Why are you right- or left-handed? New science explains why

If you’re like roughly 90 percent of people worldwide, you’re right-handed. And, obviously, if you’re among the remaining 10 percent, you’re left-handed. It’s simply a trait we’re born with, determined by a combination of genetics and other factors that hard-wire “handedness” into the brain.

Or is it?

A new scientific study has turned theories of right- or left-hand preference on their heads, so to speak. Researchers at Ruhr University in Bochum in Germany have found that it isn’t the brain where hand preference is determined at all – it’s the spinal cord.

The research team has discovered that asymmetrical genetic activity in the spinal cord, beginning in the womb, determines whether a baby will be right- or left-handed. This intriguing idea has caught the interest of scientists worldwide and left others scratching their heads (most of them with their right hands.)

The study, recently published in the journal eLife, reveals that hand preference develops as early as eight weeks in gestation, and before the motor cortex of the brain is even connected to the spinal cord. The connection between motor cortex and the spinal cord comes later, after signs of hand preference become apparent in utero. Hand preference can actually be viewed via ultrasound around the 13th week of pregnancy when unborn children can be seen sucking either their right or left thumb.

Previous studies had linked left- or right-handedness to asymmetries in the brain. To support its new theory, the Ruhr research team analyzed gene expression in the spinal cord between eight and 12 weeks of pregnancy, and found significant differences in the parts of the spinal cord that control arm and leg movement. Because the motor cortex at that stage is incapable of sending signals to the spinal cord, the team wanted to look at gene activity in the spinal cord that could account for hand preference appearing so early in human development.    

Researchers traced the cause of asymmetric gene activity and found that certain enzymes bond methyl groups to DNA, which affects or minimizes the “reading” of genes. Tests revealed a difference in the activity of genes on both sides of the body, showing that gene activity in the spinal cord is asymmetrical earlier than previously known.  

“These results fundamentally change our understanding of the cause of hemispheric asymmetries,” said the authors of the study. 

Does this settle the cause of hand preference once and for all? 

Some other theories remain tantalizing. Prenatal environment and cultural influences may play a role in handedness, and it may be partly inherited. Children of left-handed parents are more likely to be left-handed than are children of right-handed parents. Additionally, identical twins are more likely to be either right-handed or left-handed than non-identical twins, but this isn’t a hard rule. Random variation among individuals cannot be ruled out, either.

At least we know that scientists are striving to explain the mysteries of human traits. And that’s something everyone should applaud. (Using both hands.)

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