The Grapevine

Why Are Some People Left-Handed?

Is the brain responsible for determining our preferred hand in activities like writing and eating? The answer may come as a surprise.

Over the years, there have been several theories to understand "handedness" or why people develop a more efficient preference for either one of their hands, and in exceptionally rare instances, both of their hands. Ultrasound scans in the 1980s first showed that a preference of moving the left or the right hand develops in the womb as early as the eighth week of pregnancy. The sucking of the left or right thumb starts from the thirteenth week.

A team of scientists from the Netherlands, the UK, and China pursued a genetic analysis to identify what contributed to these left-right differences in the nervous system. Their study titled "Epigenetic regulation of lateralized fetal spinal gene expression underlies hemispheric asymmetries" was published on Feb 1, 2017, in the journal eLife.

The findings explained that our hand preference actually originates in the spinal cord, contradicting previous theories that the brain may be the main determinant. 

"Human foetuses already show considerable asymmetries in arm movements before the motor cortex is functionally linked to the spinal cord, making it more likely that spinal gene expression asymmetries form the molecular basis of handedness," the study stated.

The motor cortex of our brain sends signals to the spinal cord, which allows the movement of the arms and legs. But when observing fetal development in the womb, research acknowledged that the preference of hand was already decided before the motor cortex and the spinal cord were even connected. There were differences detected between the number of genes being expressed on the right or left side of the spinal cord in the eighth week.

"This seems logical since many nerve fibers cross over from one side to the other at the boundary between the hindbrain and spinal cord," explained Carolien de Kovel, lead author of the study and researcher at the Max Plank Institute for Psycholinguistics. "Around 85% of humans are right-handed; it seems the standard in human development but genetic and environmental factors may provide alternative paths of development, such as left-handedness or two-handedness."

The environmental influences may lead to enzymes bonding methyl groups to the DNA, which may affect and minimize the reading of the genes. "These results fundamentally change our understanding of the cause of hemispheric asymmetries," the authors stated.

Given that left-handed people only make up around 10% of the world population, right-handed people have always constituted the clear majority. History reveals to us that left-handedness was not accepted for a long time, considered evil and associated with the devil in some cultures. Even the word "sinister" was derived from the Latin word "sinistra" which originally meant "left".

While society may not be as harsh today, stigma still looms in certain parts of the world. Furthermore, experts have called for the inclusion of more left-handed participants in scientific research.