Why Are We Right Or Left Handed?

Handedness Facts

Evolution spent nearly 400 million years crafting these works of art, two of the most important pieces of the human evolutionary puzzle. Yet 99% of humans are good with one hand and not the other for everyday tasks like writing, high-fiving, and all-important one-handed texting. Even life seems to have chosen sides: amino acids are “left-handed,” and DNA turns into a right-handed helix. Both hands seem fully capable, connected to fully functional arms.

In the animal kingdom, where people find creatures who prefer one paw, hoof, or wing, it’s usually 50/50. Yet on average, only 1 out of 10 humans are southpaws. Why are so few people left-handed? Maybe because it’s the world that is conspiring against them.

If only someone would open a chain of stores specifically stocked with products for left-handers. That’s a money-maker. Even language hates lefties. Correcting correct is to be right, and proper is a word from “right.” Gods are full of righteousness. To be skillful is to have mastery.

Why are we right or left-handed?

The reasons behind handedness, whether someone is right-handed or left-handed, are not fully understood. However, handedness is believed to result from a complex interplay of genetic, environmental, and neurological factors. Here are some key points regarding handedness:

Genetic Factors: Studies have suggested a genetic component to handedness. While the specific genes involved are not yet fully identified, research indicates that handedness runs in families. However, the inheritance pattern is not straightforward, and it is likely influenced by multiple genes rather than a single gene determining handedness.

Brain Lateralization: Handedness is connected to brain lateralization, which refers to the division of certain functions between the brain’s hemispheres. In most people, the left hemisphere of the brain controls the right hand’s language and fine motor skills, while the right hemisphere controls the left hand’s spatial awareness and fine motor skills. The mechanisms underlying brain lateralization and its link to handedness are still under investigation.

Developmental Factors: The development of handedness is believed to occur during the early stages of brain development. Environmental factors, such as intrauterine position, birth stress, and early childhood experiences, might influence the development of handedness. However, the specific mechanisms by which these factors contribute to handedness are still being studied.

Cultural and Social Influences: Cultural and social factors can also shape handedness. Cultural practices, traditions, and educational systems may influence the prevalence of right-handedness or the expectation of right-handed behavior. This can lead to a higher frequency of right-handed individuals in certain societies.

The right lung is divided into three lobes, while the left has only two. Two areas for speech and language are found only on one lobe in the brain, usually the left. The starfish show similar internal asymmetry. The interesting thing isn’t that human bodies aren’t asymmetrical in the first place, but we all have the same asymmetry.

About 1 in 20,000 people on Earth have a genetic condition called situs inversus, where the internal organs are inverted left to right. These people usually show no negative effects from their reversed innards. Highly symmetrical bodies might signal quality genes, and looking for balanced bodies might make better offspring.

There are many symmetries and more than balanced bodies when humans pair up. To define left from right is to determine the head, tail, back, and belly. But that only sculpts our outer shell. The orientation of organs may have to go with the flow. Some reasons cause left or right-handed people:

  • For the brain.
  • For gene.
  • Natural selection.

For brain function

Left itself comes from Old English, meaning “weak.” Words for left give us sinister. But language turns out to be an essential part of the puzzle. Our brains are cross-wired, meaning the left side and vice versa control the right hand. It doesn’t mean our brain’s symmetrical.

In the 1860s, French scientist Paul Broca discovered that a region of our brains used for speech processing is usually located on one side. Language is a complex process, and the move to one hemisphere probably helps the brain deal with it more efficiently.

In 99% of right-handers’ brains, Broca’s area is located only in the left hemisphere. In left-handers’ brains, it’s also on the left 70% of the time—only 19% of lefties process speech in the right hemisphere, and 20% use both. So language and hands are not directly linked.

Two similar theories have been proposed to explain this. They suggest that as early humans evolved, a gene mutant popped up that threw a whole bunch of our brain’s functions to one side and allowed them to specialize.

For genetic behavior

Handedness is genetic, and technology has allowed us to more accurately explore the numerous genes associated with all types of human tendencies and behaviors. In 2007 researchers found a gene LRRTM1 involved in the development of handedness. There are two alleles of that gene: the “D” gene and the “C” gene.

