Why Are Some People Left-Handed? Scientists Identify Rare Genetic Variants That May Be Linked to the Trait

The variants are present in fewer than 1 percent of people, but they were 2.7 times more likely to appear in lefties than in righties

A child writes in a notebook using a pencil in their left hand
About 10 percent of the population is left-handed. Studies have identified a number of genes correlated with handedness that are related to microtubules, which help cells maintain their shapes. Dann Tardif via Getty Images

Scientists have identified a gene that may play a role in determining whether a person is left-handed or right-handed. In a large study of more than 350,000 people’s genes, published last week in the journal Nature Communications, researchers found rare genetic variants of a gene called TUBB4B that are more common in left-handed people.

Around 90 percent of people are right-handed, and 10 percent are left-handed. Which hand people rely on is a result of brain asymmetry—when it comes to hand control, left-handed people have dominant right sides of their brain, while righties have dominant left sides, per the study.

This asymmetry develops in the womb and manifests in different ways. “For example, most people have left-hemisphere dominance for language, and right-hemisphere dominance for tasks that require directing visual attention to a location in space,” Clyde Francks, a co-author of the new study and a geneticist at the Max Planck Institute for Psycholinguistics in the Netherlands, tells Reuters’ Will Dunham.

But the mechanism that drives the two hemispheres to develop differently isn’t yet known.

“Finding genes linked to asymmetries of the brain or behavior, like handedness, can give some clues,” Francks tells Courthouse News’ Sam Ribakoff.

Previous studies have found several genes that seem to be tied to left-handedness, including some related to proteins called tubulins that provide structure for cells. Tubulins make up tubular filaments known as microtubules, which act like cells’ skeletons.

These past studies typically looked at only common genetic variants, however. For the new research, the scientists looked at rarer variants. They also studied variants that coded for proteins, while previous research often focused on variants that weren’t involved in coding. The gene identified in the new work, TUBB4B, is also related to tubulins.

The team examined data from the UK Biobank—a massive biomedical database—analyzing genes from 38,043 lefties and 313,271 righties. The TUBB4B variants were present in fewer than 1 percent of people, but they were 2.7 times more likely to appear in lefties than in righties.

“This is an important and significant study” that supports the role of tubulins in determining the asymmetry between the brain’s hemispheres, Sebastian Ocklenburg, a neuroscientist at the Medical School Hamburg in Germany who did not contribute to the findings, tells Nature News’ Sumeet Kulkarni.

The findings provide further evidence that microtubules are connected to whether a person prefers to use their right or left hand, the study authors write. And the researchers offer a possible reason why: Microtubules support the structure and movement of cilia, hair-like organelles sticking out of cells that can direct the flow of surrounding fluid. An uneven fluid flow, they suggest in the paper, might play a role in the development of brain asymmetry.

While rare genetic variants only affect a small number of people, they “can give clues to developmental mechanisms of brain asymmetry in everyone,” Francks tells Nature News.

Still, handedness could come down to chance a lot of the time. “We think that most instances of left-handedness occur simply due to random variation during development of the embryonic brain, without specific genetic or environmental influences,” Francks says to Reuters.

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