Although on the outside our bodies look symmetrical, our body movements are anything but. If you’re like most people, you write, use a phone, eat, and perform just about any task that requires tactile dexterity with your right hand. A small fraction of the population, comprising around 10% of the population, is left-handed. Rarer still are those who can use either hand with equal ease for various, though not necessarily all, tasks. These people are known as ambidextrous, with fewer than 1% of the population capable of this feat.
It isn’t generally understood why some people are ambidextrous, but the limited research conducted thus far suggests it all starts in the brain. Ambidexterity isn’t as great as it sounds either, as studies have associated ambivalent handedness with poor cognitive and mental health outcomes.
What determines hand preference?
The brain is divided into the left and right hemispheres by a deep longitudinal fissure of nerves called the corpus callosum. You probably know about these hemispheres and you may have also heard that the left hemisphere handles language, learning and other analytical processes while the right hemisphere processes images and emotions, among other things. This has inevitably led to the erroneous notion that some people who are “more logical” are left-brained while those who are “more creative” are right-brained.
Despite this enduring belief, there’s no such thing as being “right-brained” or “left-brained.” We’re actually “whole-brained” since we use both hemispheres when speaking, solving math, or playing an instrument. But that’s not to say that the brain’s two regions aren’t specialized — and the actual science of how the two halves of the brain work together may be stranger than fiction.
Without going into lengthy details about how the brain performs its division of labor across all areas, we can simply observe our motor functions to see brain lateralization in action. In all vertebrates, the right hemisphere controls the left side of the body via the spinal cord and vice versa. The jury’s still out on why that is, but some scientists believe that this basic organizational feature of the vertebrate nervous system evolved even before the appearance of vertebrates.
Over 90% of humans are naturally right-handed, a proclivity that may start as early as the womb. This suggests that handedness — the tendency to be more skilled and comfortable using one hand instead of the other for tasks such as writing and throwing a ball — is genetic in nature. However, like most aspects of human behavior, it’s like a complex trait that is influenced by numerous other factors, including the environment and chance.
Until not too long ago, it was thought that a single gene determined handedness, but more recently scientists have identified up to 40 that may contribute to this trait. Each gene has a weak effect in isolation, but together their sum is greater than their parts, playing an important role in establishing hand preference.
These genes are associated with some of these brain asymmetries, especially of language-related regions. This suggests links between handedness and language during human development and evolution. For instance, one implicated gene is NME7, which is known to affect the placement of the visceral organs (heart, liver, etc.) on the left to right body axis—a possible connection between brain and body asymmetries in embryonic development.
However, handedness is not a simple matter of inheritance — not in the way eye color or skin tone is, at least. While children born to left-handed patterns are more likely to be left-handed themselves compared to children of right-handed parents, the overall chance of being left-handed is relatively low in the first place. Consequently, most children born out of left-handed parents are right-handed. Even among identical twins, many have opposite hand preferences.
According to a 2009 study, genetics contribute around 25% toward handedness, the rest being accounted for by environmental factors such as upbringing and cultural influences.
In the majority of right-handed people, language dominance is on the left side of the brain. However, that doesn’t mean that the sides are completely switched in left-handed individuals — only a quarter of them show language dominance on the right side of the brain. In other words, hand preference is just one type of lateralized brain function and need not represent a whole collection of other functions.
Since writing activates language and speech centers in the brain, it makes sense that most people use their right hand. However, most individuals do not show as strong a hand preference on other tasks, using the left hand for some, the right hand for others, with the notable exception of tasks involving tools. For instance, even people who have a strong preference for using their right hand tend to be better at grabbing a moving ball with their left hand; that’s consistent with the right hemisphere’s specialization for processing spatial tasks and controlling rapid responses.
Ambidexterity may hijack brain asymmetry — and that may actually be a bug, not a feature
This brings us to mixed-handedness, in which some people have a preference for a particular hand for certain tasks. A step above are ambidextrous people, who are thought to be exceptionally rare and can perform tasks equally well with both hands.
But if the picture of what makes people left or right handed is murky, ambidexterity is even more nebulous. We simply don’t know why a very small minority of people, fewer than 1%, is truly ambidextrous. And from the little we know, it doesn’t sound like such a good deal either.
Studies have linked ambidexterity with poor academic performance and mental health. Ambidextrous people perform more poorly than both left- and right-handers on various cognitive tasks, particularly those that involve arithmetic, memory retrieval, and logical reasoning. Being ambidextrous is also associated with language difficulties and ADHD-like symptoms, as well as greater age-related decline in brain volume. The findings suggest that the brain is more likely to encounter faulty neuronal connections when the information it’s processing has to shuttle back and forth between hemispheres.
Again, no one is sure why this is the case, nor are any of these studies particularly robust since ambidextrous people comprise such a small fraction of the general population and any study involving them will naturally involve a small sample size that invites caution when interpreting results in a statistically meaningful way. All scientists can say for now is that naturally ambidextrous people have an atypical brain lateralization, meaning they simply have brain circuitry and function that is likely different from the normal pattern we see in right-handed and left-handed people.
Of course, it’s not all bad news for the handedness-ambivalent. Being able to use both hands with (almost) equal ease certainly has its perks, which can really pay off, especially in sports, arts, and music.
Can you train yourself to be ambidextrous?
Left-handers have always been stigmatized, often being punished in school and forced to use their non-dominant right hand. However, starting with the late 19th-century, people have not only become more tolerant of left-handedness but some have actually gone as far as to praise the merits of ambidexterity and worked to actively promote it by teaching others how to use both their hands well.
For instance, in 1903, John Jackson, a headteacher of a grammar school in Belfast, founded the Ambidextral Culture Society. Jackson believed that the brain’s hemispheres are distinct and independent. Being either right or left hand dominant effectively meant that half of your brainpower potential was being wasted. To harness this potential, Jackson devised ambidexterity training that, he claimed, would eventually allow each hand “to be absolutely independent of the other in the production of any kind of work whatever… if required, one hand shall be writing an original letter, and the other shall be playing the piano, with no diminution of the power of concentration.”
Although these claims have been proven to be bogus, to this day you can find shady online programs that claim to teach you to become ambidextrous. Training involves all sorts of routines such as using your non-dominant hand for writing, brushing your teeth, and all sorts of daily activities that require the fine manipulation of a tool. Doing so would allow you to strengthen neural connections in the brain and activate both hemispheres, which may help you think more creatively — or so they claim. But that’s never been shown by any study I could find. On the contrary, if anything, ambidextrous training may actually hamper cognition and mental health, judging from studies on natural ambidextrous people.
“These effects are slight, but the risks of training to become ambidextrous may cause similar difficulties. The two hemispheres of the brain are not interchangeable. The left hemisphere, for example, is typically responsible for language processing, whereas the right hemisphere often handles nonverbal activities. These asymmetries probably evolved to allow the two sides of the brain to specialize. To attempt to undo or tamper with this efficient setup may invite psychological problems,” Michael Corballis, professor of cognitive neuroscience and psychology at the University of Auckland in New Zealand, wrote in an article for Scientific American.
“It is possible to train your nondominant hand to become more proficient. A concert pianist demonstrates superb skill with both hands, but this mastery is complementary rather than competitive. The visual arts may enhance right-brain function, though not at the expense of verbal specialization in the left hemisphere. A cooperative brain seems to work better than one in which the two sides compete.”
Handedness is a surprisingly complex trait that isn’t easily explained by inheritance. Whether you’re left or right handed, this doesn’t make you necessarily smarter or better than the other. Brain lateralization exists for a reason, and that should be celebrated.