Editor’s Note: This article was originally published in an edition of NOVA’s email newsletter, NOVA Lens, and has now been repurposed for NOVA Next. Sign up for NOVA Lens here (select “NOVA Newsletters”).
It’s back-to-school season, and, for the first time, some parents will discover that their children are left-handed.
Doesn’t sound like a big deal, right? That’s probably because times have changed.
Until the second half of the 20th century, it wasn’t uncommon to hear about parents and teachers coaxing lefties into using their right hand for daily tasks including writing, eating, and teeth-brushing. In most parts of the world, that’s no longer a standard practice—but certain myths about left-handedness persist. The truth is that left-handedness is much more than a trivial paw preference: It may be a useful lens through which to examine brain development.
About One in Ten
Patricia Cowell, a professor of cognitive neuroscience at the University of Sheffield in the United Kingdom, stresses that though left-handedness is referred to as “atypical” from time to time, there’s no need for concern.
“It’s lower prevalence. It’s not an anomaly; it’s not anything negative as such. It’s just different from the majority,” Cowell said. “I think the thing that’s really important is that it is a very constant biological variation within normal development.”
Cowell studies handedness and brain lateralization: how different abilities like language or visual processing are organized within the brain. “If someone is left-handed, their brain organization is going to be different than someone [who] is right-handed,” Cowell said.
Before the era of the laptop-filled lecture hall, Cowell would sit at the front of the room and count the number of lefties in her classes while students completed five-minute thought experiments. Although the small sample size caused some variation from class to class, around 1 in 10 students were left-handed. This distribution is present in the general population as well—it is thought that around 10% of people around the world are left-handed. “It’s pretty stable,” Cowell said. “You need a fairly big group of people to get a consistent number, but I think that you can always find a few in a big group.”
And there’s some evidence that the distribution of handedness has lasted for thousands of years, with right-handers in the strong majority and left-handers comprising a mere fraction. To study examples of handedness in history, French researchers examined Western European cave artwork dating back 10,000 to 30,000 years. Ancient artists would hold their non-dominant hands against a rock wall and use their dominant hand to blow colored dust onto it using a blowing tube, creating a negative image of their hand on the rock. The distributions of handedness found in cave art is consistent with what we see today.
Other species have demonstrated handedness, as well. Roel Willems is a senior researcher at the Centre for Language Studies and the Donders Institute for Brain Cognition and Behaviour in Nijmegen in the Netherlands.
“Other animals also tend to have a handedness preference,” Willems said. “[But] there is no other species [other than humans] that seems at the population level to be so strongly biased.”
So far, research hasn’t revealed why the distribution of handedness has stood the test of time. It’s all “one big mystery,” according to Willems.
But that could change. Scientists like Willems and Cowell are working to understand lefties’ unique brain structure—and what that could potentially tell us about brain organization and our evolution as a species.
All in the Head
Handedness is sometimes obvious, and it’s closely tied to our ability to perform complex tasks every day. But to truly study handedness, researchers have to look to the brain.
For example, development across the brain’s motor cortex—the part of the brain responsible for voluntary movement—varies in relation to handedness. To study the motor cortex, researchers often look at how much oxygen is being consumed in parts of the brain during tasks via fMRI, or through a different technique that assesses connections within the nervous system called transcranial magnetic simulation (TMS). Using these techniques, researchers have identified connections between cortex development and handedness.
“The left part of the motor cortex is more developed in right-handers than the right part of their motor cortex,” Willems said. “This imbalance we also find in left-handers. So, in left-handers, the right part of the motor cortex is better developed. But, the imbalance is much less strong.”
If motor cortex development differs based on handedness, then what would happen if a person suddenly lost the ability to use his or her dominant hand? In that case, it turns out lefties might have an advantage.
“A typical phenomenon that you see with left-handers is that they are much more capable of doing stuff with their non-preferred hands,” William said. “When you break [the dominant] arm, you have to brush your teeth with your non-preferred hand. I don’t know if you’ve ever tried it, but that’s painful. Left handers have the advantage—if they break their left arm, it’s less painful if they have to [brush] with their right arm than for right-handers [brushing with their left arm].”
Beyond movement, other brain functions like the ability to recognize faces are also tied to handedness and brain organization. While it was thought that face recognition was strongly right-lateralized, Willems said, his research has shown that it’s also less strongly lateralized in lefties.
And some abilities, like language, might have lateralization that can’t always be predicted using handedness. For serious surgeries like tumor removal, “a neurosurgeon will always test the language lateralization before he starts to operate because you cannot simply assume that that lateralization is left or right,” Willems said. But in emergency situations where cortical mapping and fMRI aren’t available, “handedness might clue [surgeons] into a patient’s brain hemisphere dominance,” Abdul-Kareem Ahmed, a neurosurgery resident at the University of Maryland, said.
No Longer Left Out
Historically, some researchers avoided left-handed subjects in neuroscience and psychology studies because the lefties could complicate the results. They’d prefer to have a homogeneous set of right-handers instead. (“It’s almost a mundane reason, but that is why,” Willems said.) One study on left-handers from 1988 contributed to the myth that left-handers have a lower life expectancy than right-handers.
“That is probably not true,” Willems says. “The good news is that this has gotten a lot of attention, and in the end, it was very likely that it was based on a statistical error.”
More reputable research began in the late 20th century, and continues today. In addition to examining the connections in the brain to handedness, researchers are now turning to genetics to demystify the origins of handedness in the first place.
“Some people seem to be born left-handed, so there is a genetic component,” Willems said. However, he notes that the research on how exactly that works is in progress. Cowell describes the development of handedness as a sort of positive feedback loop: “Presumably, there is going to be some environmental influence early on. It could be prenatal, it could be early postnatal, but once the person is in a position where they’re developing along a trajectory where they have the capacity to become a left-hander, then by virtue of being left-handed, that is in itself an influence on the brain.”
Further, if someone is naturally left-handed but forced to use their right hand, then their brain might reflect experience-dependent changes. Although it differs enormously by geographic region, there have been studies in which left-handed subjects were forced to use their right hands. Their resulting brain scans showed something striking. “In terms of lateralization of the motor functions, they are sort of half in between the right and left halves,” Willems said. In other words, their motor cortex development imbalances were in between those of left and right-handers.
With the advent of rapid and affordable genome sequencing, researchers may get closer to understanding the genetics and origins of handedness and craft a better understanding of lateralization—all with the help of lefties. By including left-handers in studies with right-handers, genetic differences between the two could be more apparent. “Otherwise, you’re looking at a system which has very minute variations, and it’s hard to find genes that correlate with those variations because they are simply too small,” Willems said.
Now, lefty brains are changing the ways scientists think about brain organization. Previous understandings of brain organization based on right-handers only may have set up the illusion that evolution led to a one-size-fits approach to wire the brain.
“However, if by-and-large 10% of people do it slightly differently, that tells you that the brain is perhaps more plastic in how it can be wired,” Willems said.
This article was written and edited by lefties.