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What Causes Left Handedness?

baby grabbing hand

Nintey percent of the world is right handed. History and culture have not been kind to left handers. This is a somewhat rare occurrence among us humans. So, after all these years, if the right hand is preferred then why do we still have a bunch of leftys out there? What causes left handedness? Listen to this Question Your World radio report produced by the Science Museum of Virginia to learn more.

For the last 10,000 years or so humans have been mostly right handed, only about 10% of us are left handed. Due to this rareness, being left handed has gotten a bad rep over time. For example, the word “left” for Anglo-Saxon speakers would translate to having weakness; the word “sinister” in Latin translates to “on the left side”; even old lore from Scotland often refers to how it's unlucky to meet a left-handed person at the start of a journey. Meanwhile, when things are correct they are "right", clearly there's been a one-handed bias throughout language and culture. So, why do we have left handed folks anyway? 

To help answer that question, some scientists recently did detailed imaging of the brains of left-handed people. These scientists dug through the genomes of about 400,000 people to look for genetic variants shared among left handed individuals. Of that big population, about 10,000 of the identified left handers had their brains photographed for this detailed cerebral information for this study. This research has shown, for the first time ever, that certain genes that guide the development of our cellular scaffolding, or the stuff that helps keep the cell’s shape also known as cytoskeleton, are associated with handedness in humans! Cytoskeletons were specifically studied in the language portion of the brain where they observed differences between left or right handed folks, the first ever visual proof of a left vs right handed brain! 

We humans are not alone in this, of course, many animal species show left versus right asymmetry. Snails also show sidedness with their shells, which, like piglet tails or a lobster’s dominant claw, develop to go either left or right. Interestingly, the curve of a snail's shell is guided by these same cytoskeleton genes!

Okay, what’s left… oh right, there is also a possibility that these differences start appearing in the womb. Any moms out there with any kids that made their soccer player debut in utero? More research is needed, but these babies could very well be kicking with their dominant foot!

So, it turns out that similar to hair color or the size of one’s nose, handedness too is an inherited trait. A great reminder that when it comes to learning more about ourselves, science is always here to give us a hand…left or right.