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.
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Margaret Alic, PhD