Why are some people left-handed? Left-handedness has long fascinated scientists, educators, and curious minds. Is it genetic? Is it shaped in the womb? Or is it a product of culture and environment?
In this article, I explore the many factors that are considered the causes of left-handedness, including brain anatomy, prenatal hormones, societal pressure, and plain old chance.
Whether you’re a lefty, know one, or just want to understand what makes us use one hand over the other, this guide breaks it all down in an easy-to-understand way.
Why are people left-handed? Let’s find out…
Genetic Factors: Is Left-Handedness Inherited?
Is left-handedness genetic? Scientists have been asking this very question for decades. And the short answer is: yes, genetics can play a role, but it’s not the whole story.
Left-Handedness in Families
Let’s start with what we see in families. If both your parents are left-handed, you’re more likely to be left-handed too — around a 24% chance, according to studies. That’s much higher than the general population, where only about 10%-12% of people are left-handed. But even with two lefty parents, most kids still end up right-handed. So it’s clear that genetics isn’t the only factor at play.
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One of the biggest studies on this was done using data from over 25,000 twins in Australia and the Netherlands (Medland et al., 2008). It found that about 25% of the differences in handedness could be explained by genetics. That means the other 75% likely comes from other things — like random chance, brain development in the womb, or even cultural influences.
The “Left-Handed Gene”? Not Exactly
You might’ve heard people talk about a “left-handed gene,” but scientists haven’t found a single gene that decides which hand you’ll use. Instead, it looks like many different genes may each play a small part — kind of like pieces of a puzzle.
A major study in 2019 looked at genetic data from over 400,000 people in the UK. It found a few specific genes linked to being left-handed, especially ones that affect how the brain is built and how nerve cells communicate. One of those genes, called LRRTM1, has even been linked to differences in brain structure and mental health.
So, Is It in Your DNA?
Kind of — but it’s not as simple as eye color or blood type. Handedness is what scientists call a polygenic trait, which means lots of genes are involved, and none of them work alone.
Plus, even identical twins (who have the exact same DNA) don’t always have the same handedness. That tells us there’s more going on than just genetics — like what happens in the womb, how the brain develops, or maybe even just random chance.
Key Takeaways
- Genetics plays a role in handedness, with heritability around 25%.
- Multiple genes—especially those related to brain structure—have been implicated.
- There is no single gene responsible for left-handedness.
- Environmental and random developmental factors also significantly influence handedness.
Brain Lateralization and Hemispheric Dominance
Okay, so we’ve talked about how genes might influence whether someone is left-handed — but what’s going on in the brain? It turns out that handedness is closely tied to how our brains are wired, especially when it comes to something called brain lateralization.
What Is Brain Lateralization, Anyway?
Our brains are split into two halves — the left hemisphere and the right hemisphere — and they each control different parts of the body. Most of the time, the left side of your brain controls the right side of your body, and the right side of your brain controls the left side of your body. This criss-cross setup is totally normal.
Now here’s where it gets interesting: in most people (even lefties), the left hemisphere is dominant when it comes to language and logic. But for many left-handed people, the brain is a little more flexible — sometimes both hemispheres are more equally involved, or the right hemisphere takes on more responsibility. It’s not better or worse, just different.
Cool fact: Studies using brain scans have shown that about 70% of left-handed people still process language mostly on the left side of the brain — but the remaining 30% are either right-dominant or use both sides more equally.
Why Does This Matter for Handedness?
The theory is that differences in how the brain organizes itself — especially during early development — might influence which hand becomes dominant. If one side of the brain is more active or develops differently, it might lead to a preference for using the opposite hand.
In other words, handedness could be one visible clue that your brain works a bit differently under the hood. That’s why researchers often study left-handed people when they want to understand more about brain symmetry, language processing, and even creativity.
Left-Handed Brains Are Just… Different
And by different, I mean diverse. Brain lateralization isn’t all-or-nothing. There’s a spectrum, and left-handed people tend to fall at different points along it more often than right-handed folks. That may explain why left-handers are slightly overrepresented in certain fields — like music, art, math, or sports that rely on fast reflexes and spatial awareness.
But let’s be clear: being left-handed doesn’t automatically make someone more creative or more talented. It just means the brain may be wired in a less typical, but totally normal way.
Key Takeaways:
- The brain is divided into two sides — each controls the opposite side of the body.
- Left-handedness is linked to how the brain is organized, especially in early development.
- Left-handers show more variety in brain lateralization patterns.
- It’s not about being “better” or “worse” — just wired a bit differently.
Prenatal Environment and Hormones
Aside from genetics and brain structure, there’s another theory that’s gained a lot of attention over the years: that being left-handed may be shaped before you’re even born! Researchers believe that things happening in the womb—like hormone levels or birth complications—might play a role in determining which hand you’ll favor.
The Role of Hormones (Especially Testosterone)
Are people born left-handed? One of the most talked-about ideas in this area is the “testosterone theory”. The basic idea is that higher levels of testosterone in the womb might slightly affect how the brain develops, possibly leading to changes in brain asymmetry — which in turn could increase the likelihood of being left-handed.
This theory was first popularized by neurologist Norman Geschwind and researcher Albert Galaburda in the 1980s. They suggested that testosterone could slow the growth of the left hemisphere of the brain during fetal development, causing the right hemisphere (which controls the left hand) to take over certain functions — like language and motor control.
This is sometimes called the “Geschwind–Galaburda hypothesis.”
It’s an interesting theory, but it’s still debated and hasn’t been definitively proven.
