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

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

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:01Speaker 00

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• 0:29Speaker 01

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• 0:57Kim 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?

• 1:42Kim Mills?

  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? 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.

• 2:24Kim Mills?

  My guest today is Dr. Sebastian Auklandberg, 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 handedness. 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. Auklandberg is the author of many peer-reviewed journal articles, as well as the book, "The Lateralized Brain: The Neuroscience and Evolution of Hemispheric Asymmetries."

• 3:06Kim Mills?

  He also writes a blog for Psychology Today called "The Asymmetric Brain: The Psychology and Neuroscience of Left and Right." Dr. Auklandberg, thank you for joining me today.

• 3:16Speaker 03

  Thank you, Kim. I'm very happy to be here.

• 3:20Kim Mills?

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

• 3:28Speaker 03

  So, the largest meta-analyses 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 is no country in the world where there are more than 20% left-handers. So, they're always a minority.

• 3:58Kim Mills?

  Why might some countries have higher percentages? What's in play?

• 4:03Speaker 03

  I think this has something to do with mostly cultural factors. So, you may remember that a couple decades ago in the US, 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 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, teacher encourage left-handed kids to write 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

• 4:47Speaker 03

  rates because in those countries also in the older generation, there are more people that are classified as left-handed in objective tests.

• 4:56Kim 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?

• 5:07Speaker 03

  We know today 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. So 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 molecular genetic techniques showed that the actual amount of handedness that

• 5:46Speaker 03

  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

• 6:30Speaker 03

  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 This controlling the left half of the body and vice versa. And handedness 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 the hands. It's more complicated than like drawing a baseball or things like that. And people have a dominance for that.

• 7:10Speaker 03

  So left-handers, for these people, the right motor cortex is dominant for my fine motor behavior. And for right-handers, the left. So um, this is the 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 could show that men are more often left-handed than women, which suggests there is a role of sex hormones like testosterone.

• 7:50Speaker 03

  And there are also a couple of prenatal factors that have been associated with left-handedness. For example, being breastfed plays a role, being in the twin birth plays a role and several other factors. And also here hormonal effects are discussed.

• 8:09Kim 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?

• 8:17Speaker 03

  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 that question. So people used very high-tech ultrasound recordings to look at unborn babies in the mother's womb and looked at which thumb they like to suck or which arm they like to interact with. And there you can see, even 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 reborn and after they went to primary school. And you test them at like around age six or so, when you start to learn writing.

• 9:02Speaker 03

  You can predict the preferred hand for writing with a more than ninety six percent chance from the hand that is preferred in the ultrasound for like thumb sucking or movement. So we can tell from that research that for a large, um, large amount of people, handedness is already pretty much determined at birth or even before.

• 9:29Kim Mills?

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

• 9:35Speaker 03

  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 is much more common than in humans. So while animals also show handedness, this very skewed distribution with the ten point six left-handers

• 10:17Speaker 03

  and eighty nine point four percent right-handers in humans seems to be pretty unique. And there have been several psychological theories why it could be the case that we have this, yeah, 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, right? So for 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.

• 10:56Speaker 03

  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 humans.

• 11:19Kim Mills?

  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?

• 11:31Speaker 03

  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 has a stroke in the left hemisphere, they often have severe language issues and

• 12:16Speaker 03

  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'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, the human would be needed to be born even earlier as they are now and so on and so on.

• 13:01Speaker 03

  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.

• 13:14Speaker 03

  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 head in this 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.

• 13:59Speaker 03

  And there's some that are in between, right? So you may know some people that, for example, write with the left and they draw with the left, but then they throw a baseball with the right and use a broom with the right and so on. 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 who qualifies 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

• 14:44Speaker 03

  about 0.1% of the population, so roughly 1 in 1,000 people. And it often has a lot to do with people training this actively, right? So there are some people who wanna be ambidextrous. For example, it's something like tennis players sometimes do, they actively train the nondominant 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 they're ambidextrous, 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-handed.

• 15:28Kim Mills?

  We're going to take a short break now. When we return, I'll talk with Dr. Ocklenberg about whether lateralization is also common in animals and how to tell if your pet is left or right pawed.

• 15:42Speaker 04

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• 16:10Speaker 01

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• 16:38Kim Mills?

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

• 16:53Speaker 03

  Yes, so people thought back in the day that tannedness is 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 partners 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 partners in cats or dogs, or flipperedness in turtles, there is a wide variety of these asymmetries.

• 17:33Speaker 03

  And it can show, obviously, on a behavioral level, but it can also show in other things. For example, the most primitive organism has been shown to show asymmetries is actually warm, that doesn't even have limbs, say, elegance.

• 17:51Speaker 03

  And here it could be shown that neurons that were relevant for olfactions, 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.

