Left-handedness may start in the cell's scaffolding
Tiny differences in the cell's highway network may pile signals toward one side and snowball into a dominant hand.
Quanta Magazine · Why Am I Left-Handed?Ask what makes someone left-handed and the intuitive answer is genes for the brain. The real answer may be stranger, and it is still only a hypothesis.
In large studies, researchers have found about 40 gene variants that each slightly raise the odds of being left-handed, or a southpaw in baseball slang. The more of these variants a person carries, the more likely that person is to favor the left. Most belong to one family, the genes for tubulin, a protein that builds the cell's internal scaffolding. “I don’t think I had this [gene family] on my list 10 years ago, to be honest,” said Sebastian Ocklenburg, who works in behavioral psychology and co-wrote the 2017 eLife study of early spinal cord asymmetry and a 2025 review of the findings.
To see why cell scaffolding might matter, picture microtubules, the long filaments assembled from tubulin. They act as both a cell's skeleton and its highway network, with other proteins latching on and hauling chemical freight to where it is needed inside the cell.
One proposal centers on neural progenitor cells, the starter cells that divide to make neurons for the spinal cord. Under this unproven idea, perhaps a signaling molecule rides along those highways and collects more toward one edge of such a cell. When the cell splits, one daughter cell would inherit a little extra of the signal. Across repeated cycles of division, that small inequality could grow into a mismatch between the two halves of the cord as it forms.
In the proposed mechanism, feedback loops would then amplify the difference, boosting activity in the favored half while quieting it opposite, usually in a way that structurally favors the right. Variants tied to left-handedness might change how proteins grip the highways, shifting the pileup to the other side, or they might weaken the usual rightward lean so much that chance fluctuations let the left win.
If the idea holds, a dominant hand may begin as freight gathering toward one edge of a cell. Only later, under this account, would the brain join in, when sensory feedback arrives at the developing motor cortex and reinforces the neural picture of that limb.
Questions people ask
When in development does this happen?
Very early, roughly 8 to 12 weeks after conception. Tissue from that window shows uneven gene activity between the two halves of the spinal cord, and the arm a tiny fetus moves more tends to predict later handedness with high accuracy.
What else do microtubules do?
They do many jobs in almost every cell. They help cells keep their shape, pull chromosomes apart when cells divide, and serve as tracks for moving nutrients, proteins, and other cargo. Nerve cells also rely on them to grow long extensions and transport materials along those branches.
Why are the same genes linked to dyslexia, autism, and schizophrenia?
The connection is statistical, not destiny. People with those conditions are somewhat more likely to be left-handed or mixed-handed, which suggests that subtle differences in early brain and spinal cord development can affect both handedness and how brain circuits are organized. Having the variants raises the odds slightly; it does not cause either outcome on its own.