Why are most people right-handed?
If you’re reading this on your phone, there’s a nine out of ten chance you’re scrolling with your right hand. About 85 to 90 percent of people are right-handed, while only 10 to 15 percent are left-handed and a small percentage are ambidextrous.
Although the exact numbers vary slightly from place to place, there is no human population in which left-handedness is more common than right-handedness, says Paul Rodway, a psychology lecturer at the University of Chester in the United Kingdom, who studies right-handedness.
So why do humans overwhelmingly prefer one hand over the other?
Nature or culture?
The environment you grow up in often plays a role in whether you are right-handed 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 coercion and punishment. “Cultures where there is strong social pressure against left-handers have lower rates of left-handedness,” says Rodway.
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But even in societies that show no prejudice, leftists clearly remain a minority. “This stable prevalence rate across cultures suggests a biological influence,” says Rodway.
In fact, scientists believe 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, professor of brain genomic imaging at the Max Planck Institute and Radboud University Medical Center in the Netherlands. Ultrasound scans showed that from 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 result of early brain development, as encoded by the genome,” according to Francks.
Research suggests that dozens of genes, perhaps as many as 40, play a role in hand formation. Rather than determining it directly, these genes construct the brain in a way that generally favors the right hand as the dominant hand, Francks explains.
But if right-handedness is the brain’s default setting, what makes some people left-handed? “We believe that most cases of left-handedness occur simply due to random variations during embryonic brain development, 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 scalable advantage
Some scientists believe that most people are right-handed because it gave our ancestors an advantage.
One theory links manipulation to tool use and the transmission of skilled movements across generations, says Paul Rodway, a psychologist at the University of Chester. Archaeological evidence supports this: a 2011 study found that humans have strongly favored their right hand for using tools for at least half a million years.
Rodway and his collaborators proposed another theory. “Right-handedness may have evolved, in part, because of human fighting with sharp weapons,” he says.
“When facing an opponent, a right-hander is more likely to stab the opponent’s left chest, where the majority of the heart is located,” Rodway explains. So, say in a medieval duel, “a right-hander may be more likely to kill an opponent than a left-hander, because it penetrates the heart. This may result in a relative survival advantage for right-handers, and this may be one of the forces that led to the predominance of right-handed humans.”
Yet left-handers may have had their own advantages. Their rarity makes their movements more difficult to predict, which can be useful in both combat and sports. “This may have allowed left-handers to persist in human populations,” says Rodway.
“With such opposing advantages and disadvantages,” says Rodway, “evolution often strikes a balance. This may have happened with the proportions of left-handers and right-handers in populations. However, further research is needed, because the picture is complex.”
Takeaways
Cultural pressure may 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 reinforced this tendency.
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