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May 8, 2020

Why are most people right-handed?

sinister, adj.: giving the impression that something harmful or evil is happening or will happen. From Latin sinister, meaning “on the left.”

For centuries, left-handedness has been associated with deviation: indeed, Spanish and French use the same word for “right” and “straight”, while “sinister” and “awkward” are synonyms for “left.” Likewise, the biblical right hand of God is for the chosen, whereas those cast aside are on his left.

A common myth about lefties (propagated in part by right-handed siblings) is that they die about ten years earlier. It originated with a 1991 survey that asked family members whether deceased relatives in California had been left or right-handers, and it found that people who died younger were more likely to be left-handed. But the study was flawed, because there were simply fewer very old people who were left-handed.

Older people were more likely to be right-handed, because in Europe and the United States, left-handedness was suppressed during the 19th and early 20th centuries, and decreased from its natural occurrence in ~10% of the population to about 3%. During this time, people were forcibly made right handed in schools where everyone learned to write with their right hand, and to accommodate factory work where machines were set up for right handers only. So, the study suggesting that lefties die early simply reflected the fact that people who’d died young were much more likely to be left handed.

While this particular theory has been debunked, the reason that handedness exists in the first place isn’t wholly understood, and the reason that about 10% of people prefer the “wrong” hand is less clear still.

Handedness is a manifestation of brain lateralization—one side of the brain or the other being responsible for a certain cognitive function. Lateralization was discovered in 1865 when French physician Pierre Paul Broca noticed that patients with lesions on the left side of their brains had language deficits. For a century, it was thought that humans were the only creatures with lateralized brains, which explained their cognitive superiority to other animals. But since the 1970s, research has shown that almost all animals have some degree of lateralization, with some animals also displaying handedness.

Neuroscientist Onur Güntürkün believes that brain lateralization conserves the brain’s energies, which are significant—the brain accounts for about 2% of body weight, but consumes 20% of our calories. Likewise, neuroscientist Giorgio Vallortigara says that the “specialization of one hemisphere for a cognitive function avoids useless duplication of cognitive functions between the two hemispheres.”

While it’s widely thought that people inherit handedness, genes don’t entirely account for this particular type of brain lateralization. Indeed, a recent study of twins showed that handedness is only about 25% heritable, with the remaining factors being environmental or unexplained. Several studies support the environmental influence hypothesis: one concludes that handedness in mammals results from social pressures, as they coordinate their behaviors for collective survival. Another notes that mammals show asymmetric tendencies even in the womb: most fetuses preferentially turn to the right, which may provide more sensory input to that side, reinforcing the preference. And yet another possible contributor to handedness is biological sex: there’s a small but significant difference in the number of female vs. male lefties—about 12% of men, but only 10% of women, are left handed. So it might be that high fetal levels of testosterone are connected to left-handedness.

While most lefties are healthy, of course, left-handedness is associated with a variety of disorders. About 32% of people with dyslexia, for instance, are left handed, suggesting a linkage between the two. And, there is also a correlation between schizophrenia and left-hand dominance. These suggest that there may be common epigenetic factors—or, environmental influences that directly impact gene expression—influencing lateralization and such diseases. Güntürkün notes that, "There are almost no disorders of the human brain that are not linked to brain asymmetries.” So, understanding how lateralization takes place in the developing brain may help us understand neurological diseases.

Despite some health implications, associations between ‘left’ and ‘evil’ are for the most part, passé. Indeed, there is some exceptionalism associated with left-handedness today, including advantage in sports, and increased creativity.

The sports advantage has a clear logic: opponents aren’t as experienced dealing with left handedness, so their reaction times increase when the anticipated right-handed play is instead a left-handed one. Accounting for the connection to creativity, according to Alan Searleman, are studies suggesting that lefties have greater fluid intelligence and larger vocabularies. Then again, there’s also evidence that strong handedness increases language processing abilities, but it doesn’t matter which hand is dominant.

A lefty myself, it’s cheering to think that left-handedness confers some effortless superiority, but the idea wilts a bit when all things are considered. Still, lefties are in good company, with 5 of the last 8 U.S. presidents being left-handed. And Oprah.

By Aimee Fountain











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