There is a number that neuroscientists have been studying for decades with fascination and perplexity: 90. It is the percentage of humans who, across all cultures, time periods studied, and corners of the globe, preferentially use their right hand. That 90% is a figure so consistent, so universal, so round, and so strange that the question isn’t even why left-handers exist. It’s why there are so few of them — and why this has been so consistently the case across time and space.

Handedness — that is, having a dominant hand and a non‑dominant hand, as we colloquially put it — is not unique to our species. Cats have a preferred paw and parrots always hold food with the same foot. Almost every animal with a brain shows some bias toward one side or the other. What is truly unusual in humans is that nearly the entire species leans the same way from our very origins. “We call it a population preference,” says Miquel Llorente, a researcher at Girona University who specializes in comparative ethology and the evolution of language. “It’s not that each individual has a favorite hand — that always happens — but that almost all individuals in the species share the same one. In chimpanzees, for every left‑hander there are two right‑handers; in us, for every left‑hander there are eight or nine. It’s that jump that needs explaining, and there’s still no clear answer."

Scientists have spent years searching for that explanation, which does not have a single cause. It is an amalgam of several factors, interwoven over millions of years.

Handedness is, above all, a story about brains. Although the two hemispheres may look like twins at first glance, they actually divide labor very unevenly: the left is preferentially responsible for language; the right, for tasks such as face recognition or spatial orientation. In more than 95% of right‑handed people, language resides in the left hemisphere. In the human brain, hand and speech travel together.

That connection is the most interesting clue so far as to why the proportion of right‑handed humans is so stable. “The hypothesis with the most weight today is that language didn’t originate as speech but as gesture,” Llorente says. “Manual signs would have been the first form of complex communication among our ancestors, and only later did they shift to the voice.” Because we tend to gesture with the right hand, controlled by the left hemisphere, that same hemisphere would have specialized both in moving the skilled hand and in producing speech, the expert adds. The brain areas for language and for control of the right hand are not only related: they are literally adjacent, sharing circuits.

Parrots, which are also highly vocal and social animals, are almost the only nonhuman species in which something like that population asymmetry is observed. The coincidence does not seem accidental.

The snowball

How do we know this lateral bias has persisted for millions of years in our ancestors? The brains of our predecessors don’t survive, but the skull leaves an internal imprint (the endocranium, a sort of cast of the brain pressed into bone) that allows reconstruction of its shape and asymmetries. Even in australopithecines, three to four million years ago, the brain was not symmetrical. When the first worked stone tools appear, about two and a half million years ago, those asymmetries become more pronounced. And from the way the oldest flakes are knapped, archaeologists can infer that most of those hominins struck with the right hand.

Making tools requires very precise coordination: one hand holds, the other strikes at a specific angle, force, and sequence. And that is learned by watching others, adding a cultural layer. Tools, gestures, language, teaching across generations… “The image that works best is not a single cause but a snowball,” Llorente says. “We started with a mild right‑hand bias, similar to that of today’s great apes,” he adds. Bipedal walking, tools, communicative gestures, language and culture added layer upon layer over millions of years, the researcher explains. And at the end of the process: nine out of 10 people are right‑handed, in every corner of the world.

And the genes?

All of this happened because something in our biology allowed it and encouraged it. So one of the goals of science has been to try to identify exactly which genes are responsible for handedness. But the research has turned out to be much more complicated than scientists expected.

“When I started working on this, we thought we might find maybe 10 genes that explained everything,” says Silvia Paracchini, a geneticist at the University of St. Andrews (Scotland) who has spent years studying lateralization and its relation to language and dyslexia. Years ago her team identified the gene PCSK6 as a promising candidate. Later, studies involving more than a million people complicated the picture: 42 genes surpassed the threshold of statistical significance, and even so they do not explain everything. “Biology is complicated,” Paracchini says. “And the more we know, the more we appreciate that complexity.”

What is beginning to emerge is the type of biology involved in lateralization. Several of those genes are related to cilia, tiny cellular organelles involved in establishing left‑right asymmetry in the body from the earliest stages of embryonic development. Genes linked to neurodevelopment also appear, and there is a striking association with variants tied to conditions such as dyslexia, autism, and schizophrenia. The effect is small — being left‑handed does not significantly increase the risk of anything — but the signal is consistent: the same biological circuits that set which side is left and which is right in the body appear connected to how the brain organizes itself for language.

Why aren’t we all right‑handed?

That leaves the most counterintuitive question: if being right‑handed is so advantageous, why didn’t left‑handers disappear millions of years ago? One of the most ingenious hypotheses comes from game theory: being left‑handed gives a tactical advantage in close‑quarters combat precisely because it is unexpected. A right‑hander trained to fight other right‑handers is thrown off by a left‑hander, as fans of The Princess Bride know well, in which a duel between two legendary left‑handed swordsmen becomes even more interesting when both admit they are using their off hand. But that advantage only works while left‑handers remain a minority: if everyone were left‑handed, the surprise would vanish. Evolution would thus reach a stable equilibrium around 10%. That left‑handers are overrepresented in contact sports such as boxing or fencing (between 13% and 20%, according to studies) gives some empirical support to the hypothesis. Although it remains only that: a hypothesis.

Almost everything in this field remains, in fact, a matter of debate. “Handedness seems simple because it’s binary: left or right,” Paracchini reflects. “That leads everyone to think it should be easy to explain. But it turns out to be one of the most complex and fascinating windows we have into how the human brain evolved.” A non‑dominant hand, a dominant hand, and two million years of questions that still lack a definitive answer.