It's down to the spinal cord — not the brain
Research since the 1980s has shown that our preference for being left or right-handed is probably determined in the womb before we are born, as early as the eighth week of pregnancy, according to ultrasound scans. From week 13 in the womb, babies tend to suck either their right or their left thumb.
It was previously thought that the genetic differences in the left and right hemisphere of the brain would determine whether someone was born left or right handed. But a study from 2017, published in the journal eLife, has found that the answer could lie in the spinal cord.
The research was led by Dr Sebastian Ocklenburg, Judith Schmitz, and Prof Dr H. C. Onur Güntürkün from Ruhr-Universität Bochum, along with other colleagues from the Netherlands and South Africa. They found that gene activity in the spinal cord is already asymmetrical in the womb, and this could be what causes left or right-handedness.
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