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Southpaw Minority

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Why are left-handed people in the minority?


Transcript

Why are lefties in the minority? I'm Bob Hirshon and this is Science Update.

Listener Jessica from Washington, D.C., wrote to ask why only about ten percent of people are left-handed. We asked handedness expert Chris McManus of University College London. He says no one knows for sure, but the stability of that percentage suggests that a gene for left-handedness may be most beneficial in small doses. He compares it to the sickle-cell gene: having one copy protects you from malaria, but having two copies, which is much less common, causes sickle-cell anemia.

McManus:

And I suspect the same thing is happening with left-handedness. We're pretty sure there are genes underlying it, and it's not having two genes for left-handedness that's necessarily a good thing, it's probably having one.

But we can't test that hypothesis until we confirm that there is such a gene, and then figure out what else it controls besides handedness. I'm Bob Hirshon for AAAS, the science society.


Making Sense of the Research

In the United States and Western Europe, roughly 10 percent of people are left-handed, though that number varies considerably within smaller populations. It runs in families and tends to emerge spontaneously in early life, both of which suggest that the causes are biological. Recently, scientists identified the first gene known to increase the odds of being left-handed, called LRRTM1; however, this gene alone can't account for all cases of left-handedness. It's also possible that LRRTM1 doesn't actually cause left-handedness, but is simply linked to it. Some scientists believe that other factors, like exposure to high levels of the hormone testosterone in the womb, can also increase the odds of being left-handed.

McManus' answer doesn't relate specifically to this gene, but rather, it's a broader genetic explanation for why lefties remain a substantial but small minority. If left-handedness posed a severe evolutionary disadvantage, you would expect that over time, the trait would shrink to tiny levels or disappear entirely. Yet evidence of left-handedness goes all the way back to prehistoric times. On the other hand (sorry!), if left-handedness were beneficial, you would expect that it would become more common over time, until it was at least equal in prevalence to right-handedness. But that hasn't happened either.

That's why McManus thinks the genes that control left-handedness may be most advantageous in limited amounts. He compares this to a gene that causes the disease sickle-cell anemia. In Africa, for example, about 16 percent of people carry the gene for this disease. That's a lot for a disease that can be fatal if untreated, until you consider that having one copy of the gene doesn't cause sickle-cell anemia and provides protection against malaria, another deadly disease.

People with only one copy of the sickle-cell gene (along with one copy of another, normal version of the gene) are called heterozygotes, and the higher level of fitness that they enjoy is called a heterozygote advantage. A heterozygote advantage will generally keep a gene stable in a population at low levels—so it's common enough to give a fair number of people extra protection against malaria, but rare enough that relatively few people get sickle-cell anemia. In areas without malaria, like Western Europe, the sickle-cell gene is much less common.

Going back to handedness, McManus' hypothesis implies that there's some kind of heterozygote advantage to the gene or genes that cause left-handedness. It's not clear what that advantage might be. Some scientists have speculated about advantages to left-handedness itself: for example, that left-handed people had an advantage in hand-to-hand combat, because it gave them an element of surprise against mostly right-handed opponents. (Left-handers today enjoy some advantages in sports for similar reasons.) A 2004 French study found support for the combat advantage hypothesis. What's more, the advantage would persist only if left-handedness remained relatively uncommon—another possible contributor to lefties' minority status.

Many of the disadvantages of left-handedness are social, ranging from difficulty with tools like scissors designed for right-handers to an association between the left side and evil that has created prejudice against lefties throughout history. Interestingly, McManus notes that left-handedness seems to be more common now than it was a century ago, even accounting for old social prejudices that forced southpaws to switch hands. That's because a hundred years ago, left-handers actually had fewer children, perhaps owing to greater social isolation that may have led them to marry later in life than right-handers. In fact, McManus says social prejudice may explain why left-handedness remains less common today in other societies—for example, in the Middle East and Asia—which have a stronger bias against lefties.

Now try and answer these questions:

  1. What's significant about the percentage of left-handers in society?
  2. What is a heterozygote advantage? How might it apply to left-handed people?
  3. How does the sickle-cell anemia gene exemplify a heterozygote advantage?
  4. Suppose equal numbers of people were left-handed and right-handed. How might the genetic explanation change?

You may want to check out the April 18, 2008, Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: the exclusively human genes, transitional fossils, and the history of left-handedness.


For Educators

Prof. MacManus' official website, RightHandLeftHand.com, includes questionnaires and tests related to handedness and other forms of lateralism.

Sidedness, from Neuroscience for Kids, has several experiments (appropriate for all ages) that can help students find out if people have a dominant hand, foot, eye, and ear.

The National Geographic News article Most People Kiss the Right Way reports on a study that examined which way lovers turn their heads when kissing. The findings are related to other aspects of lateralism.

The BBC News article Lefthandedness Common in Ice Age describes evidence for prehistoric left-handedness.


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