Handedness is the preferred use of the right hand, the left hand, or one or the other depending on the task.
Handedness is defined and categorized in different ways. Most people define handedness as the hand that one uses for writing. Within the scientific community some researchers define handedness as the hand that is faster and more precise for manual tasks. Others define it as the preferred hand, regardless of its abilities. Whereas some people always use their right hand or their left hand for most activities, others use one hand or the other depending on the activity. Still other people can use either hand for most functions.
Lefthanders usually prefer using their left hand for delicate tasks; however, there is no good method for predicting which hand a lefthander will choose for a specific task. Although left-handed children usually are more flexible in their hand usage than right-handers, this may be because they are forced to function in a world designed for right-handers.
There is no standard measure for determining degrees of handedness. Some scientists believe that there are only two types of handedness: right and non-right. These researchers believe that true left-handedness is rare and that most lefthanders are really mixed-handed. Others believe that ambidexterity—the equal use of both hands—is a third type of handedness, and some think that there are two types of ambidexterity. Other scientists believe that handedness should be measured on a continuum from completely right-handed to completely left-handed.
It is commonly estimated that about 10 percent of the human population is left-handed or ambidextrous. Boys are about 1.5 times more likely than girls to be left-handed. Archeological evidence indicates that the proportion of left-handed to right-handed people was about the same 30,000 years ago as it is in the early 2000s.
In the past there were many social and cultural biases against left-handed children. In particular, left-handed children often were forced by parents or teachers to use their right hand for eating and writing. In the early 2000s the frequency of left-handedness appears to be on the increase. This may be due to the increased acceptance of children determining their own hand preferences. Left-handedness appears to be rarer in restrictive societies as compared with more permissive societies.
Basis of handedness
The physical basis of handedness is not well-understood. Through the centuries left-handedness has been attributed to numerous physical, psychological, and supernatural causes.
Each hemisphere of the brain has some specialized functions, a poorly-understood phenomenon called brain lateralization. In the late nineteenth century Paul Broca, a French neurosurgeon, identified an area of the left hemisphere that has a major role in the production of speech. Carl Wernicke, a German neurologist, identified another region in the left hemisphere that was responsible for language comprehension. Broca suggested that people's handedness was the opposite of their language-specialized hemisphere, so that a person with left-hemisphere language specialization would be right-handed. Thus until the 1960s, handedness was believed to be indicative of brain lateralization. Between 70 and 90 percent of humans have language specialization in their left hemispheres. The remainder may have right-hemisphere specialization or no real distinction between the two hemispheres in language specialization. However, among lefthanders, about 50 percent process language on both sides of their brains, 10 percent process language primarily in their right brains, and the remainder process language primarily in their left brains.
The 1987 Geschwind-Behan-Galaburda (GBG) Theory of Left-Handedness suggested that left-handedness was a result of some brain injury or trauma or chemical variations in the fetal environment, such as high levels of the male hormone testosterone.
For decades during the twentieth century scientists argued about whether there is a genetic basis for handedness. Children of left-handed parents have a 50 percent chance of being right-handed and 18 percent of identical twins differ in their handedness. Furthermore, right-handed twins are equally as likely as their left-handed twins to have left-handed children. A 2003 study appeared to identify a single gene that controls both handedness and the direction that hair spins on the scalp. An individual possessing at least one copy of the dominant form of this gene is both right-handed and has a clockwise hair spiral. However, when an individual has two copies of the recessive form of the gene—one copy from the mother and one copy from the father—the gene does not determine handedness. Thus 50 percent of these individuals are right-handed and 50 percent are non-right-handed. Furthermore, these individuals have a separate 50 percent likelihood of hair that spins clockwise or counterclockwise.
Handedness determines few if any lateralized behaviors other than fine finger dexterity. However, one study showed that right-handers preferred turning to their left side and non-right-handers preferred turning to their right side. Turning to the right or left is strongly correlated with turning toward the side of the brain that has less dopamine, an important brain hormone.
In his pioneering work on child behavior, the American developmental psychologist Arnold Gesell claimed that infants as young as four weeks display signs of handedness and that right-handedness is clearly established by age one. However, it was as of 2004 commonly believed that babies are born ambidextrous. Although a hand preference may seem apparent towards the end of the first year, this is not necessarily due to right- or left-handedness and may change several times over the subsequent few years.
Toddlers usually go through phases of using one hand for some activities and the other hand for other activities. Although many children exhibit clear left- or right-handedness from the age of two—and handedness usually is determined during the third year—it is not unusual for a child to repeatedly switch hand preferences well into their preschool years. Early hand preference may be due to a pathological problem (e.g. stroke ).
