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Whether we are left- or right-handed is most obvious when we write. Left-handed people write with their left hand. Right-handed people write with their right hand. Researchers at the Ruhr University in Bochum, Germany, have discovered that this development begins in the womb.
For a long time, science assumed that the brain determined the preferred side. However, we now know that it is the spinal cord that determines whether a person will be left-handed or right-handed. This is the conclusion reached by experts of biopsychology at the Ruhr University in Bochum, Germany, together with colleagues from the Netherlands and South Africa.
In 2017, the international team was able to prove that gene activity in the spinal cord is already asymmetrical in the fetus in the womb. This asymmetry may explain why the left side of the body is more pronounced in some people and the right side in others.
Whether we are left-handed or right-handed is already decided in the womb
Even in the womb, unborn babies show whether the right or left hand will dominate later in life. Ultrasound studies in the 1980s revealed this: From the eighth week of pregnancy, one side becomes more pronounced based on movements. By the 13th week of pregnancy, fetuses suck on either the left or right thumb, depending on which side they prefer.
The Ruhr University Bochum report states:
“Arm or hand movements are initiated by the motor cortex in the brain. It sends an appropriate signal to the spinal cord, which converts the command into movement.” Whether the left or right side of the body is more dominant has long been associated with the hemispheres of the brain. “But the motor cortex is not connected to the spinal cord from birth. Preliminary stages of handedness are visible even before the connection is formed,” the study continues. The researchers therefore suspect that the preferred side originates directly in the spinal cord.
Environmental influences determine left- or right-handedness
Asymmetric gene activity is the result of epigenetic factors. They reflect environmental influences: “These influences can, for example, cause enzymes to attach methyl groups to DNA and thus cause genes to be read at a reduced rate. Since this happens to a different extent in the left and right spinal cords, the genes are active to different degrees on both sides,” the research team explains.
Left- or right-handedness also exists in the animal kingdom.
It’s not just humans who have different dominant sides of the body. Comparable differences in preference can also be found in the animal kingdom. In a 2018 study of domestic cats, Deborah Wells observed that house cats are left- or right-handed. They also have a preference for one side of the body over the other, for example, when reaching for food.
Right-handedness and left-handedness occur in some species. Researchers have observed preferred sides of the body in whales, elephants, and primates, among others.