  • The D gene is more frequent in all populations and promotes right-handed preference.
  • The C gene does not promote left-handedness but is a chance gene.

It appears that 50% of people with this a left-handed, and 50% are right-handed. The discovery also helps explain some of the well-known phenomena regarding handedness. More specifically, you can teach someone to use their non-dominant hand and become ambidextrous. The leading theory is that it affects the asymmetry of the brain. The LRRTM1 gene also slightly increases a person’s risk of developing schizophrenia.

If both parents are leftists, there’s a 26% chance that their baby will be more than double the normal odds. Premature babies are more likely to be left-handed. There’s also evidence that ultra-sonic scans increase the chances of a baby being a left-hander. All of which is an elaborate way of saying nobody knows.

Language and hand dominance move to the left hemisphere if someone has two copies of the Right Shift mutation. That shift is slightly less if they carry one copy and if they don’t carry the Right Shift mutation at all. Language and handedness could go to the right or left by chance. It means instead of their being a gene for left-handedness, it’s the lack of a gene.

A genetic influence makes sense since parents sometimes give their kids handedness. While two right-handed parents only have a 9% chance of having a left-handed child, one parent being left-handed raises that to 19%, and two to 26%. Research has found evidence of right-hand preference in captive chimps.

For natural selection

There’s even another theory. Human bodies tell right from left when it comes to guts. The whole brain-hand-sidedness can be traced back to embryonic days. In 2013, researchers reported that some of the same genes tell spleens and stomachs which way is which are linked to brain asymmetry.

If being right-handed and shifting our speech and coordination to one side was such an excellent thing for evolution, why do lefties still exist? A simpler theory is that nature enjoys variety. Chris McManus believes that left-handers may have what he calls “random cerebral variation,” their brains are more shuffled up.

It might lead to more unexpected connections, cross-talk between the right and left hemispheres and maybe more creativity. What’s clear is that having diverse brains has made humans a much more exciting species.

How do humans become right and left-handed?

The brains of our primate relatives work the same way, but scientists believe the human brain is more asymmetrical. So it may not be a coincidence that the side of our brain in charge of speech controls the hand used to produce written language. Natural selection could make most righties because the left brain tells our right hands how to write.

This theory is helped by studies that have found that many lefties tend to process language more evenly across both hemispheres of their brains. In some cases, entirely on the right side. So it could be that lefties don’t exactly inherit their hand dominance. Instead, they inherit a lack of neurological bias toward the dominant left brain.

Handedness, or hand dominance, is unique to humans among primates. Close relatives like chimps and gorillas do not consistently favor one hand over the other. While many cats and dogs favor one paw over another, the breakdown between righties and lefties is roughly equal. Though many believe one exists, scientists have yet to discover a gene determining the dominant hand. Though handedness runs in families, that doesn’t necessarily mean it’s hereditary.

Identical twins share the same genes and don’t always have the dominant hand. Many theories believe that our right-handed world is an evolutionary byproduct of human cooperation. According to one study released in 2012, cooperative acts, like sharing tools, have favored the presence of a dominant hand within a group. So everyone can use the same tools and stuff.

The more widely accepted theory is that hand dominance is connected to the asymmetry of our brains. Our brains are contralateral, which means the left hemisphere of the cerebral cortex. Language and logical processing are usually localized and control our body’s right side. The right hemisphere, where spatial recognition usually takes place, controls the left side of our body.

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Holder MK. “What does Handedness have to do with Brain Lateralization (and who cares?)”.
Papadatou-Pastou, Marietta; Ntolka, Eleni; “Human handedness: A meta-analysis.” Psychological Bulletin. 146 (6): 481–524.
Santrock JW, “A Topical Approach to Life-Span Development.”

Julia Rose

My name is Julia Rose. I'm a registered clinical therapist, researcher, and coach. I'm the author of this blog. There are also two authors: Dr. Monica Ciagne, a registered psychologist and motivational coach, and Douglas Jones, a university lecturer & science researcher.I would love to hear your opinion, question, suggestions, please let me know. We will try to help you.

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