Picking Sides: How Genes Help Us Decide Between Left and Right

By Sharon Moalem | April 15, 2014 8:30 am

genes of handedness

Some people call left-handers southpaws. Others call them mollydookers or corky dobbers. Scientists still often call lefties sinister, which in Latin originally just meant “left” but later came to be associated with evil.

Wondering about the medical implications of being born a corky dobber? It may surprise you that left-handed women were found to be twice or more likely to develop premenopausal breast cancer than right-handers. And a few researchers believe this effect may be linked to exposure to certain chemicals in utero, affecting your genes and then setting the stage for both left-handedness and cancer susceptibility, thus opening up another probability of nurture changing nature.

Goofy Feet

When it comes to our hands, feet, and even our eyes, most human beings are right-side dominant. Now, you might think that footedness and handedness are always aligned, but as it turns out that’s not always the case for right-handed people, and it’s even more infrequent for left-handed people. Lots of people aren’t congruent.

In board sports, being left-foot dominant is termed goofy – a goofy-footed surfer stands with her left foot on the back of board instead of her right. There are an amazing number of theories as to why some of us are goofy-footed. But the term itself is often said to have originated with an eight-minute long Walt Disney animated short, called Hawaiian Holiday, that was first released to theaters in 1937. The color cartoon stars the usual suspects: Mickey and Minnie, Pluto and Donald, and, of course, Goofy. During the gang’s vacation in Hawaii, Goofy attempts to surf, and when he finally catches a wave and heads back to shore atop its short-lived crest, he’s standing with his right foot forward and his left foot back.

If you’re wondering if you might be goofy and would like to find out before hitting the beach, then imagine yourself at the bottom of a staircase that you’re about to ascend. Which foot moves first? If you’re taking that first imaginary step with your left foot, then it’s likely that you’re a member of the goofy-footed club. And if you find out that you aren’t goofy, then you’re in the majority.

Genetics of Handedness

Why some of us are born left-handed, right-handed, or goofy-footed is thought to relate to an important and early time in the formation of our brain. One of the most popular explanations for lateralization, which is the term given to this phenomenon, is that each side of our brain has evolved for functional specialization. This division of labor allows us to perform multiple complex tasks.

Do you whistle while you’re at work? Your coworkers can thank your brain’s remarkable lateralization for that. Are you able to drive and talk on the phone at the same time? That’s lateralization, too.

So why the predominance of righties? For our species, one of the most important tasks is communication, which is generally processed on the left side of the brain. And some scientists think that’s the reason why we’re right-side dominant, because, as you’ve probably heard, the left side of the brain generally controls the muscles on the right side of the body (which is why a stroke suffered on the left side of your brain is more likely to result in impairment to the arm and leg on the right side of the body).

So why should you care if you’re goofy? Well, it’s the same question that many people have posed to Amar Klar, a senior investigator of the Gene Regulation and Chromosome Biology Laboratory at the National Cancer Institute. He has been interested in the genetics of handedness for more than a decade.

Klar is a proponent of a direct genetic cause for handedness, perhaps even a single gene—a discovery we’ve thus far managed to miss as we’ve combed the human genome. The theory, which Klar’s team has backed with a predictive model of dominant and recessive traits that would make Gregor Mendel proud, even explains the fact that monozygotic twins don’t always share the same handedness. This might seem to be an argument against genetic inheritance, but what Klar and several other respected geneticists have proposed is that this theoretical gene carries two alleles, a dominant one that orders up right-handedness and a recessive one. Someone who inherits a pair of recessive alleles has a 50-50 chance of going either way. More than a decade after he started looking for that elusive gene, Klar hasn’t found it yet, but he’s still holding out hope.

Environmental Exposure

As an alternative to an exclusively genetic cause of handedness, a different line of thinking suggests that left-handed individuals experienced some neurological insult, or damage, during development or delivery that affected the way their brains are wired. Marshaling evidence for “the insult theory,” some people have pointed to studies that found a correlation between children who are born premature and left-handedness. A Swedish meta-analysis found an almost twofold increase in left-handed children who were born premature.

Discovering more of the biology behind handedness, tracing it to genetics, exposures, or both, could give us a lot more knowledge than simply whether we should line up our kids on the left or right side of the tee-ball batter’s box. That’s because left-handedness has also been associated with higher rates of dyslexia, schizophrenia, attention deficit hyperactivity disorder, some mood disorders, and, as we’ve discussed, even cancer. Indeed, adding handedness into the mix has helped Danish researchers identify which children who had symptoms of ADHD at the age of eight (when, let’s face it, just about every kid is a little bit on the rambunctious side) would still have it at the age of 16.

