Happy Left-Handers’ Day! What Science Says About Handedness

It's a day to celebrate the 10% who always feel they're being elbowed out by the other 90%

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PhotoAlto / Michele Constantini / Getty Images

Left-handed? Today is your day — International Left-Handers’ Day, that is — a day to celebrate the 10% who always feel they’re being elbowed out by the other 90%.

While the body is remarkably symmetrical in many ways, in some key areas, it’s decidedly one-sided. About 10% of people are left-handed, a proportion that, for whatever reason, has remained relatively stable throughout human history (scientists have even determined that some cave painters etched their masterpieces with their left hands).

People who study the brain, and early development, have a number of theories about what drives handedness, but are at a loss to explain why humans are the only species with handedness. One theory holds that hand dominance is established in the womb by the hand that babies prefer hold to their mouths, while another says higher testosterone in utero can increase the chances of becoming a lefty.

But regardless of what sets the pattern, handedness can play a role in how we think, behave and interact with others. The brain, after all, is asymmetrical, with many thinking and intellectual skills centered in the left hemisphere, while emotional and mood-related functions are concentrated in the right. Some studies, for example, find that left-handed people tend to be more vulnerable to negative emotions such as depression and anger, possibly because southpaws, as they’re known, engage the right sides of their brains more actively.

(MORE: Are You a Hyperpolyglot? The Secrets of Language Superlearners)

An interesting trial published earlier this year even showed that right-handed people who clenched their right fists before memorizing a group of words, thereby activating the left sides of their brains, performed better on recall tests than right-handers who balled their left hands into a fist (in righties, the left side is home to the regions responsible for encoding information, and the right for recall).

Scientists have also found that lefties may be more prone to fear than righties, and therefore may be vulnerable to posttraumatic stress disorder.

Clearly, these associations aren’t absolute — otherwise no left-handers would ever jump out of a plane or brave New York City streets as a taxi driver. But scientists have found that left-handers have more symmetry between the right and left sides of their brains compared with right-handers. And that can have implications for everything from language to motor skills.

“The majority of people who are right-handed are left-hemisphere language dominant,” says Dr. Daniel Geschwind, a professor of neurology, psychiatry and human genetics at the University of California, Los Angeles. That means most of their language processing occurs in the left side of the brain.

“Almost 10% of [left-handers] have that flipped, and have right-hemisphere language dominance, and many have almost equal distribution of language skills in both hemispheres,” Geschwind says. “The notion is that left-handers are less constrained when it comes to brain asymmetry, so their skills are most randomized and less specified” to one side of the brain or the other.

(MORE: Brain Science: Does Being Left-Handed Make You Angry?)

That can have advantages, particularly after a stroke. Right-handed patients who have a stroke on the left side of the brain tend to recover their speaking abilities more slowly than left-handers, since they don’t have the wider distribution of language networks throughout both sides of the brain.

But there’s also a reason why left-handedness isn’t more common, even if it provides such advantages.

“Having more distributed language abilities probably makes the system more complicated, so it may increase the susceptibility to developmental [abnormalities] and neurodevelopmental disorders,” says Geschwind. That could explain some studies that correlated risk of autism, ADHD and dyslexia with being left-handed or with being ambidextrous.

So which comes first? Does being left-handed contribute to a different organization of the brain that makes it less asymmetrical, or does the brain’s layout veer someone toward left- or right-handedness?

“It’s fundamentally related, but it’s more like it happens together,” says Geschwind. “Left-handedness is a marker for how the brain is organized in a more symmetric than less symmetric way.”

Appreciating how that lack of symmetry translates into benefits — among high test scorers at Massachusetts Institute of Technology, for example, about 22% are lefties, which is double their proportion of the population — or risks is still a work in progress.

Which means that, in the meantime, lefties should celebrate their differences — or, in the case of their brains, the lack of difference between their right and left sides.