Why is a person's dominant hand so much better at writing, throwing or using tools? A new study from UCLA Health researchers suggests the answer isn't a hardwired brain advantage — the dominant hand has simply had more practice.
The study , published in the journal Proceedings of the National Academy of Sciences, challenges a decades-old hypothesis in neuroscience that the dominant hemisphere of the brain is inherently better at controlling movement.
Previous studies have shown a limb side preference develops in the womb and can predict which side will become dominant after birth. Why the preferred side is more skilled has been debated since the early 2000s, with one influential view holding that the dominant brain hemisphere has built-in specializations that make it better for skilled motor control.
But UCLA research led by neurologist Dr. Ahmet Arac , the study's corresponding author and assistant professor of neurology at the David Geffen School of Medicine at UCLA, sought to test whether this superior skill is the result of more practice with the preferred side over time.
The team found that the skill gap between a person's dominant and non-dominant limbs largely disappears under the right conditions and reappears only when a task draws on the kind of complex, curved hand and arm movements that come from years of practice handling tools, pens and other objects.
Arac and his team tracked the 3D arm movements of healthy adults using motion-capture cameras as they reached toward targets under three conditions: normal reaching, reaching while wearing a four-pound wrist weight and reaching while holding a lightweight stick strapped to the forearm to mimic tool use.
In a second experiment, participants wrote letters and numbers using a pen first with their hand, then with a pen taped to their elbow, which is an effector they'd never used for writing before.
During simple reaching, and even when a weight was added to make the movement physically harder, the dominant and nondominant arms performed about the same.
Once the stick was added, requiring the kind of curved, controlled trajectory needed to use a real tool, the dominant arm clearly outperformed the nondominant arm.
When participants wrote with their elbow instead of their hand, the usual dominant-side advantage vanished entirely; both elbows wrote equally poorly at first.
But after practice, both the dominant and nondominant elbow improved by the same amount, and both got better than the nondominant hand had ever performed.
"The dominant arm isn't more capable because one hemisphere of the brain is simply better at controlling movement," said Arac. "It is because we've spent a lifetime practicing the specific, complicated movements that tools and handwriting demand. Take away that practice by switching to a body part like the elbow that's never done the task before and the advantage disappears."
The results may be of interest to clinicians who work with stroke survivors and others relearning motor skills, as well as researchers studying how skilled movement is learned and represented in the brain.
The research, conducted at UCLA, also involved coauthor Nicolas Y.H. Jeong Lee and senior author Dr. John W. Krakauer of Johns Hopkins University and the Santa Fe Institute. It was supported by the National Institutes of Health, the US-Israel Binational Science Foundation, NVIDIA and UCLA's Department of Neurology.
The full study, "Arm Dominance Is an Emergent Effect of Practice Executing Complex Trajectory Shapes Required by Tools and Objects," is available in PNAS at https://www.pnas.org/doi/10.1073/pnas.2601569123 .
