Why Are Some People Left-Handed?

Whether they’re writing their grocery lists or using a pair of scissors, about 90% of folks favor their right hand. Across cultures, continents, and historical eras, lefties have always been in the minority.

But despite decades of research, scientists still haven’t found a specific reason why roughly one in 10 people are naturally left-handed. Instead, evidence suggests handedness — or your preference to use one hand over the other — is shaped by multiple factors.

Genetics plays a role, but it’s not as simple as being born with a single, specific “southpaw gene” — it doesn’t exist. So let’s get right (or left) to business and dig into what gives you your handedness.

Is Left-Handedness Genetic?

Somewhat confusingly, the left and right hemispheres of your brain control your motor skills on the opposite sides of your body, meaning the right side of your brain is responsible for your left arm and hand. But because they rarely control behaviors equally, you typically prefer one hand over the other. This is called cerebral lateralization.

For right-handed folks, motor control over their hand is better developed in the left side of the brain, while the opposite is true for lefties. Language, on the other hand, is dominated by the left hemisphere for most people, regardless of how they write. 

That means, in the brains of most right-handed people, hand control (writing) and language are more closely connected, neurologically speaking. This could explain why most humans prefer to write with their right hands.

You could even have a preference for your left hand before you’re born. Studies have shown fetuses will suck one thumb more than the other as early as 15 weeks.

Research shows that your genes play a big role in whether you’ll be right- or left-handed. We’ve known for decades that left-handedness tends to run in families: Studies show that when both of your parents are left-handed, you’re much more likely to be, too. 

If your parents are right-handed, meanwhile, you’re very unlikely to be left-handed. This suggests that handedness could be passed on from parents to their children, but no “left-handed gene” has been discovered. Instead, like many human traits, it’s believed to be a combination of dozens of different genes, as well as which ones are turned “on” or “off.” 

Plus, if handedness was purely genetic, then identical twins — who usually share 100% of their DNA — would always be both right-handed or left-handed. However, that’s not the case, as one twin may be a lefty and the other a righty. 

That said, a recent meta-analysis of twin studies found that identical twins are more likely to use the same hand compared to fraternal twins (who share just 50% of their DNA). This suggests genetics still plays a significant role in the development of handedness.

Nature vs. Nurture

While genetics plays a role, so does epigenetics — which is the way your environment and habits can physically alter the way your genes work. That’s why genetically identical twins can look pretty different as they age, as different genes are turned on and off based on external factors. 

Epigenetics may also play a role in handedness, especially since lefties live in a world dominated by righties and often have no choice but to use tools and equipment designed for right-handedness — everything from scissors and gear shifts to musical instruments and sports equipment.

But given a plethora of left-handed products are more easily available to purchase these days, shopping opti, this is less likely to be a factor in modern developed societies.

Using your right hand out of convenience is one thing, but many lefties have been forced to do it. In many cultures, left-handedness was a social stigma, and children who showed signs of being left-handed were often made to write with their right hand instead. Many righties are actually lefties in disguise. At least two U.S. presidents — Ronald Reagan and Harry Truman — were lefties forced to write with their right hand. 

Studies show that as the rate of “enforced right-handedness,” which varies between cultures, has declined in recent decades, left-handedness in those populations has increased. 

Still, a “southpaw revolution” is unlikely to occur anytime soon, as natural left-handedness remains relatively rare. So you can safely assume that the pair of scissors you’re buying off the shelf is designed to be used with your right hand.