A group of researchers may have found a new set of genes that help to explain why people are usually right handed.  Image courtesy of Wikimedia Commons.
A group of researchers may have found a new set of genes that help to explain why people are usually right-handed. Image courtesy of Wikimedia Commons.

Scientists have struggled for a long time to explain why 85-90% of people are right-handed. They’ve known genetics plays an important role in people occasionally ending up left-handed, but they also know it is not the whole story. The consensus appears to be that around 25% of handedness can be explained by genes.

Of course, there is a big difference between knowing genetics is involved and identifying the actual genes. But now it looks like scientists may finally be starting to make some progress in identifying some of these handedness genes.

In a new study looking at people with dyslexia, researchers found that genes involved in making our brains and bodies have a distinct left and right side may also be involved in handedness too. If these genes are really important for handedness, then they may also provide an explanation for why most people are right-handed—because language and handedness evolved together, hand in hand.

There were hints in earlier research that this might be the case. Speech is housed in the left part of the brain in 95% of right-handed people but only 70% of left-handed people. Clearly there is a connection but it is by no means the whole story as only 30% of lefties use the right side of their brain for language.

Right handedness might have been a useful side effect of evolution sort of like this beautiful spandrel.  Image courtesy of Wikimedia Commons.
Right-handedness might have been a useful side effect of evolution sort of like this beautiful spandrel. Image courtesy of Wikimedia Commons.

So one way to think about right-handedness is that it is simply a side effect of language development (an evolutionary spandrel). Sort of like our big brains possibly being a side effect of the smaller jaws we needed once we changed our diet. If this is the case, then there is no need to invoke some disadvantage to being a lefty. (And no, it is an urban myth that lefties die early.)

Of course the coevolution of language and handedness is just a theory at this point. There are definitely other theories that claim that being a lefty was an evolutionary disadvantage. For example, one group proposed that lefties are rare because being left-handed impedes cooperation. Since humans do best with a lot of cooperation, most of us would favor one hand over the other. In this scenario we could have ended up left or right-handed…the luck of the draw made us right-handed.

These two ideas are not easy to tease apart. For example, there is some evidence that Neanderthals favored their right hand too. But they almost certainly had language and cooperated with one another as well.

So the bottom line is that we still don’t know why we evolved to be mostly right-handed. Was it just something that came along with learning to talk or was it an active process or something we haven’t thought of yet? We just don’t know.

What this new study does provide is some genes that may be involved in determining handedness. But these genes are almost certainly not all of the possible handedness genes (click here for a bunch more). And besides, handedness isn’t even mostly genetic! We may never be able to simply look at someone’s DNA and predict whether they are a righty or a southpaw.

Even left handed parents only have left handed kids 26% of the time.  Data taken from here.
Even left-handed parents only have left-handed kids 26% of the time. Data taken from here.

Now if I had to predict, though, I’d predict right-handed every time. As you can see on the right, I’d be right most of the time even if both parents are lefties!

In the table, a right-handed parent is represented by an R and a left-handed parent with an L. As you can tell, even two left-handed parents have a right-handed child more often than not. Whatever genes are involved, they are not something Mendel could have easily figured out!

Throughout this piece, I keep mentioning the environment. The idea is that we are all born to be right-handed but something in the environment occasionally pushes someone to be left-handed. Genes are involved by making us more or less likely to respond to whatever the environmental triggers are.

Scientists do not yet have a great feel for what in the environment might turn someone into a lefty. One idea that has been around for a long time is that difficult pregnancies make being left-handed more likely. Other factors that make someone more likely to be a lefty are if he is a boy or if he or she is one of a pair of twins. There is also some evidence that being either the first born or being the fourth born or higher increases you chances of being a lefty.

While we are making progress, we still have a long way to go in figuring out why people are usually right-handed. It’ll be interesting to see if being right-handed was selected for, being left-handed selected against or if it is simply a consequence of language development.

Why Are So Many People Right-Handed? Genetic Research May Hold The Clues 23 September,2013Dr. Barry Starr

  • ilupper

    my hypothesis is that humans advanced over other hominids because of our throwing skills. So, because 90% of world population lives in the northern hemisphere, 90% of us are right-handers.