It may be hard for right-handers to appreciate the daily problems confronting non-right-handers. Although most of these difficulties are simply annoying or frustrating, others can cause physical injury or serious life-long problems. Most systems and tools are designed for right-handers and so have an intrinsic bias. Many items, such as screws and light bulbs, require a left-to-right turning that is easier for a right-hander. Items designed specifically for right-handers include:
- cooking utensils
- can openers
- computer keyboards
- sports equipment
- musical instruments, especially stringed instruments
Non-right-handed children must either learn to use tools with their right hand, which can be awkward, inefficient, and frustrating, or to use tools backwards with their left hand. The latter can be dangerous for a child.
In the past left-handed children often were forced to write with their right hand. In the early 2000s, a non-right-handed child may still feel pressure to conform to a right-handed world. Most parents and teachers as of 2004 probably accept that it is wrong to attempt to suppress or change a child's handedness. Nevertheless, lefthanders may still suffer at school. A teacher may label a lefthander's writing as "sloppy" because of an unconscious reaction to handwriting that looks different. Left-handed children may hook their wrists while writing in order to see the paper. However, hand or wrist twisting can reduce legibility and writing fluency. This problem is avoidable with correct positioning of the paper for the lefthander. In art and science lefthanders may struggle with tools, instruments, and equipment designed for the right-handed majority. Some left-handed children may seem clumsy as they try to adapt to a right-handed world.
At various times in the past, left-handedness has been wrongly associated with numerous physical, mental, emotional, and behavioral disorders. However, there is some evidence that left-handed people may be more at risk for schizophrenia , bipolar disorder , or language-processing disorders, including dyslexia and stuttering .
Many children repeatedly switch hand preferences until at least the age of three. This is normal unless it seems to interfere with the child's fine motor skills . By the age of two most children should be able to do the following:
- hold a fork and spoon well enough to feed themselves
- handle small objects well
- hold the paper in place while drawing
Once a child's handedness becomes apparent, parents or caregivers should never try to change it. Parents can assist left-handed children by the following:
- placing their table settings according to their handedness
- providing left-handed scissors
- helping them find the easiest ways to handle paper and pencil
- assuring that teachers and caregivers treat their lefthanders appropriately
- not dwelling on their child's non-right-handedness
Left-handed children can become very frustrated when they are trying to imitate a right-handed parent or sibling, particularly with activities such as shoe-tying. In these cases the parent or sibling should sit across from the child—rather than next to or behind the child—so as to be the child's mirror.
If handedness is not apparent by the time a child enters school, the teacher must determine which hand the child should learn to write with. Observing which hand the child consistently uses for various activities—or whether the child switches hands when repeating the activity—can help the teacher make this determination. Example of such activities include:
- holding a spoon
- cutting with scissors
- playing with puppets
- using a lock and key
- hammering nails
- screwing lids on jars
- throwing a ball
Teachers should help lefthanders to hold a pencil and place the paper in ways that are appropriate for left-handed writing.
When to call the doctor
A child who exhibits a strong preference for one hand at about one year of age—too early to clearly express handedness—may have a weakness or neuromuscular problem in the other arm or hand.
Ambidextrous —Equally competent with either hand.
Brain lateralization —A function that is dominated by either the left or the right hemisphere of the brain.
Intrinsic bias —An assumed bias that favors one group over another; as in systems and hand implements that assume that all people are right-handed.
Annett, Marian. Handedness and Brain Asymmetry: The Right Shift Theory. New York: Taylor & Francis, 2002.
Healey, Jane M. Lefties: How to Raise Your Left-Handed Child in a Right-Handed World. New York: Pocket Books, 2001.
Brodie, Chris. "Head in Hand." American Scientist 92, no. 1 (January/February 2004): 27.
"Handed Down from Distant Generations." New Scientist 180, no. 2415 (October 4–10, 2003): 21.
Vlachos, F., and F. Bonoti. "Handedness and Writing Performance." Perceptual and Motor Skills 98, no. 3 (June 2004): 815.
Handedness Research Institute. Hillcrest Psychology Research Center, 674 East Cottage Grove Avenue, Suite 222, Bloomington, IN 47408. Web site: <handedness.org>.
Hackney, Clinton. "The Left-Handed Child in a Right-Handed World." Zaner-Bloser Inc. Available online at <www.zaner-bloser.com/html/HWsupport1.html> (accessed December 27, 2004).
Holder, M. K. Gauche! Left-Handers in Society. Available online at <www.indiana.edu/~primate/lspeak.html> (accessed December 27, 2004).