Scientists still have a long way to go to understand why we prefer one hand and foot over the other—whether because of genes or because of environment or, as is often the case in biology, some combination of both. But the fascinating thing about handedness is that it opens a window onto the workings of our brains themselves. The laterality of our table manners or our tennis swings can help researchers understand how our brains are wired and what factors contribute to create each individual’s distinctive biology. Now that’s something both sides can get behind.



Excerpted from INHERITANCE by Sharon Moalem, MD, PhD. Copyright © 2014 by Sharon Moalem. Used by arrangement with Grand Central Publishing. All rights reserved.

Top image by MaryMo / Shutterstock

MORE ABOUT: genes & health
  • Nader

    So, don’t we have handedness in animals (which don’t speak)? I always thought that being righthand would cause less stress on heart!

    • yal taha

      There is lateralization in animals…. at least mammals. For example dogs tend to be right paw….somewhere i heard that right paw dogs have more comunicational habilities with their owner and are more susceptible for becoming helping dogs for blind people.

  • LaNell

    What about ambis, of which I have always been?

    • SeeToSee

      Many or most left-handers are somewhat ambidextrous, or so I’ve been told. So my guess is that being ambidextrous is a form of left-handedness. Right-handers are more connected to the left brain for speech, but left-handers use both hemispheres for at least language processing. More left-handed women have ambidextrous traits than left-handed men, which I think is part of the testosterone exposure in the womb theory. My info is dated, having been left-handed for over 50 years, so feel free to bash it.

  • http://www.mazepath.com/uncleal/qz4.htm Uncle Al

    Somebody who leads with his left presents his heart closer to his enemy. We then expect heterosexual men to be strongly right-handed and women to be biased left-handed from breast-feeding babies at their heart.

  • Alan Russell

    My parents are both left handed. So if genetics really is at play, both have two copies of the recessive allele, and when it came to making my sister and I, the only alleles available would be that for being left-handed. But her and I are both righties. Even if we’re heterozygous righties, the question remains Where did the right-handed allele come from?

    • yoursitesucks

      I know that a lot of genetics and hereditary traits skip a generation. i.e if my grandfather has a genetic disease or red hair, I am more likely to get it than my mother or father. Were your grandparents right handed?

      • Leo Adams

        what you said is included in the theory Alan had said, and it’s impossible here.

        • yoursitesucks

          clearly I am ignorant on this subject

    • squall_lionheart

      The answer to your question is actually mentioned on the article: “Someone who inherits a pair of recessive alleles has a 50-50 chance of going either way. ” (If you get a dominant, you become right-handed, but if you get both recessives, you have a 50% chance of being a leftie. But you still could be a rightie.) :)

  • Lori Lynne Van

    Then what happens to a person that left side of brain is white matter and the peron is both handed including writing with both?

  • Tony

    Interesting I am the eldest of 5 children. 3 including myself are lefties. Only mum was left handed. None of 4 children are left handed. What I did notice in teaching them to use their left to kick a soccer ball was that picked it up at different rates. PS the “sinister” bit would in part explain why the nuns didn’t like mum using her right hand at school. (She is 86 now so awhile ago)

  • Johnny

    I write & draw with my left hand however I throw & bat a ball with my my right. I kick ball with my right foot. I am a graphic artist & I also do technical work.

    • Trixie

      Is there a term for that? It is not ambitextorious

      • Johnny

        Hi Trixie, I don’t know what term or if there is a term but it is kinda weird though!

    • Le Blanc

      Yeah man, I shoot and play tennis with my left. I write with my right.

  • http://destinylegends.com/ Shaun

    A better way to find out if you are goofy footed is to run and slide across a floor in your socks. If you lead with your right foot you are goofy.

    • Leo Adams

      what do you mean “across a floor in your socks”?

      • http://destinylegends.com/ Shaun

        Put socks on. Find a nice smooth wood or tile floor. Run a few steps then slide. You will use the stance you are more comfortable with to balance yourself, goofy or regular.

    • http://blogs.discovermagazine.com Michael Cole

      Incorrect Shaun. In your sliding experiment, someone who leads with their right foot is NOT likely to be goofy-footed. He has essentially run and jumped off of his left foot. Just as any right side dominant athlete would do if she were taking a lay-up in basketball or long-jumping in T&F. The strong leg and arm are used to drive upwards (or forwards).

      • http://destinylegends.com/ Shaun

        Oh really? Because I’m goofy footed and that’s how I slide. And guess what, when I walk up a staircase I lead with my right foot which debunks the other method used above. Doing a lay up is not the same as snowboarding or surfing goofy footed. As an example in baseball I throw righty and use my right leg to drive power but I bat lefty and use my left leg to drive the power when hitting. When I go snowboarding I am goofy footed and this is a common trick to figure it out.

