the answer is yes. So there have been large-scale meta-analyses, for example, on people in autism spectrum disorders and people on the spectrum typically have a higher chance of being left-handed or being mixed handed than the general population. And why this is 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, 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 are trying to understand why in research.

Mills: What got you interested in this line of research and are you a lefty or a righty?

Ocklenburg: I’m not left-handed at all. I’m really like 100% right-hander. I’m generally really interested in how the brain generates behavior. And I find it extremely fascinating that these evolutionarily very old structural organization principles, like all brains have two halves, if you look at the human brain, it’s the most striking visual thing about it. It has two halves, why does it have two halves? 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 very old structural 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 in front more emotionally warm or emotionally positive. So people naturally without cognitively reflecting on the psychological science behind this go into a 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.

Mills: 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?

Ocklenburg: That happens and it is a problem. 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 a 100% of people. So almost everyone. So if you now look at left-handed people, you will fight this left-sided activation of the brain in only 75% of left-handers, 25 of them would show a unusual right hemispheric activation. This is just normal diversity. This is how human brains are organized. There’s nothing bad or wrong with that.

But if you now 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 activations 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 less clear, it’s 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 we systematically exclude a group of people?

What happens? Maybe if for example, their clinical intervention planned on things like that, they might not be optimal for left-hand people. They might not be great for left-handed people. There might even be detrimental for lefthand people. That’s the big issue that like brain stimulation techniques, right? So there’s psychologists who use stimulation of certain brain areas as tools. This for example, the idea in depression that it could be helpful. And what happens there is that the stimulation in those studies and 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, that’s also more common, 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 lefthanders should be included. And we just need a different way of analyzing the data. We shouldn’t just really mash all people together, but we should more look at the individual variation and also reflect that in our statistics. So you can say there’s some people like 75% show it on the left and the other on the right, and we show these groups differentially or something like that. But we definitely should not continue to exclude left-handed people in psychology.

Mills: And just to wrap up, what are you working on now? What are the big questions you’re still trying to answer?

Ocklenburg: What we’re working on now is we are trying to understand what happens when people are training their non-dominant hand. So we have studies where people volunteer to train their non-dominant 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. What are the molecular processes over which these environmental factors are affecting handedness. 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 is actually happening on a biological level.

Mills: Well, Dr. Ocklenburg, I want to thank you for joining me. It’s been a very interesting conversation. Thanks.

Ocklenburg: Thank you, Kim. I was happy to be here.

Mills: You can find previous episodes of Speaking of Psychology on our website at speakingofpsychology.org or on Apple, Spotify, YouTube, or wherever you get your podcasts. And if you like what you’ve heard, please follow us and leave a review. If you have comments or ideas for future episodes, you can email us at speakingofpsychology@apa.org. Speaking of Psychology is produced by Lea Winerman.

Thank you for listening. For the American Psychological Association, I’m Kim Mills.