Right
and left-handedness in humans
Why do humans,
virtually alone among all animal species, display a distinct left or
right-handedness? Not even our closest relatives among the apes possess such
decided lateral asymmetry, as psychologists call it. Yet about 90 per cent of
every human population that has ever lived appears to have been right-handed.
Professor Bryan Turner at Deakin University has studied the research literature
on left-handedness and found that handedness goes with sidedness. So, nine out
of ten people are right-handed and eight are right-footed. He noted that this
distinctive asymmetry in the human population is itself systematic. “Humans
think in categories: black and white, up and down, left and right. It’s a
system of signs that enables us to categories phenomena that are essentially
ambiguous.’
Research has shown
that there is a genetic or inherited element to handedness. But while
left-handedness tends to run in families, neither left nor right handers will
automatically produce off-spring with the same handedness; in fact, about 6 per
cent of children with two right-handed parents will be left-handed. However,
among two left-handed parents, perhaps 40 per cent of the children will also be
left-handed. With one right and one left-handed parent, 15 to 20 per cent of
the offspring will be lefthanded. Even among identical twins who have exactly
the same genes, one in six pairs will differ in their handedness.
What then makes
people left-handed if it is not simply genetic? Other factors must be at work
and researchers have turned to the brain for clues. In the 1860s the French
surgeon and anthropologist, Dr Paul Broca, made the remarkable finding that
patients who had lost their powers of speech as a result of a stroke (a blood
clot in the brain) had paralysis of the right half of their body. He noted that
since the left hemisphere of the brain controls the right half of the body, and
vice versa, the brain damage must have been in the brain’s left hemisphere.
Psychologists now believe that among right-handed people, probably 95 per cent
have their language center in the left hemisphere, while 5 per cent have right
sided language. Left-handers, however, do not show the reverse pattern but
instead a majority also have their language in the left hemisphere. Some 30 per
cent have right hemisphere language.
Dr Brinkman, a
brain researcher at the Australian National University in Canberra, has
suggested that evolution of speech went with right-handed preference. According
to Brinkman, as the brain evolved, one side became specialized for fine control
of movement (necessary for producing speech) and along with this evolution came
righthand preference. According to Brinkman, most left-handers have left
hemisphere dominance but also some capacity in the right hemisphere. She has
observed that if a left-handed person is brain-damaged in the left hemisphere,
the recovery of speech is quite often better and this is explained by the fact
that left-handers have a more bilateral speech function.
In her studies of
macaque monkeys, Brinkman has noticed that primates (monkeys) seem to learn a
hand preference from their mother in the first year of life but this could be
one hand or the other. In humans, however, the specialization in (function of
the two hemispheres results in anatomical differences: areas that are involved
with the production of speech are usually larger on the left side than on the
right. Since monkeys have not acquired the art of speech, one would not expect
to see such a variation but Brinkman claims to have discovered a trend in
monkeys towards the asymmetry that is evident in the human brain.
Two American
researchers, Geschwind and Galaburda, studied the brains of human embryos and
discovered that the left-right asymmetry exists before birth. But as the brain
develops, a number of things can affect it. Every brain is initially female in
its organization and it only becomes a male brain when the male fetus begins to
secrete hormones. Geschwind and Galaburda knew that different parts of the
brain mature at different rates; the right hemisphere develops first, then the
left. Moreover, a girl’s brain develops somewhat faster than that of a boy. So,
if something happens to the brain’s development during pregnancy, it is more
likely to be affected in a male and the hemisphere more likely to be involved
is the left. The brain may become less lateralized and this in turn could result
in left-handedness and the development of certain superior skills that have
their origins in the left hemisphere such as logic, rationality and
abstraction. It should be no surprise then that among mathematicians and
architects, left-handers tend to be more common and there are more left-handed
males than females.
The results of
this research may be some consolation to left-handers who have for centuries
lived in a world designed to suit right-handed people. However, what is
alarming, according to Mr. Charles Moore, a writer and journalist, is the way
the word “right” reinforces its own virtue. Subliminally he says, language
tells people to think that anything on the right can be trusted while anything
on the left is dangerous or even sinister. We speak of lefthanded compliments
and according to Moore, “it is no coincidence that lefthanded children, forced
to use their right hand, often develop a stammer as they are robbed of their
freedom of speech”. However, as more research is undertaken on the causes of
left-handedness, attitudes towards left-handed people are gradually changing
for the better. Indeed, when the champion tennis player Ivan Lendl was asked
what the single thing was that he would choose in order to improve his game, he
said he would like to become a lefthander.
Geoff
Maslen
Questions 1 – 7
A Dr Broca
B Dr Brinkman
C Geschwind and Galaburda
D Charles Moore
E Professor Turner
Example Answer
Monkeys do not show a species-specific
preference for left or right-handedness. B
1 Human being started to
show a preference for right-handedness when they first developed language.
2 Society is prejudiced
against left-handed people.
3 Boys are more likely to
be left-handed.
4 After a stroke,
left-handed people recover their speech more quickly than righthanded people.
5 People who suffer
strokes on the left side of the brain usually lose speech their power of.
6 The two sides of the
brain develop different functions before birth.
7 Asymmetry is a common
feature of the human body.
Questions 8 – 10
Using the information in the passage, complete the
table below. Write your answers in boxes 8 – 10 on your answer sheet.
|
Percentage of children left-handed |
|
|
One parent
left-handed |
...
(8) ... |
|
Both parents
left-handed |
...
(9) ... |
|
Both parents
right-handed |
...
(10) ... |
Questions 11 – 12
Choose the appropriate letters A – D and write
them in boxes 11 and 12 on your answer sheet.
11 A study of monkeys has
shown that.
A monkeys are not usually right-handed.
B monkeys display a capacity for speech.
C monkey brains are smaller than human brains.
D monkey brains are asymmetric.
12 According to the writer, left-handed
people.
A will often
develop a stammer.
B have undergone hardship for years.
C are untrustworthy.
D are good tennis players.