——. What does Handedness have to do with Brain Lateralization (and who cares?). Available online at <www.indiana.edu/~primate/brain.html> (accessed December 27, 2004).
Shoemaker, Carma Haley. "When Left Is Right: Left-Handed Toddlers." Toddlers Today. <http://toddlerstoday.com/resources/articles/lefthand.htm> (accessed December 27, 2004).
Margaret Alic, PhD
This dominance of the right hand is a characteristic of the human species. Other animals may show a systematic preference for one hand or paw when reaching for things, or using implements, but over the population of the species as a whole there is typically no overall preference for one or other hand or paw. Even individual mice typically show a systematic preference for one paw when reaching for food in a glass tube, and the preferred paw usually has the stronger grip; but there are as many left-pawed as right-pawed mice. Chimpanzees in the wild have been observed to use sticks to extract termites from their nests, and individual animals vary with respect to which hand they prefer to use to hold the stick, but again there appears to be no overall preference. There is evidence for a slight majority of right-handers among captive chimpanzees for some activities, such as feeding, but the overall bias is only about 67%, significantly lower than the proportion of right-handed humans. The only species known to show a bias comparable to that in humans is the parrot, since roughly 90% of them prefer the left foot when picking up small objects or bits of food!
Right-handedness appears to be universal among humans. There is slight variation across cultures, but this is largely restricted to activities in which there are strong sanctions against using the left hand. For instance, in some traditional cultures it is forbidden to eat with the left hand, which in Hindu society, for instance, is regarded as ‘the hand of the privy’. Until quite recently, there were strong pressures to force left-handed children to write with the right hand, although it is now generally recognized that this is likely to do more harm than good. Forced right-handers may be more than usually prone to stuttering, and may fail to realize their full potential for manual and verbal skills.
It was once thought that the ancient Israelites must have been predominantly left-handed because Hebrew is written from right to left. This Eurocentric view ignored the many other right-to-left scripts: until about ad 1500 there were about as many right-to-left scripts as left-to-right ones. The present-day prevalence of left-to-right scripts almost certainly owes more to European expansionism than to handedness. Again, it has been argued that the ancient Egyptians were left-handed, because Egyptian art typically portrays humans in right profile, whereas the natural tendency of right handers is to draw faces and bodies in left profile. But the preference for right-sided profiles in ancient Egypt was probably due to a belief that the left side of the body is inferior and should be hidden. Indeed, the very universality of such beliefs is testimony to the universality of right-handedness itself. Evidence from works of art suggests that the proportion of right-handers has been roughly constant for at least 5000 years. Close examination of the design and wear of ancient stone tools suggests that right-handers may have been in the majority throughout the Stone Age, going back two million years or more.
Given the overwhelming majority of right-handers, it is all too easy to overlook the left-handed, or to infer that left-handedness is in some way pathological. Yet there is virtually no indication that left-handers are in any way inferior in intellect or skill, and indeed they often seem to excel. The Italian Renaissance artist and inventor Leonardo da Vinci, one of the greatest geniuses of all time, was left-handed, and confused the right-handed world by writing, in mirror-image fashion, from right to left. Several US presidents, including Harry Truman, Gerald Ford, Ronald Reagan, and Bill Clinton, join such historical figures as Alexander the Great, Julius Caesar, Cicero, Charlemagne, and King David in being left-handed. Left-handers in the entertainment world include Charlie Chaplin, Harpo Marx, Rock Hudson, Paul McCartney, and Ringo Starr. A disproportionate fraction of prominent sportspersons have been left-handed, especially in sports involving racquets and bats: Rod Laver, Jimmy Connors, John McEnroe, and Martina Navratilova in tennis; Babe Ruth in baseball; David Gower in cricket.
It is not always clear whether the prominence of left-handers in some walks of life has some biological cause, or whether being left-handed simply makes people want to prove themselves in a right-handed world. The advantage they often seem to show in competitive sport may also be due to the unexpectedness, to right-handers, of their movements and actions.
It is sometimes suggested that the prevalence of right-handedness is simply due to environmental pressure. We live in a right-handed world. The location of door handles, the way pages are arranged in books, the construction of objects such as scissors, corkscrews, nuts and bolts, golf clubs, and even access to the zipper on men's trousers (to the wearer of the trousers, that is), are all designed for the convenience of right-handers and are a source of frustration to left-handers. But the very universality of right-handedness, extending to cultures that have been isolated for tens of thousands of years, suggests that causation is the other way round: it is the biological disposition toward right-handedness that is responsible for the cultural pressure. Moreover, the stubborn persistence of left-handers as a roughly constant proportion of the population in all human societies suggests that handedness may be under genetic control.