    • petergkinnon

      That one of the components of the predominance of our line was throwing skill is an interesting possibility. Although I suspect that belligerence could be a stronger candidate. But I am afraid I don’t follow your reasoning re northern hemisphere.
      Surely not coriolis effect ? :>)

      • ilupper

        Thanks Peter, it’s coriolis effect as a principle but the eastward movement of wind as a practical matter. In fact, it’s 1 of the tenets of the Google internet via balloon projects; that we can predict with good surety where winds will move.

        So, the theory is that for the time that the human thrower/hunter has to project a rock towards the sun (south in northern hem..), the slight angle gives the right-hander an advantage. That given a million or so years of evolutionary advantage, the predominance steers very slowly away from lefties.

        The geographic angle is that most civilizations emerged in the north; maybe because there’s more land on the top-side. So, greater population eventually overwhelms the lesser gene pools down south. Egypt, fertile cresent, india, china, aztec/mayan, and europe are all northern hem societies.

        Please let me know if there are any details you like me to specify. Happy Holidays!

        • petergkinnon

          Its an intriguing notion, ilupper, and top marks for original lateral thought!
          However, I would personally give the prevailing wind effect very little weight compared with competing hypotheses, particularly that of a possible association with the evolution of language.

  • Tlaxcalli

    Okay……… but why is it that I have yet to see anyone suggest that we are mostly right handed because the heart is on the left side? This seems like a better evolutionary explanation. Genetic explanation is good but it still fails to explain why most people are ultimately this way. The left side is usually a little bigger….. this seems obviously to protect the heart. My left ribs, breast, and shoulder in particular are bigger. It probably makes sense evolutionarily to have some left handers as well. First, the left handers would balance things out when you are hunting in a group. Furthermore, many sports have advantages for the left handers…. perhaps this makes them skilled at battling other humans as well, for similar reasons. Left handers are better in sports competition against right handers and equal to other left handers. The position of parts in the brain left vs right could just be a result of these evolutionary benefits, rather than the left vs right handedness merely being a result of the position of the brain parts. The brain is a versatile thing. I suspect left handers have other advantages that are less perceptible, considering they tend to be presidents much more than right handers.

    • YesLup YesMa

      Good theory, the key after figuring out the specific mechanics of how a right-hander benefits in different kinds of physical actiivity is to work out the proportions of right to left-handers. Like, why is it currently set around 90%-10% rather than 75-25%. How does our environment decide that 10% left-handers is statistically a good proportion for a left-handed advantage? Why not 25% because it would still be unique somewhat. That’s a good sticking point.

    • Nagendra

      Good theory, just wanted to raise one point left handed peple re good in one to one sports, beacuse they get right handed people 90% of teh time while doing practice whereas right handed people get 10% of left handed poeple while on practice 🙂 thats why lefty is better practiced for righty but righty has the disadvantage.

    • John Steele

      Your heart is in the center of your chest. Not on the left. The left side of the heart is closer to the chest wall than the right, so it makes it feels like it’s on the left when you place your hand on your chest.

    • Kevin

      ur irrelevant

  • Ditiro

    I cannot wait to find out the real reason why some people are left handed, my maternal side, there are ten girls and none of them are left handed including their parents and out of 25 grandchildren only one is left handed.

  • Kurt

    Catholics made people write with the right hand in schools. My school St. Leo’s in northeast Philadelphia PA. They said right is might. So if the brain is a muscle then it is by simple muscle memory. Most of us are right handed cause the church forced it on us. It is now in the DNA cause as people were educated they were religious and forced to learn that way.


Dr. Barry Starr

Dr. Barry Starr (@geneticsboy) is a Geneticist-in-Residence at The Tech Museum of Innovation in San Jose, CA and runs their Stanford at The Tech program. The program is part of an ongoing collaboration between the Stanford Department of Genetics and The Tech Museum of Innovation. Together these two partners created the Genetics: Technology with a Twist exhibition.

You can also see additional posts by Barry at KQED Science, and read his previous contributions to QUEST, a project dedicated to exploring the Science of Sustainability.

Sponsored by

Become a KQED sponsor