About one in ten people are left-handed. That number is strangely stubborn: it holds at roughly 10 percent across cultures, across continents, and as far back as we can trace, in cave paintings and ancient tools. Righties have always been the overwhelming majority, lefties the small, steady minority. You'd think something this consistent, this old, would have a clean explanation by now. It doesn't. After more than a century of study, the honest answer to why some people are left-handed is this: we have good clues, and we still can't say for sure.

What we can say with confidence

Here's the most truthful short answer. Handedness is set very early, almost certainly before birth, and it reflects how an individual brain divides its labor between its left and right halves. It's shaped partly by genes, but not by a single "handedness gene," and partly by something closer to developmental chance. No one chooses it, parents don't cause it, and you can't reliably train it out of a child. By the time a baby is born, the preference is largely already there.

That much is solid. Almost everything past it gets blurry, which is exactly what keeps the question alive.

The genetics: many tiny pushes, no master switch

For a long time people assumed handedness ran in families through one gene. It doesn't. Large studies put the heritability, the share of the variation explained by genetics, at only around a quarter, and that effect is spread across many, many genes that each tip the scale by a hair (PNAS). There's no on-off switch to find.

The most intriguing recent clue came in 2024, when researchers scanned the genes of more than 350,000 people and found that rare variants in a gene called TUBB4B were noticeably more common in left-handers (Nature Communications). TUBB4B helps build microtubules, part of the internal scaffolding that gives cells their shape and helps organize a growing body. The leading idea is that these tiny structural proteins help set up the brain's left-right axis very early in development, long before a hand preference could exist (Smithsonian Magazine). It's a real lead. But even the scientists who found it are careful to say they don't yet know how a cellular scaffold becomes a preference for the left hand.

It's really a question about the brain

Underneath the genetics, handedness is a brain story. The two halves of your brain don't do identical jobs; they specialize, a feature called lateralization. Each side controls the opposite side of the body, so a left-hander's dominant hand is run by the right hemisphere, a right-hander's by the left. And handedness turns out to be linked to subtle left-right differences in the brain's structure, including the areas that handle language (PNAS). In most right-handers, language lives mainly in the left hemisphere; in left-handers, the layout is more varied. So the hand you write with is really a visible clue to a much deeper question: how your particular brain decided to split up the work.

The honest ending

Put it all together and you get a satisfying frame around an unsolved center. We know handedness is set before birth. We know it's weakly heritable through a crowd of small-effect genes, with a fresh and fascinating thread leading to the cytoskeleton. We know it reflects the brain's deep left-right asymmetry. What we still can't fully explain is the thing you actually asked: why some specific people land on the left, and why the human species has held that ratio at about nine righties to one lefty for as long as there have been humans to count.

Some questions in science are thrilling because they're answered. This one is interesting because it isn't, not completely. The next time you watch a left-handed person sign their name, you're watching the visible tip of a process that began in the dark, months before they were born, by rules we've spent a hundred years almost, but not quite, working out.

Keep wondering: the body is full of traits we're still decoding, from why identical twins don't share fingerprints to why people have different eye colors and why we dream at all.