• Posted July 2, 2026

Why Are You Right- Or Left-Handed? Experiments Suggest Surprisingly Simple Explanation

Why is your right hand so much better at everyday tasks like writing, throwing or using tools if you’re a righty, but your left hand if you’re a lefty?

A new study argues that your dominant hand is made, not born — in other words, practice determines whether you’re left- or right-handed.

There’s no hardwired brain advantage that determines whether someone is left-handed or right-handed, researchers reported June 30 in the Proceedings of the National Academy of Sciences.

Instead, the dominant hand simply has developed more skill because it’s had more practice, researchers said.

The new findings challenge a decades-old hypothesis, which holds that one hemisphere of the brain is dominant in every person, and that makes it inherently better at controlling movement.

"The dominant arm isn't more capable because one hemisphere of the brain is simply better at controlling movement," lead researcher Dr. Ahmet Arac, an assistant professor of neurology at the David Geffen School of Medicine at University of California, Los Angeles, said in a news release. "It is because we've spent a lifetime practicing the specific, complicated movements that tools and handwriting demand.”

Previous studies have shown that a limb side preference develops in the womb and can predict which side will become dominant after birth. This led to the theory that a person’s dominant brain hemisphere has built-in specializations that make it better for skilled motor control, researchers said.

To test this hypothesis, researchers developed a series of experiments designed to test the skill gap between a person’s dominant and non-dominant arms and hands.

The team used motion-capture cameras to track the 3D arm movements of healthy adults as they reached toward three targets under different conditions — normal reaching, reaching while wearing a four-pound wrist weight and reaching while holding a lightweight stick strapped to their forearm to simulate using a tool.

During simple reaching, a person’s dominant and non-dominant arms performed about the same. Even with extra weight added, performance was similar between the two limbs.

But once the stick was added — which required more control and precision — the dominant arm clearly outperformed the other.

A second experiment asked participants to write letters and numbers with a pen using first each hand and then with a pen taped to each elbow.

When writing with their elbow, the usual dominant-side advantage disappeared entirely. Both elbows were terrible at writing, as one might expect.

“Take away that practice by switching to a body part like the elbow that's never done the task before and the advantage disappears,” Arac said.

After practice, both elbows improved by the same amount, and both got better than the non-dominant hand had ever been, researchers said.

These results could provide new insights into helping stroke survivors and people with brain injuries who must relearn motor skills, researchers said. It also might help people better understand how skilled movement is learned and processed by the brain.

More information

The American Psychological Association has more on handedness.

SOURCE: UCLA, news release, June 30, 2026