Right-Handed or Left-Handed: Unlocking the Secrets of Brain Asymmetry

Discover why some individuals favor their right hand while others prefer their left, how this relates to brain asymmetry, and whether retraining left-handed children is beneficial or detrimental.

1. Are people truly born right- or left-handed, or is it learned?
2. How is handedness inherited genetically?
3. Do right-handed individuals always outnumber left-handed ones?
4. Are there right- and left-handed animals, and which is more common?
5. What evolutionary advantages does hand preference provide?
6. Is there truth to the idea that left-handed people are more creative or talented?
7. Should left-handed individuals be retrained to use their right hand?

Are Handedness Traits Innate or Acquired?

Human brains exhibit lateralization, meaning functions are divided between the left and right hemispheres. This leads to dominance of one hand, foot, ear, and eye. The most visible sign of this brain lateralization is hand preference.

Approximately 90% of the global population is right-handed.

The left hemisphere, which controls language centers, governs the right hand's movements.

Ultrasound studies reveal that by the ninth week of gestation, about 75% of fetuses begin moving their right hand, and by the fifteenth week, they tend to suck their right thumb. Near birth, the fetus often turns its head to the right side.

This right-side preference may partly arise from anatomical factors. For instance, in birds, egg positioning exposes the embryo's right eye to light through the shell, influencing brain lateralization. Eggs incubated in darkness produce chicks without this asymmetry.

Similarly, mammalian embryos have specific uterine positioning and circulatory development contributing to the dominance of the left hemisphere and right hand.

However, about 10% of people predominantly use their left hand. In the 1980s, researchers Geschwind and Galaburda proposed that elevated prenatal testosterone might inhibit left hemisphere development, shifting some functions to the right hemisphere, leading to left-handedness.

This hormonal theory explains why stress during pregnancy correlates with higher rates of left-handed children.

Other factors include genetics, higher left-handedness among twins, premature babies, children born to older or smoking mothers, and a slight male predominance among left-handers (12 men to 10 women).

How Is Handedness Passed Down?

Left-handedness is hereditary. Children with left-handed parents have a higher likelihood of being left-handed themselves. Around 40 genetic loci have been linked to hand preference, including PCSK6, involved in early left-right body symmetry, and LRRTM1, which affects synapse organization in specific brain neurons.

Are Right-Handers Always More Numerous?

Assessing hand preference can be complex because people may use different hands for different tasks. The dominant hand usually performs more complex actions.

The Edinburgh Handedness Inventory, a widely used test from 1971, asks about 20 activities such as writing, drawing, using scissors, combing hair, brushing teeth, sweeping, throwing, opening boxes, and dealing cards.

Each task is scored +1 or -1 depending on the hand used. A positive total indicates right-handedness; negative indicates left-handedness.

However, some tasks may not be relevant across all ages or cultures. For example, children may not play cards, older adults may not use tennis rackets, and some tasks like sweeping have become less common due to vacuum cleaners.

Cultural influences also affect results; in some societies, like China and historically the Soviet Union, writing with the left hand is discouraged, and in Muslim cultures, the left hand is considered unclean.

To minimize cultural bias, studies focus on universal actions like throwing and hammering.

Analysis shows Papua New Guinea has the highest proportion of left-handers—around 25%—while countries like the USA and UK have about 10%. Despite variations, right-handers remain the majority worldwide.

Handedness in the Animal Kingdom

For a long time, brain asymmetry was thought to be uniquely human due to language centers in the left hemisphere. However, since the 1970s, lateralization has been observed in lab animals like rats and chicks.

Even ancient Cambrian creatures, such as trilobites, showed asymmetry in predator bite marks favoring one side.

Brain lateralization is now recognized as a fundamental trait across animal species, including simple organisms like the nematode Caenorhabditis elegans.

Different species display varied limb preferences. Chimpanzees are predominantly right-handed, as shown through termite fishing tests. Frogs and chicks tend to be right-limbed, while parrots favor their left foot. Dogs show facial lateralization rather than paw preference, and cats display mixed preferences but are generally considered left- or right-pawed depending on the task.

Evolutionary Purpose of Brain and Hand Asymmetry

While the exact mechanisms behind hand preference remain unclear, the benefits are evident. Brain lateralization allows for more efficient function by reducing duplication.

Animals with asymmetrical brains respond faster and more accurately to environmental stimuli compared to symmetrical individuals.

Studies on fish and birds highlight this: chicks can simultaneously scan for food with one eye and watch for predators with the other. Fish bred to lack asymmetry react slower to threats.

In humans, lateralization may have enhanced tool use and technological development by focusing skills in one hand. Archaeological evidence shows early tools were crafted for right-handed use.

Despite this, a stable proportion of left-handers persists, possibly due to the 'fighting hypothesis'—left-handers may have an advantage in combat by surprising right-handed opponents.

Are Left-Handed People More Creative or Talented?

Supporting the fighting hypothesis, left-handers are overrepresented in contact sports like boxing, fencing, football, and baseball. However, in individual sports like running or gymnastics, no such advantage exists.

Left-handers also contribute intellectually; they are more common among gifted children with above-average IQs. Some research suggests left-handers excel in architecture and mathematics, possibly due to enhanced right hemisphere development.

Additionally, studies indicate that left-handed men with college degrees may earn slightly higher incomes than their right-handed peers.

On the downside, left-handedness is more frequent among individuals with autism or schizophrenia. Average life expectancy for left-handers is slightly lower, possibly due to accidents in a right-handed world. Left-handedness is also more common among homosexual individuals.

Should Left-Handed Individuals Be Retrained?

In the past, left-handed children, especially in Soviet and Chinese schools, were often forced to switch to the right hand, viewed as a disciplinary issue.

Fortunately, such prejudices have diminished, and education systems now aim to provide equal opportunities. Specialized training programs exist for left-handed students pursuing careers like surgery.

Retraining is not only unnecessary but harmful, leading to psychological stress and reduced learning ability. Left-handed individuals should embrace their uniqueness and explore fields like architecture, mathematics, and contact sports while awaiting a more left-hand-friendly world.

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