Birth Complications and Left-Handedness
Some researchers have also noticed that left-handedness is slightly more common in people who experienced complications around birth — such as premature delivery, low birth weight, or a lack of oxygen during delivery. The thinking is that these kinds of stresses could affect how the brain organizes itself early on.
A 1986 study found that individuals who experienced perinatal complications were more likely to be non-right-handed. However, this doesn’t mean birth complications cause left-handedness — just that there might be a weak correlation.
Important note: These are small effects. Most left-handed people were born without any complications, and many people with rough births are right-handed. So nothing here is set in stone.
Random Developmental Factors
Even without hormones or complications, brain development in the womb isn’t perfectly symmetrical — and sometimes, nature just leans one way over the other. That’s why some scientists believe random variation during brain development might also help explain handedness. It’s not always about outside forces — sometimes it’s just how your brain happened to wire itself.
Key Takeaways:
- Hormone levels, especially testosterone, may influence handedness during fetal development.
- Some studies link birth complications to a slightly higher chance of being left-handed.
- Random variation during brain development may also play a role.
- These are all possible contributors, but not proven causes.
Cultural and Environmental Influences
While genetics, brain structure, and prenatal factors all play a role in handedness, we can’t ignore the impact of culture and environment. Believe it or not, which hand you use can be influenced by the society you grow up in, the tools you’re taught to use, and even the teachers and parents around you.
A History of Right-Hand Bias
For a long time, many cultures around the world saw left-handedness as something “wrong” or “unnatural.” In some cases, left-handed children were forced to switch hands for writing or other tasks — a practice known as “hand-switching” or retraining.
In Western countries, this was pretty common until the mid-20th century. Teachers would make left-handed students write with their right hand, sometimes even tying their left hand behind their back. This didn’t make them right-handed — it just caused frustration, awkward handwriting, and sometimes even stuttering or confusion.
Studies suggest that people who were forced to switch hands often retain their natural hand preference in other activities, like throwing or brushing their teeth — showing how deeply ingrained handedness really is.
Handedness Around the World
Interestingly, the number of left-handed people varies depending on the country — not because of biology, but because of cultural attitudes.
In places where left-handedness is more accepted, like the U.S., Canada, or the U.K., about 10–12% of the population is left-handed. But in countries like China, India, or Indonesia, rates are much lower — sometimes as low as 2–4% — because left-handedness is often discouraged from an early age.
For example, a 2007 study from Taiwan found that 59.3% of naturally left-handed children were forced to switch to their right hand, often due to cultural expectations and parents’ education levels.
Environment and Everyday Tools
Beyond social pressure, the physical environment can influence hand use. Most tools, instruments, and workspaces are designed for right-handed people — from scissors and can openers to desks and computer mice. Over time, this can cause some left-handed individuals to adapt and use their right hand for certain tasks simply out of convenience.
And in some cultures, there are taboos around using the left hand — particularly in contexts like eating or greeting others. In these cases, children may be subtly or overtly encouraged to use their right hand, even if their natural inclination is otherwise.
Key Takeaways:
- Many cultures historically discouraged left-handedness, especially in school settings.
- Cultural pressure can lower reported rates of left-handedness, especially in parts of Asia and the Middle East.
- The design of tools and environments can influence how people use their hands, especially if they face barriers using their dominant one.
- Today, cultural stigma is decreasing — but it still affects handedness in some parts of the world.
A Mix of Factors – Why There’s No Single Cause
What causes left-handedness? By now, you’ve probably noticed a pattern: there’s no one reason why someone ends up a lefty. Genetics, brain structure, prenatal hormones, culture, the environment — they all play a part. But none of them tell the whole story on their own.
It’s Not Just Nature vs. Nurture — It’s Both
We love neat explanations, but handedness isn’t that simple. Think of it more like a recipe with lots of ingredients. Some people might be left-handed mostly because of genetics, while others might be influenced more by prenatal factors, or even cultural pressures.
For example:
- One person might have a family history of left-handedness and inherit a subtle brain wiring pattern that leads them to favor their left hand.
- Another might be born in a country where left-handedness is discouraged, so even though their brain naturally leans that way, they switch early on.
Randomness Plays a Role, Too
One important thing scientists have discovered is that random variation during brain development — especially in the womb — might explain a lot about why someone turns out left-handed. The brain is incredibly complex, and small differences early in development can lead to different outcomes.
In other words, sometimes it just happens, and there’s no clear “why.”
Why This Matters
Understanding that there’s no single cause for left-handedness helps us move away from outdated myths — like the idea that lefties are “broken,” “bad,” or even “gifted” by default. Being left-handed is just a natural part of human diversity, shaped by a bunch of different factors working together.
Key Takeaways:
- Handedness doesn’t have one single cause — it’s influenced by genes, brain structure, prenatal factors, culture, and chance.
- Even identical twins can have different dominant hands.
- Scientists estimate only about 25% of handedness is explained by genetics — the rest is a mix of environmental and random developmental factors.
- The complexity of handedness reflects how wonderfully unique human development really is.
Conclusion: Left-Handedness Is a Fascinating Puzzle
So, why are some people left-handed? As it turns out, there’s no simple answer — and that’s what makes it so interesting. Handedness is shaped by a blend of biology, environment, and a bit of randomness.
Genetics might point the way, but things like brain development, hormones in the womb, cultural attitudes, and even life experiences all help determine which hand you’ll end up favoring.
What we do know is this: being left-handed is completely natural and part of the rich variety of human traits. Whether you’re a lefty, a righty, or somewhere in between, handedness is just one small piece of what makes each of us unique.
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