        Have you ever been snowboarding or surfing or done any sport that actually has goofy footed stances?! This is pretty common method to find out which one you are. Or are you speaking from your expertise in this field. Or maybe there is no expertise in this field and all of these claims are bogus.

        • Soye

          I agree Shaun and find your assessment correct. As a goofy footed lad myself but right handed, I do layups with my right leg leading and that is because I throw and feel more comfortable using my right hand. And by the way, you are correct with the ‘sock and slide’ test. I’m definitely left footed 😀

  • Jyernazrie Rulida

    I’m not goofy though. But I have one question.
    Does being left-handed affect anything how our brains function or even in our limbic system or something?

  • Arvindh Mani

    Left-handed monozygotic twin here. Lots of questions, no answers!

  • breed7

    Yeah, it’s just a lot of guessing. I’m left-handed, I wasn’t born prematurely, and I didn’t suffer an injury to the left side of my brain as a young child. My triplets — born VERY prematurely — are all right-handed.

    I subscribe to the theory that there is a gene for right-handedness. About 80% of the population has that gene, making 80% of the population right-handed by default. The remaining 20% of people have an equal chance of becoming left- or right-handed, resulting in about 10% of the population in general being left-handed.

    • http://blogs.discovermagazine.com Eric Lipps

      I’m left-handed, the son of two left-handed parents–but both of my sisters are right-handed.

  • balen

    why most time use right but tow hands at the same time?Cause one hand useing is saveing energy! But,why we have two hands other than one hand?Cause some time Right needs the help of Left.

  • imnottellingu

    I would love to figure this out. I consider myself left handed as I write left handed, throw a ball left handed, but I golf and bat right handed, am more comfortable shooting right handed. I guess I am goofy footed as I put my right foot in front when I snowboard. When I draw my left hand is better at perspective drawing (copying things, drawing specific items I am looking at), while my right seems better at creative drawing. My mother was ambidextrous and my daughter is strictly left handed. I do have to temper this with the fact I had a stroke when I was 6 months old. I had a neurologist tell me they could find out whether I was meant to be left handed or the stroke affected it by sticking electrodes in my brain while I was awake to see what happened. I declined.

  • mary Wms

    You can see if your right or left “eyed” with a piece. Start with the paper at arm’s length and draw it to your face…the eye that it’s drawn to is your dominant eye

  • http://blogs.discovermagazine.com Eric Lipps

    What about the ambidextrous?

  • Carolyn Stein

    What would make sense is if left-handedness were a recessive trait. It shows up about every other generation and even then sporadically. My grandfather was left-handed but they forced him to be right-handed (you never could read anything he wrote).
    When I started school in 1966, the sisters asked Mother if she wanted them to force the left-handedness out of me – she told them, emphatically, “NOT ON YOUR LIFE!! You are NOT doing that to my child!” Thank you MOM! But the sisters were always critical of my penmanship. :(
    None of my 5 siblings are left-handed, neither of my children are, but so many of my male students are it’s crazy! And they seem amazed when they realize that I’m a lefty!
    However, I am right-eyed. Left-footed. Learned to play baseball either handed but am MUCH better with the left. HS gym teacher tried to teach me to golf right – what a disaster! Definitely golf left. It’s safer for all involved!
    The world is definitely designed for right handers! I’ve learned to adapt but righties don’t get it!

  • Dan Jones

    Maybe it is because I am left handed, but I read a negative undertone racing through this article. Words like goofy, recessive, insult, ADHD, premature, and a range of mental disorders were named with nothing positive to say about “lefties”. I started to write a list of famous left-handers, but realized that it would be longer than the article. Just Google it and a list of great artists, musicians, athletes, scientists, and politicians will pop up.

    I did find the observation regarding breast cancer in women interesting since even though my left handed brother and I (age 74 and 65) are fine, our ambidextrous mother died as a result of breast cancer that had been diagnosed at age 59.

    One final observation, in my 40 year career in electronics I noticed an inordinately high percentage of left handed electrical engineers and programmers. At one company, over 50% of the programmers were left handed. Rather than an anecdotal observation I would like to see these scientists investigate that phenomenon.

  • Michael Mata’matic

    Im right handed but I dribble and use my left hand for other things. But I write with my right hand

  • Soye

    Another way to test if you are goofy is to strike a soccer ball with both legs. I hit balls stronger and feel more balanced / comfortable using my left leg over my right.

  • zachymo

    Im a right handed on everything I do in terms of sports including writing. But im left eye, left ear and dominant leg is also left. Ive found with data Ive collected if you are right handed and left footed you are a well balance, natural tremendous athlete and your dominant eye and ear will line up accordingly to match the balance of feet and hand all together in an athletic perspective.


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