No one has yet located a gene for handedness, if indeed there is one (or perhaps there are several); but there have been reasonably plausible attempts to capture some of the facts about human handedness in terms of a hypothetical gene. Children are more likely to be left-handed if one or both of their parents are left-handed, but handedness does not ‘breed true’, since the proportion of left-handers born to two left-handed parents is somewhat less than half. It has therefore been suggested that the handedness gene does not simply determine whether one will be right-handed or left-handed. Rather, one form (or allele) of the gene, which we may call D for dextral, may code for right-handedness, while the other allele, which we may call C for chance, leaves the direction of handedness to chance. Different versions of this idea have been proposed by Marian Annett of the University of Leicester, and by Christopher McManus of the University of London. The chance element may also explain why some people are ambidextrous rather than clearly right- or left-handed. The children of left-handed parents would then have a chance of receiving two C alleles, one from each parent, which perhaps explains why only about 50% (or less) are left-handed. The C allele also helps explain why identical twins, like Tweedledum and Tweedledee in Lewis Carroll's Through the Looking Glass, are often of opposite handedness, although other factors are likely to contribute as well.
Besides being right-handed, most of us are right-footed when kicking, right-eyed when aiming, and right-eared when listening on the telephone, and for most people it is the left side of the brain that controls speech and language. Since it is also the left brain that controls the right hand, this has led to the idea that the left side of the brain exerts a general dominance, especially over skilled actions like tool use or speech. This view has been tempered, however, by discoveries that the right brain is the more involved in other functions, such as spatial attention and orientation, and in non-verbal activities, such as art and music.
Nearly all right-handers are left-brained for language, suggesting that the D allele controls both handedness and brain dominance for language. Left-handers show a much more mixed pattern. Some are left-brained, some right-brained, and some appear to have speech represented on both sides. This is further evidence that the C allele, which is assumed to be present in the great majority of left handers, does not have a directional influence.
Throughout recorded history, and in virtually all human cultures, handedness has invited myth, superstition, and of course prejudice. One reason for this may be that handedness is not evident in the hands themselves. It is difficult to tell whether a person is left- or right-handed by physically inspecting the hands — although there may be some give-away signs like rings, or a watch, or differential signs of wear. Handedness is really only evident from the way people do things — a reflection of brain activity rather than of how the hands themselves are formed. The hidden sources of handedness may explain why it is often linked to supernatural or extrabodily sources. The right hand is often the hand of God or virtue, while the left hand is the hand of the Devil or wickedness. In the Bible there are over 100 favourable references to the right hand, and about 25 unfavourable references to the left.
The true nature of handedness is still not fully understood, and there is no reason to suppose that our modern theories are entirely free of age-old superstitions.
Michael C. Corballis
See also language and the brain; symmetry and asymmetry.
A person's preference for one hand when performing manual tasks.
The term handedness describes a characteristic form of specialization whereby a person by preference uses one hand for clearly identified activities, such as writing. For example, a person 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. Because left-handed people who are forced to write with their right hand sometimes develop the ability to write with both hands, the term ambidexterity is often used in everyday parlance to denote balanced handedness.
An often misunderstood phenomenon, handedness is a result of the human brain's unique development. While the human mind 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 our mental functions. The brain's fundamental structure is dual (there are two cerebral hemispheres), and this duality is an essential quality of the human body. Generally speaking, 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. When scientists started studying the brain's anatomy, they learned that the two hemispheres are not identical. In fact, the French physician and anthropologist Pierre Broca (1824-1880) and the German neurologist and psychiatrist Carl Wernicke (1848-1905) produced empirical evidence that important language centers were located in the left hemisphere. Since Broca's findings were based on right-handed subjects, and since right-handedness is predominant in humans, psychologists felt prompted to develop the notion of the left hemisphere as the dominant part of the brain. Furthermore, Broca formulated a general rule stating that the language hemisphere is always opposite of a person's preferred side. In other words, the left hemisphere always controls a right-handed person's language abilities. According to Broca rule's, left-handedness would indicate a hemispheric switch. Handedness research, however, uncovered a far more complex situation. While Broca's rule works for right-handers, left-handed people present a rather puzzling picture. Namely, researchers have discovered that only about two out of 10 left-handers follow Broca's rule. In other words, most left-handed people violate Broca's rule by having their language center in the left hemisphere. Furthermore, the idea of clearly defined cerebral dominance seems compromised by the fact that some 70% of left-handed people have bilateral hemispheric control of language.
