Why are most people right-handed?

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📓 The short answer

While we don't know for sure, it's theorized that humans predominantly became right-handed due to the development of the left brain hemisphere associated with language and tool use. Additionally, right-handedness may have offered advantages in fighting and soothing infants.

📚 The long answer

From scissors to can openers, and school desks to pants zippers, we live in a right-handed world.

Roughly 90% of the human population prefer using their right hands. While some animals do show a preference for using their left or right hands (or paws), humans' overwhelming right-handedness sets us apart. In fact, no human population has ever reported a majority of left-handed individuals.

Our right-handedness is not a recent evolutionary development. The earliest evidence of right-handedness dates back as far as 1.8 million years ago, from fossil records of Homo habilis, an early human species.

Researchers discovered microscopic marks on the teeth of these early humans. The hypothesis went that these marks were made when early humans held materials, like animal hides, in one hand and used their teeth to secure the other side, freeing a hand to use stone tools, which sometimes hit their teeth (ouch). The angle of the marks indicates that these early humans were predominantly right-handed.

Drawing showing teeth marks on an early human, with an illustration showing how they may have hit their teeth with tools using their right hand — Drawing: Jay Smith. Photos: David Frayer and Luca Bondioli. Source: Discover Magazine

So we've been mostly using our right hands for a long time. It raises the question: why did the right hand become dominant and not the left?

While this question has been the subject for plenty of scientific debate, the fact is that we don't have a definitive answer as to why humans are mostly right-handed. But here are three leading theories.

Reason #1: Humans evolved to use language.

You've probably heard before that the left side of your brain controls the right side of your body, and vice versa.

But you know what else is on the left side of your brain? Language.

This hypothesis states that as humans developed more complex spoken language, the left brain hemisphere evolved. The preference for using the right hand, controlled by the left brain, may have intensified as linguistic processing advanced.

Reason #2: Humans evolved to use tools.

Another thing that sets humans apart from many other animals is our ability to create and use tools.

Brain scans have shown that a specific region in our left brain hemisphere, called Brodmann area 44 (BA44), fires up when we use tools for various tasks.

GIF showing how the ba44 brain region is in the left hemisphere — "Brodmann area 44 animation" by Anatomography is licensed under CC BY-SA 2.1 JP.

Similar to the language hypothesis, the idea is that increased specialization in tool use led to more activity in the left brain hemisphere. Since the left side of our brain controls the right side of our body, this tool-making behavior may have contributed to the preference for using the right hand.

Reason #3: There are some behavioral advantages to being right-handed.

Finally, there are some advantages associated with right-handedness. The fighting hypothesis suggests that a fighter who held his spear in his right hand improved his chances for survival by better protecting the heart, located on the left side of the body. This would lead the right hand to be used for more skilled movement.

Illustration of an early human holding a spear with his right hand and protecting his heart with his left hand — Source: M. Larsson et al., *Symmetry*, 2023

Another hypothesis suggests that mothers holding infants in their left arms could better soothe them with their heartbeat, freeing the right hand for complex tasks.

Of course it is possible that the predominance of right-handedness happened before these behaviors.

I guess we're left to wonder, which one of these theories is right.

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All my best,

Caitlin

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📖 Book of the week

East of Eden by John Steinbeck

East of Eden is an epic book following two families across multiple decades. There were lots of characters, but somehow you're able to really get to know each one, as they are painted with great care and detail. It's a book covering impactful themes, like good and evil, fathers and sons, and will and destiny.

Steinbeck regarded this book as his masterpiece stating, "I think everything else I have written has been, in a sense, practice for this."

I enjoyed this book so much that I spent a good chunk of time afterward reading analyses of the symbols and themes, like I was back in school taking a literature class.

Check out the full list of books I've recommended here.