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.
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
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
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
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
years. Early hand preference may be due to a pathological problem (e.g.
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:
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
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
, or language-processing disorders, including
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:
Once a child's handedness becomes apparent, parents or caregivers
should never try to change it. Parents can assist left-handed children by
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:
Teachers should help lefthanders to hold a pencil and place the paper in
ways that are appropriate for left-handed writing.
—Equally competent with either hand.
—A function that is dominated by either the left or the right
hemisphere of the brain.
—An assumed bias that favors one group over another; as in
systems and hand implements that assume that all people are
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