Handedness

Handedness is a person's preference for using one hand when performing manual tasks.

Handedness describes a characteristic form of specialization whereby an individual prefers the use of one hand over the other for specialized coordinated activities such as writing and eating. Accordingly, an individual who uses his or her right hand for activities requiring skill and coordination (e.g., writing, drawing, cutting) is defined as right-handed. Roughly 90% of humans are right-handed, with approximately nine right-for every one left-hander. Fossil records show that this strong preference for the right hand has been with humans for at least a half a million years. Lefthanded people who have been forced to write with their right hand may develop the ability to write with both hands. The term ambidexterity is often used to denote balanced handedness. Some scientists believe that the direction of handedness is a matter of chance, with lefthandedness arising from a lack of bias toward the right hand rather than from a genetic source.

KEY TERMS

Ambidexterous—
The ability to use the right and left hands equally well.
Cerebral hemispheres—
The left and right parts of the cerebrum in the brain of vertebrates.
Handism—
Aform of discrimination against people based on whether they are left-or right-handed.
Sensory receptors—
In the body's sensory system, a sensory nerve ending that responds to stimuli in the internal or external environment of an organism, including humans and other vertebrates.

An often misunderstood phenomenon, handedness is a result of the human brain's unique development. While the human brain is intuitively understood as a single entity, research in brain physiology and anatomy has demonstrated that various areas of the brain control different mental aptitudes and that the physiological structure of the brain affects mental functions. The two cerebral hemispheres make the brain a dual structure, and this duality is an essential quality of the human body as each hemisphere is connected to sensory receptors on the opposite side of the body. In other words, the right hand is controlled by the left hemisphere of the cerebral cortex.

While humans and most animals are considered bilaterally symmetrical, meaning the same on both sides, humans have a clearly defined type of hemispheric dominance compared to animals. Animals may favor right or left paws, but only humans are predominantly right-handed. Primates have shown only a weak preference for the right hand, suggesting that evolution has directed both humans and animals which way to go. The American developmental psychologist Arnold Gesell (1880–1961), known for his pioneering work in scientific observation of child behavior in the early twentieth century, discovered that infants display signs of handedness as early as the age of four weeks. At that age, Gesell reported, right-handed children assume a fencing position, right arm and hand extended, and by the age of one, righthandedness is clearly established. The one-year-old child uses the right hand for a variety of operations and the left for holding and gripping.

The evolution of human handedness is associated with hemispheric specialization for language. Investigation of gestural communications in primate species have reported a strong degree of population-level right-handedness compared to manual, noncommunicative activities, which were primarily left-handed in the primates studied. Researchers have hypothesized that the evolution of gestural communication in these human ancestors might have affected lateralization and may constitute precursors of the hemispheric specialization for language.

Predominant right-handedness in humans has led researchers to determine whether right-handedness is genetically coded. Although handedness has been reported in studies of families and twins to be a genetic trait, no single gene has been found to be responsible. Instead, scientists consider handedness to be a multifactorial trait determined by many genetic and environmental factors. The question has been asked: If handedness is due to a single gene and right-handedness dominates the human species, why has left-handedness not disappeared? But if multiple genes are involved, a trait such as handedness would be more difficult to erase completely. A British study of 4,000 twins provided little support for genetic theories and no links were found to specific genes. Another piece of the left-handed gene puzzle is that an individual with two left-handed parents has only a 35% chance of being left-handed. Nevertheless, the search for genetic links to handedness continues, and the focus is on genes known to help determine development of leftright asymmetry in bilateral species.

For many years, left-handedness was wrongly associated with mental deficiency, as well as emotional and behavioral problems. This erroneous thinking was unsupported by research and led to the popular belief, strengthened by folklore, that left-handed people are somehow flawed. Even language contributes to maligning left-handed people. The word left evolved from the Anglo-Saxon lyft, which means weak. The Latin word sinister, meaning left and unfavorable, is still used to denote something evil; and gauche, the French word for left, generally indicates awkwardness. In addition, lefthandedness has been associated with immunological problems and a shorter life span. While not devoid of foundation, these ideas are based on inconclusive and sometimes even deceptive evidence. For example, statistics may indicate a shorter life-span for left-handers, but statistical analysis omits the fact that higher mortality is associated with accidents occurring in an often dangerous right-hand world.

Bias against left-handers takes many forms, including the design of tools solely for use by right-handers. However, an even greater challenge to left-handers than right-handed scissors and can openers is what contemporary psychologist Stanley Coren (1942–) calls handism, an underlying belief that right-handedness is better than left-handedness. The idea that left-handers need to conform to a dominant standard was at one time translated into punitive educational practices whereby lefthanded children were physically forced to write with their right hand. While awareness has grown among educators and parents that left-handedness should not be suppressed, the left-handed child is still exposed to a variety of pressures to conform, some subtle, such as “the right hand of God” favoring right handedness, and some crude, including an association between the left hand and personal hygiene. In contrast, certain cultures such as the indigenous people of the Andes consider left-handers to possess spiritual abilities, including magic and healing powers. The left hand represents wisdom in tantric Buddhism, and the Russian word for lefthander, levsha, is a common noun for skilled craftsman.

See also Left-brain hemisphere ; Right-brain hemisphere .

Resources

BOOKS

Coren, Stanley. The Left-Hander Syndrome: The Causes and Consequences of Left-Handedness. New York: Vintage Books, 1993.

Wolman, David. A Left Hand Turn Around the World: Chasing the Mystery and Meaning of All Things Southpaw. Cambridge, MA: De Capo Press, 2006.

PERIODICALS

Frayer, D. W., et al. “More than 500,000 Years of Righthandedness in Europe.” Laterality 17 (January 2012): 51–69.

Mequerditchian, A., et al. “On the Origins of Human Handedness and Language: A Comparative Review Of Hand Preferences for Bimanual Coordinated Actions and Gestural Communictcation in Nonhuman Primates.” Developmental Psychobiology 55 (September 2013): 637–50.

WEBSITES

Scientific American. “Why are more people right-handed?” http://www.scientificamerican.com/article/why-are-more-people-right/ (accessed August 6, 2015).