Only about 10 percent of the world's population is left-handed, according to Live Science, but why is it so rare?

A new study published online in Biological Psychiatry has examined the gene identity linked to left- or right-handedness, which develops as an embryo.

Read: International Left-Handers Day 2016: Lefties Face Psychological Battles, But Are More Creative Thinkers

After four to eight weeks of overall growth, an embryo’s left and right sides of the spinal cord develop at different paces. The team discovered this after "searching for genes that contribute to left-right differences in the nervous system," according to a press release from the Netherlands’ Radboud University.

The body’s nervous system is a network of cells and fibers responsible for transmitting nerve impulses throughout, so the spine is crucial. The study showed that the left side of the spinal cord matures slightly faster than the right, meanwhile, "sets of key genes that control growth and maturity were found to reach a more advanced profile of activity on the left side than the right," the release noted.

Embryos of eight weeks already tend to move their right arms more often than their left, but these signals are not sent from the brain and are only from the spinal cord, the press release reported.

Researchers think these differences in development help determine if you’re a leftie or rightie — these very early left-right differences in the spinal cord could later trigger asymmetries in the brain, which eventually determines the more powerful brain hemisphere and your dominant hand.

Read: Why Is Left-Handedness So Rare?

"This seems logical, since many nerve fibers cross over from one side to the other at the boundary between the hindbrain and spinal cord," lead study author Carolien de Kovel said, according to the press release. "How exactly this left-right genetic difference in the spinal cord leads to right-handedness is, however, not yet clear."

Source: de Kovel CGF, Lisgo S, Karlebach G, Ju J, Cheng G, Fisher SE, Francks C. Left-right asymmetry of maturation rates in human embryonic neural development. Biological Psychiatry. 2017.

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