Ask Smithsonian: What Makes Us a Righty or a Lefty?

Humans are symmetrical—two eyes, two arms, two legs—but it is rare to have equal function between the parts. There is usually a dominant eye or hand or leg, which is in tune with the asymmetry of our internal organs. But why we have asymmetry, including why we tend to have a preferred hand, is still an unknown.

Almost 90 percent of us are right-handed. The rest are either left-handed or ambidextrous, and are labeled “non-right-handed.” Right-handed dominance can be traced back to our early human ancestors. Perhaps because non-righties have been such a minority, they have been persecuted or shamed over the centuries, labeled as evil or forced at an early age to give up their left-handed proclivities.

Rather than chalk it up to some sort of witchery, scientists have become increasingly interested in studying why some of us are non-right-handers. In part, because it can tell them more about how the brain develops. And it may also reveal something about how humans have evolved.

“Handedness is mentioned in the Bible, but the biological basis of how you develop preference is not known,” says Amar Klar, head of the developmental genetics section at the Center for Cancer Research at the National Cancer Institute.

The brain has two hemispheres, but they are usually not equal. Right-handed people tend to have a more highly developed left hemisphere. Left-handers have a better-developed right hemisphere, while those who are ambidextrous seem to have some crossover between hemispheres. The dominant hemisphere processes language and does some math functions, while the non-dominant is the site of spatial perception, intuition and creativity, according to Klar. 

Understanding brain lateralization—that is, how each side develops and for what reason—is key to knowing handedness, but it may also give a window into how some diseases arise. Klar has been studying handedness preference for decades, focusing in on its possible relationship to mental illness and cancer. 

There are many schools of thought on handedness, some having little to do with genetics. In the mid-1980s, Harvard neurologists Norman Geschwind and Albert Galaburda proposed the idea that a surge of testosterone during pregnancy caused the brain’s right hemisphere to grow, leading to a left-handed preference.

Others—mostly psychologists—have posited that children choose to do things with one hand or the other based on the feedback they get from their environment.

Many have rejected the idea that handedness is inherited, because it hasn’t closely followed the rules of Mendelian genetics. Two left-handed parents might not necessarily have a left-handed child, and studies of monozygotic, or identical, twins have shown that they don’t always have the same hand preference.

No specific gene for right-handedness or left-handedness has been discovered, but there have been some tantalizing clues. In 2007, Clyde Francks of Oxford University and a team of scientists from around the world said that they’d pinpointed a gene, called LRRTM1 (Leucine-rich repeat transmembrane neuronal 1), that they were pretty sure was responsible for some left-handedness—and that it also appeared to be associated with schizophrenia.  A more recent study cast doubt on that schizophrenia hypothesis.

Six years later, another Oxford-led group reported that it had discovered a link between handedness and a network of genes that was involved in establishing asymmetry in developing embryos. They thought the same mechanism might be at play in determining left and right positioning of internal organs and hand preference; but, they said, culture and the environment might still play a role in determining the dominant hand.

Klar believes that, ultimately, a single gene will be tied to handedness, although he seems to be in the minority. He bases his theory in part on his observations of the direction hair grows on people’s heads. He’s found that most right-handers have a hair whorl that is in a clockwise direction, while most lefties have a random mixture of counterclockwise and clockwise growth patterns. The hair whorl and hand dominance are so closely tied that there must be a genetic link, he says.

And, he adds, humans may have evolved to need one side in control. Dominance was necessary to allow us to walk—otherwise, we might hop, he says. And, as humans evolved, our brain became more multi-layered and sophisticated—which created a need for one side of the brain to essentially be in charge.

For most of us, the right side (left hemisphere) won out. So why are there still non-right-handers? Left-handedness and being ambidextrous might still confer some benefits, says Klar. In various studies, non-right-handers have been found to be more creative and flexible thinkers, more intelligent, and to have advantages on the playing field.

Klar, for one, says that he’d rather not play lefties when he goes out for a tennis match.

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Alicia Ault | | READ MORE

Alicia Ault is an independent journalist whose work has appeared in the New York Times, the Washington Post, Medscape Medical News, WebMD and Wired. She splits her time between St. Petersburg, Florida, Washington, D.C., and New Orleans, exploring the rich variety of food, music, art and culture on offer in those cities.