Why Are Some People Left-Handed? | Science | Smithsonian
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Why Are Some People Left-Handed?

Being a righty or a lefty could be linked to variations in a network of genes that influence right or left asymmetries in the body and brain

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President Barack Obama is left-handed, as well as at least six former presidents. Photo via Wikimedia Commons.

For the left-handed people of the world, life isn’t easy. Throughout much of history, massive stigmas attached to left-handedness meant they were singled out as everything from unclean to witches. In Medieval times, writing with your left-hand was a surefire way to be accused of being possessed by the devil; after all, the devil himself was thought to be a lefty. The world has gotten progressively more accepting of left-handed folk, but there are still some undeniable bummers associated with a left-handed proclivity: desks and spiral notebooks pose a constant battle, scissors are all but impossible to use and–according to some studies–life-expectancy might be lower than for right-handed people.

What makes humanity’s bias against lefties all the more unfair is that left-handed people are born that way. In fact, scientists have speculated for years that a single gene could control a left-right preference in humans. Unfortunately, they just couldn’t pinpoint exactly where the gene might lie.

Now, in a paper published today in PLOS Genetics a group of researchers have identified a network of genes that relate to handedness in humans. What’s more, they’ve linked this preference to the development of asymmetry in the body and the brain.

In previous studies, the researchers observed that patients with dyslexia exhibited a correlation between the gene PCSK6 and handedness. Because every gene has two copies (known as alleles), every gene has two chances for mutation; what the researches found was that dyslexic patients with more variance in PCSK6–meaning that one or both of their PSCK6 alleles had mutated–were more likely to be right-handed.

The research team found this especially interesting, because they knew that PCSK6 was a gene directly associated with the development of left-right asymmetry in the body. They weren’t sure why this would present itself only in dyslexic patients, as dyslexia and handedness are not related. So the team expanded the study to include more than 2,600 people who don’t have dyslexia.

The study found that PCSK6 didn’t work alone in affecting handedness in the general population. Other genes, also responsible for creating left-right asymmetry in the body, were strongly associated with handedness. Like PCSK6, the effect that these genes have on handedness depends on how many mutations the alleles undergo. Each gene has the potential for mutation–the more mutations a person has in any one direction (toward right handedness or left handedness) the more likely they are to use that hand as their dominant hand, or so the researchers speculate.

The hypothesis is a logical response to a key question: If handedness is genetic and if right-handedness is such a dominant trait, why hasn’t left-handedness been forced out of the genetic pool? In reality, the research suggests that handedness could be more subtle than simple “dominant” or “recessive” traits–a whole host of genes might play significant roles.

What’s especially exciting is that these genes all relate to the development of left-right asymmetry in the body and brain, creating a strong case for correlation between the development of this symmetry and the development of handedness. Disrupting any of these genes could lead to serious physical asymmetry, like situs inversus, a condition where the body’s organs are reversed (heart on the right side of the body, for example). In mice, the disruption of PCSK6 resulted in serious abnormal positioning of organs in their bodies.

If physical asymmetry is related to handedness, then people with situs inversus should favor one hand more often than what you’d find in the general population. Studies show that this isn’t the case–individuals with this condition mirror the general population’s split in handedness–leading the researchers to postulate that while these genes certainly influence handedness, there might be other mechanisms in the body that compensate for handedness in the event of major physiological asymmetries.

Other animals, such as polar bears or chimpanzees, also have handedness–chimpanzees have been known to prefer one hand to the other when using tools or looking for food, but the split within a population hangs around 50/50. Humans are the only species that show a truly distinct bias toward one hand or the other: a 90/10 right/left split throughout the population.

One predominant hypothesis for this bias relates to another distinct human trait: language ability. Language ability is split between the different hemispheres of the brain, much like handedness, which suggests that handedness became compartmentalized along with language ability, For most, the parts of the brain that govern language are are present in the left-side of the brain–these people tend to be right-handed. The few that have language skills focused in the right side of the brain tend to be left-handed.

However, William Brandler, a PhD student at Oxford University and the paper’s lead author, isn’t convinced that this theory holds much stock, as correlations between language and handedness in research aren’t well established. Brandler is more interested in learning how the permutations and combinations of genetic mutations play into humans’ likelihood to be right-handed. “Through understanding the genetics of handedness, we might be able to understand how it evolved,” he says. “Once we have the full picture of all the genes involved, and how they interact with other genes, we might be able to understand how and why there is such a bias.”

And he’s confident that even if environmental factors (like the continued hatred of lefties by two-thirds of the world) place pressure on handedness, any baseline bias still boils down to genetics. “People think it’s just an environmental thing, but you’ve got to think, why is there that initial bias in the first place, and why do you see that bias across all societies? Why aren’t there societies where you see a bias to the left?” Brandler asks. “There is a genetic component to handedness, hundreds of different genetic variants, and each one might push you one way or the other, and it’s the type of variance, along with the environment you’re in and the pressures acting on you, which affect your handedness.”

But until a larger population can be tested–hundreds of thousands, by Brandler’s estimates–a full genetic map of what controls handedness and why our population isn’t evenly split between righties and lefties can’t be determined. “It’s going to take a bit of time before these materialize—but it will happen,” Brandler says. “There’s been a whole revolution in genetics such that, in a few years time, we’re really going to start to understand the genetic basis of complex traits.”

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