Most people favor one hand, and that hand tends to be the better one at writing, at throwing, at managing chopsticks. The long-standing view is that the dominant hand is "born" more capable, its skills rooted in a brain hemisphere specialized for motor control. A new study in PNAS argues that this difference in skill is not innate at all. It is the consequence of a lifetime of practice, and it surfaces only when we pick up a tool.
The paper , by SFI External Professor John Krakauer (Johns Hopkins University) with Ahmet Arac and Nicolas Y. H. Jeong Lee at the University of California, Los Angeles, separates two ideas — preference and dominance — which are often conflated under "handedness." Preference, meaning which hand you instinctively favor for most tasks, appears before birth and has biological roots, prior research shows. Dominance, the skill gap between the two hands, is the question the study takes on.
In a series of experiments using 3D motion capture, the researchers compared people's two arms during ordinary reaching, reaching with a weight on the wrist, and reaching with a lightweight stick fixed to the forearm. Ordinary reaching revealed no clear advantage for the dominant arm, and the added weight made no difference either. The gap appeared only with the stick, when the nondominant arm struggled to control the more complex, curved path it demanded. A further test sharpened the point: when participants wrote with their elbows, using a pen strapped to the arm, the dominance vanished, and with equal practice both elbows improved equally, each ending up better than the untrained nondominant hand.
"You don't prefer your dominant hand because it's more skilled," Krakauer says. "It becomes more skilled because you prefer it. And you wouldn't notice any difference between your two hands without tools and objects in the world that demand practice to use well."
Arac takes the idea a step further. "Since humans are uniquely prolific tool users and makers, handedness may be a byproduct of our inventiveness," he says. In that sense, he adds, handedness "can be seen as a fingerprint of human tool-use culture."
The study reframes arm dominance as something that emerges over a lifetime — skills built asymmetrically through practice rather than wired from the start. The asymmetry isn't stored in a gene or a hemisphere; it accumulates from using the tools we ourselves invented. It is a reminder that even a trait as seemingly fixed as handedness can be emergent, arising from the interplay of biology, behaviour, and culture rather than from any single cause.
They point to a natural next step: studying people whose preference and practice diverge, such as left-handers forced to use tools with the right hand, stroke survivors whose hand preference shifts, and amputees who develop skill with unconventional effect.
Read the paper, "Arm dominance is an emergent effect of practice executing complex trajectory shapes required by tools and objects," by Ahmet Arac, Nicolas Y. H. Jeong Lee, and John Krakauer in PNAS. DOI: 10.1073/pnas.2601569123
