If you’re reading this on your phone, there’s a nine-in-10 chance you’re scrolling with your right hand. Roughly 85 to 90 percent of people are right-handed, while just 10 to 15 percent are left-handed, and a small percentage are ambidextrous.
Although the exact figures vary slightly from place to place, no human population exists in which left-handedness is more common than right-handedness, says Paul Rodway, a senior lecturer in psychology at the University of Chester in the United Kingdom, who studies handedness.
So why do humans overwhelmingly favor one hand over the other?
Nature or nurture?
The environment you grow up in often plays some role in whether you’re right or left handed. In some Asian, Arab, and African countries, the left hand is considered “unclean” and children with a dominant left hand are forced to become right-handed through restraint and punishment. “Cultures where there is strong social pressure against left-handedness have lower rates of left-handedness,” says Rodway.
Related Ask Us Anything Stories
But even in societies that show no bias, lefties remain a clear minority. “This stable prevalence rate across cultures suggests a biological influence,” says Rodway.
In fact, scientists think that the tendency to prefer one hand over the other begins before birth.
“This preference is already visible in the movements of unborn fetuses,” says Clyde Francks, a professor of brain imaging genomics at Max Planck Institute and Radboud University Medical Center in The Netherlands. Ultrasound scans have shown that by the 10th week of gestation, most fetuses move the right arm more than the left, and from the 15th week most suck the right thumb rather than the left.
“It is likely that right-handedness is the default outcome of early brain development as encoded by the genome,” according to Francks.
Research suggests that dozens of genes—perhaps up to 40—play a role in shaping handedness. Rather than determining it outright, these genes build the brain in a way that typically favors the right hand as the dominant one, Francks explains.
But if right-handedness is the brain’s default setting, what makes some people left-handed? “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,” says Francks. “For example, random fluctuations in the concentrations of certain molecules during key stages of brain formation” could influence which hand you write and throw with.
Possible evolutionary advantage
Some scientists think most people are right-handed because it gave our ancestors an edge.
One theory links handedness to tool use and the passing down of skilled movements across generations, says Paul Rodway, a psychologist at the University of Chester. Archaeological evidence backs that up: A 2011 study found that humans have strongly favored their right hand for tool use for at least half a million years.
Rodway and his collaborators have proposed another theory. “Right-handedness may have evolved, in part, due to human fighting with sharp weapons,” he says.
“When facing an opponent, a right-hander is most likely to stab an opponent’s left thorax, where the majority of the heart is,” Rodway explains. So, say during a medieval duel, “a right-hander may be more likely to kill an opponent than a left-hander, because they penetrate the heart. This can result in a relative survival advantage for right-handers, and this may be one of the forces that led to the prevalence of right-handed humans.”
Still, left-handers may have had their own advantages. Their rarity makes their movements harder to predict, which can be useful in both combat and sports. “This may have enabled left-handers to persist in human populations,” says Rodway.
“With such opposing advantages and disadvantages,” Rodway says, “evolution often settles on an equilibrium. This is what may have happened with the proportions of left- and right-handers in populations. However, more research is needed, because the picture is complex.”
The takeaway
Cultural pressure can influence which hand people use, but the preference likely begins before birth. Dozens of genes involved in brain development create a natural bias toward the right hand, and over time, evolutionary forces such as tool use and combat may have strengthened that tendency.
In Ask Us Anything, Popular Science answers your most outlandish, mind-burning questions, from the everyday things you’ve always wondered to the bizarre things you never thought to ask. Have something you’ve always wanted to know? Ask us.
2025 Holiday Gift Guide