While hemispheric dominance can be observed in animals, only humans have a clearly defined type of dominance. In other words, while animals may be right or left "pawed," only humans are predominantly right-handed. The American developmental psychologist Arnold Gesell (1880-1961), known for his pioneering work in scientific observation of child behavior, noted that as early as the age of four weeks infants display signs of handedness. At that age, according to Gesell, right-handed children assume a "fencing" position, right arm and hand extended; by the age of one, right-handedness is clearly established, the child using the right hand for a variety of operations, and the left for holding and gripping. Predominant right-handedness in humans has led researchers to define right-handedness as genetically coded. If left-handedness also had a genetic basis, was it possible to establish inheritance patterns? However, empirical studies, even studies of identical twins , have failed to establish left-handedness as a genetic trait. For example, a person with two left-handed parents has only a 35% chance of being left-handed.
In the past, left-handedness was associated with mental deficiency, as well as emotional and behavioral problems, which led to the popular belief, strengthened by folklore, that left-handed people were somehow flawed. In addition, left-handedness has also been associated with immunological problems and a shorter life span. While not devoid of any 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 what statistics omit is the fact that higher mortality should probably be attributed to accidents in an often dangerous right-hand world.
An even greater challenge than right-handed scissors and can openers is what psychologist Stanley Coren calls "handism," the belief that right-handedness is "better" than left-handedness. The idea that left-handers need to conform to a dominant standard has traditionally been translated into punitive educational practices whereby left-handed children were physically forced to write with their right hand. While there is a growing awareness among educators and parents that left-handedness should not be suppressed, the left-handed child is still exposed to a variety of pressures, some subtle, some crude, to conform. These pressures are reinforced by a tradition of maligning left-handed people. Major religious traditions, such as Christianity, Buddhism, and Islam, have described left-handedness in negative terms. Current language is also a rich repository of recorded animosity toward left-handers. For example, 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. The numerous expressions which imply that left is the opposite of good include a left-handed compliment.
Coren, Stanley. The Left-Hander Syndrome: The Causes and Consequences of Left-Handedness. New York: Vintage Books, 1993.
Temple, Christine. The Brain. London: Penguin Books, 1993.
handedness, habitual or more skillful use of one hand as opposed to the other. Approximately 90% of humans are thought to be right-handed. It was traditionally argued that there is a slight tendency toward asymmetrical physiological development favoring the right side of the body, and that the center of gravity is to the right of the body's midline. This, however, would seem to be the consequence of greater dependence upon the right hand rather than the cause of right-handedness.
The neurological argument holds that since the right and left sides of the body are controlled by the opposite hemispheres of the brain, the greater development of the left hemisphere results in right-handedness. Anatomical studies have demonstrated that Broca's center, the area of the cerebral cortex that controls speech and muscular coordination, is almost always better developed in the left hemisphere in right-handed individuals; in 70% of left-handed individuals these centers are located in the right brain. Psychologists have raised the possibility of a cultural explanation. Although young children can be trained to prefer the right hand against a natural inclination, there is evidence that handedness is hereditary and that denser neurological connections extending from one side of the brain or the other are present from birth. A cultural explanation is also challenged by the evidence that some other vertebrates demonstrate a preference for one hand or paw over the other.
Although it is not clear that culture is a causative agent in handedness, it is certain that the high incidence of right-handedness has shaped human society in almost every conceivable aspect. Tools, machinery, and even clothing are largely designed for the right-handed, and until fairly recently, many left-handed individuals were strongly encouraged to switch to right-handedness. In some cultures the left-handed were thought to be evil or to bring bad luck.
The term "handedness" typically refers to a person's preference for the use of a particular hand in familiar, unimanual tasks such as handwriting and throwing a ball. Depending on the criteria used, 65 to 90 percent of adults are right-handed, about 4 percent are left-handed, and the rest are mixed-handed, preferring the right hand for some tasks and the left for others. The tendency to prefer the right hand exists across cultures, and fossil evidence suggests that this preference dates to prehistoric times. Some bias toward the use of the right hand is evident even before birth, but within individual infants, hand preference often varies across time and tasks. It is not until sometime in the second year after birth that handedness becomes clearly established for the majority of children. Because the preferred hand is controlled by the opposite cerebral hemisphere, handedness brings up important questions about how functional differences develop between the two sides of the brain.
Young, Gerald, Sidney Segalowitz, Carl Corter, and Sandra Trehub, eds. Manual Specialization and the Developing Brain. New York: Academic Press, 1983.