• 18:33Speaker 03

  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 is 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. And it's quite interesting, you can also, if you're interested as a listener of the podcast, if you want to look at into that in your pet, you could, for example, do something like a food reaching task.

• 19:13Speaker 03

  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 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 size. So if the cat curls up to the left or curls up to the right side, if you put them into their like little basket and they're sleeping, and many other things.

• 19:58Speaker 03

  So that is something that's not only affecting our lives with humans a lot, but also those of our pets and many other animals.

• 20:07Kim Mills?

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

• 20:15Speaker 03

  Yeah, they're different structurally in terms of the brain areas that are directly relevant to handedness. So they are the motor cortex, he says. They are 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,

• 20:58Speaker 03

  they show changes. Many other brain areas typically don't.

• 21:04Kim 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?

• 21:20Speaker 03

  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, wow, 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. There are also these sensory biases. So people have a favorite ear to listen to a door, like a favorite

• 22:00Speaker 03

  eye to look to a microscope. What I find very fascinating is also that people have side 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 their head to one side. It's quite interesting.

• 22:25Speaker 03

  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 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 may be a little bit different if you chew it on the left or 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 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.

• 23:08Speaker 03

  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 always do the right side. So these are very deeply ingrained behavioral preferences.

• 23:37Kim Mills?

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

• 23:47Speaker 03

  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...

• 24:23Speaker 03

  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 you're left-handed. 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, yeah, modern, highly controlled psychological science.

• 25:07Kim Mills?

  Now, you've written about how left-handed athletes tend to be more successful at their sports than right-handedness. What's behind that?

• 25:17Speaker 03

  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 like higher ability in sports. For certain types of sports, such as racket sports like tennis and like combat sports, like for example boxing,

• 25:41Speaker 03

  left-handers have a advantage and that advantage is surprise. Like, 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 if somebody comes up and they're left-handed, they might have played against left-handed or fought against the left-handed 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 that 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.

• 26:25Speaker 03

  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 calm as right-handers. But it doesn't mean they're like generally better at sports. It's just the, the system basically gives them an advantage due to their rarity.

• 26:48Kim 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?

• 26:58Speaker 03

  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, 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 and this, but this is just a really, really tiny amount of variance.

• 27:41Speaker 03

  But the general finding is quite strong. So the effects are quite large and we're trying to understand why in research.

• 27:51Kim Mills?

  What got you interested in this line of research? And are you a lefty or a righty?

• 27:56Speaker 03

  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? It has two halves. Why does it have two halves, right? So in understanding this and understanding how this affects psychology and how this affects

• 28:37Speaker 03

  our everyday lives, our everyday behavior is something I always felt incredibly fascinating because these are like 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, more emotionally positive. So people like naturally, without this cognitively reflecting on the psychological signs behind this,

• 29:20Speaker 03

  go into 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.

• 29:42Kim Mills?

  Now, I've heard 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?

• 29:55Speaker 03

  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 ninety-five to a hundred percent of people.

• 30:39Speaker 03

  So almost everyone. So if you now look at left-handed people, you will find this left-sided activation of the brain in only seventy-five percent of left-handers. Twenty-five of them would show a unusual right hemispheric activation.

• 30:59Speaker 03

  This is just normal diversity. This is how how human brains are organized. There's nothing like bad or wrong with that. But if you now would do a psychological experiment, if you 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

• 31:44Speaker 03

  the 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 systematically exclude a group of people? What happens maybe if, for example, there are like clinical intervention planned on things like that? They might not be optimal for for left-handed people, right? They might not be great for left-handed people. They might even be detrimental for left-handed people.

• 32:27Speaker 03

  That's the big issue at like brain stimulation techniques, right? So there's psychologists who use stimulation of certain brain areas as therapeutic tools. For example, the idea in depression that that 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, and that's also more common with left-handers, they might not benefit from a treatment that is good for the majority of people.

• 33:12Speaker 03

  So what needs to be done is both basic psychological research and clinical psychological research, especially if it comes to brain stimulation of one half of the brain. Is that left tenders 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. You can say like, there's some people like seventy five percent sure on the left and the other on the right. And be like, sure, these groups differentially or something like that.

• 33:52Speaker 03

  But we definitely should not continue to exclude left tended people in psychology.

• 34:00Kim Mills?

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

• 34:06Speaker 03

  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 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. Like what are the molecular processes over which these environmental factors are affecting handedness? There's a lot of neuroimaging actually going on.

• 34:51Speaker 03

  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 happen on a biological level.

• 35:07Kim Mills?

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

• 35:13Speaker 03

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

• 35:16Kim 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 subscribe and leave us a review. If you have comments or ideas for future podcasts, you can email us at [email protected]. Speaking of Psychology is produced by Lee Weinerman. Thank you for listening. For the American Psychological Association, I'm Kim Mills.