Are left-handed people smarter?

Left-handed people comprise only around 10% of the global population, but a quick glance reveals that many key movers and shakers are lefties. 

For instance, three out of the last six American presidents were lefties: George H.W. Bush, Bill Clinton and Barack Obama. Plus, an eclectic slew of outliers who've rocked the world in one way or another had dominant left hands: Michelangelo, Leonardo da Vinci, Paul McCartney, David Bowie, Jimi Hendrix, James Baldwin, Nikola Tesla, Oprah Winfrey, Bill Gates and Steve Jobs, according to a 2019 report and The New York Times. 

It's an impressive roster, but what does the data say? Are left-handed people smarter than righties? 

Related: Why are people left- (or right-) handed?

To investigate this question, researchers looked at the differences in mathematical achievement between more than 2,300 right- and left-handed students between the ages of 6 and 17 in Italy. While there was no difference when looking at the easier math problems, left-handed students had a significant edge on the more difficult problems, such as associating a mathematical function to a set of data, according to the 2017 study in the journal Frontiers, led by Giovanni Sala, an assistant professor at the Institute for Comprehensive Medical Science at Fujita Health University in Japan.

But why would a person's dominant hand have anything to do with mathematical ability? Left-handedness is associated with some surprising differences in the architecture of the brain. A 1995 meta-analysis of 43 studies in the journal Psychobiology determined that left-handers possess a significantly larger corpus callosum — the bundle of nerve fibers connecting the two hemispheres of the brain — than right-handers do.

"A possibility is that the stronger connection between the two hemispheres allows the [left-handed] subject to have stronger spatial abilities, and we know that spatial abilities are connected to mathematics because mathematics is often conceptualized throughout space," said Sala, who conducted his research while at the University of Liverpool in the U.K.

In some cases, it might depend on how a person becomes left-handed. "Handedness is a very complex trait, and specifically left-handedness may be an advantage or a disadvantage depending on what the cause is," Sala told Live Science. Sometimes, "left-handedness can be caused by some kind of brain damage, when the right hemisphere has to take over because there is some kind of damage in the left hemisphere." 

This type of damage could be caused by a hemispheric lesion that occurs prenatally, according to a 1985 study in the journal Brain and Cognition. If the lesions occur in the left hemisphere of the brain, then this could lead the individual to predominantly use the right half of their brain. Since the hemispheres of the brain are cross-indexed (meaning the left hemisphere controls the right side of the body, and vice versa), a dominant right hemisphere can lead to left-handedness. The study refers to this as "pathological left-handedness," and noted that it can lead to learning difficulties. In other words, sometimes being lefty is associated with learning problems.

It's complex, but Sala's study paints a picture of left-handers being over-represented at both the bottom and top of the cognitive spectrum. "Once you see that the subject is not intellectually challenged ... then left handedness seems to be a predictor of intellectual ability," specifically mathematical ability, according to his study, Sala said. However, he cautions that his results are not the final word, and that further studies must be done. 

What's more, other data shows that righties have a slight intellectual edge over lefties. A 2017 study in the journal Neuroscience and Behavioral Reviews reviewed 18 other studies which included data from over 20,400 people and found that right-handers had negligibly higher IQs than left-handers do, on average. "The jury is still out when it comes to the questions of whether it is degree of hand preference that is associated with intelligence and whether there is a relationship between relative handskill and intelligence," the authors wrote in the study. 

Lefty history

Left-handedness has not always been seen in such a positive light. In the 1936 pamphlet "The Prevention and Correction of Left-Handedness in Children," by J.W. Conway, left-handedness was described as a "disease" as serious and problematic as rickets and pneumonia. 

Prejudice against left-handers has deep roots and is built into our very language. To be someone's "right-hand man" is good, while having "two left feet" or receiving a "left-handed compliment" is bad. The word itself comes from the old English "lyft" meaning weak or broken, while the word "right" is given the additional honors of meaning factually correct, morally justified, or a "moral or legal entitlement." 

These slights are not unique to English, either. The French word for left is "gauche," which in both English and French also means "awkward or tactless," while ,"droite," the French word for right, translates to "adroit." Going back further still, the Roman word for left was "sinister," while their word for right, or "dexter," gives us the English word dexterity.

Even modern research acknowledges some downsides to being of the lefty persuasion. It's been statistically associated with schizophrenia, dyslexia, and breast cancer, to name a few. (However, an association does not prove that one causes the other.)

But these prejudices just make those numerous left-handers who have become historical and contemporary icons all the more impressive. Not bad for a group of supposed gauche and sinister abnormals, right?

Originally published on Live Science.