Handedness
Handedness, or hand preference, refers to an individual's tendency to favor one hand over the other for various tasks such as writing, throwing, and eating. The dominant hand is primarily determined by genetic factors present before birth, with about 85 to 90 percent of the global population being right-handed and 10 to 15 percent left-handed. Evolutionary studies suggest that right-handedness has been favored for hundreds of thousands of years, likely due to the brain's left hemisphere controlling motor skills and language functions, which are essential for survival.
A small percentage of individuals are ambidextrous, meaning they can use both hands equally well. The genetics of handedness involve two alleles: the D gene, which promotes right-handedness, and the C gene, which allows for random determination of handedness. Cultural perceptions of left-handedness have varied significantly throughout history, often leading to stigma and attempts to change left-handed individuals to right-handedness. Despite these challenges, left-handed individuals may excel in certain sports due to their unique hand skill set. Overall, handedness remains a fascinating aspect of human biology and cultural understanding.
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Handedness
Handedness, or hand preference, is an individual's inclination to use one hand over the other to complete tasks such as writing, throwing, or eating. Handedness is determined by the presence of certain genes in a person's deoxyribonucleic acid (DNA) before birth. One gene will make the person right-handed, while the other will randomly select whether a person becomes right-handed or left-handed. About 85 to 90 percent of people in the world are right-handed, while the other 10 to 15 percent are left-handed.
The evolution of humans by natural selection likely determined the human race's skew toward right-handedness; for reasons that are not fully known, the body's speech and motor skills became delegated to the left hemisphere of the brain, which controls the right side of the body. The complexity of human language skills indicates a strong left part of the brain. This gave rise to a preference for the right hand in most of the world population, although left-handedness survives due to heritable genes. An extremely small percentage of people are ambidextrous, or skilled in using the right and left hands equally well to perform the same tasks.
Background
Handedness in an individual is determined by genes, meaning it is left largely to chance. A 2011 study found that humans have strongly favored using their right hands for tool use for at least the last five hundred thousand years. This conclusion came from the study of early human fossils discovered near Atapuerca, Spain. The researchers believed these humans were the ancestors of the Neanderthals of Europe, demonstrating that humanity's general preference for using the right hand is one of the oldest surviving traits of human beings.
A 2012 study illustrated this point. That year, an international team of researchers analyzed the skeleton of a Neanderthal man who died tens of thousands of years ago in France. Scientists had initially claimed, after the skeleton was discovered in 1957, that the larger muscles on the man's right arm indicated that the man had been right-handed. The 2012 researchers attempted to confirm this by studying scratch marks on the man's teeth. Neanderthals used their mouths like extra hands, the scientists argued, resulting in their teeth wearing down from continually grinding against hard objects.
The research team discovered that the teeth of the Neanderthal were scratched from the right angle. This, combined with the muscularity of the man's right arm, confirmed to the scientists that the Neanderthal had been right-handed. Studies of other Neanderthal teeth showed similar results, with few Neanderthal teeth being scratched from the left.
The research team later wrote that the pattern of right-handedness in Neanderthals indicates a trend of dominance by the left hemisphere of the brain, since the left hemisphere controls the right side of the body, and the right hemisphere controls the left side. The scientists argued that this left-hemisphere dominance could mean that Neanderthals were also proficient in language use, given that the left side of the brain also controls language skills in human beings.
The abundant evidence for people deriving their motor skills principally from the left hemisphere of the brain suggests that the human race has been mostly right-handed for hundreds of thousands of years. Scientists have found only inconsistent hand preference in tool use in humans' closest living relatives, the other great apes, which include chimpanzees and gorillas.
Impact
In the 2020s, approximately 90 percent of people in the world were right-handed, while about 10 percent were left-handed. Less than 1 percent of people were ambidextrous. Scientists have researched genetics—the study of genes and heredity, or the passing of biological traits from parent to offspring—to discover how a person's handedness is determined before birth, and why, in terms of human evolution, left-handedness has survived as a heritable trait at all, considering its rarity; two left-handed parents have only about a 25 to 30 percent chance of producing left-handed offspring.
One theory of how an individual's hand preference is produced involves two alleles, or two variations of the same gene, at the same genetic location in a person's DNA. One allele is called a D gene, meaning dextral. The other is called a C gene, meaning chance. The presence of the D gene creates right-handedness in a person. This gene is simply more common in human beings, the theory states, and it is therefore more likely to be passed down to offspring, who will then become right-handed themselves.
The C gene, meanwhile, occurs in people less frequently. It decides handedness in an individual entirely randomly. People with the C gene have a 50 percent chance of becoming either right-handed or left-handed. Additionally, these people can, in some cases, be influenced by their environment to stop using their naturally preferred hand in favor of the other hand. Children who naturally prefer their left hands, for instance, may be taught by their parents to use their right hands while growing up and could eventually become right-handed.
Other scientific research shows that the genetic material that controls handedness also determines the body's overall symmetry, or its quality of being made up of identical parts that face one another across an axis. Any disturbance of these genes can skew this symmetry before birth and cause a person to be born with a condition called situs inversus, which reverses the placement of the body's internal organs. The heart, for instance, in an individual with situs inversus is located on the right side of the body rather than on the left.
However, the development of this condition before birth has not been shown to skew people's natural hand preference, despite the disturbed genes being responsible for determining handedness. This suggests to scientists that other genetic material compensates for the genes that have been disturbed to maintain the body's natural hand preference.
Throughout human history, the right-handed majority of people regarded left-handed people with suspicion. In the Middle Ages and earlier, left-handed people were believed to be unclean, witches, or possessed by the devil. In the mid to late twentieth century, some parents and teachers tried to force naturally left-handed children to write with their right hands. Into the twenty-first century, left-handed children in schools may struggle to use right-handed scissors or writing desks. At the same time, left-handed people might become more skilled in some sports, such as boxing and racquet sports.
Bibliography
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