2016.5.21 雅思阅读回忆left-handed or right-handed

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Being Left- handed or Right-handed?

The world is designed for right-handed people. Why does a tenth of the population prefer the

left?

A.

The

probability

that

two

right- handed

people

would

have

a

left- handed

child

is

only

about 9.5 percent. The chance rises to 19.5 percent if one parent is a lefty and 26 percent if both

parents are left-handed. The preference, however, could also stem

from an infant’s imitation of

his parents. To test genetic influence, starting in the 1970s British biologist Marian Annett of the

University of Leicester hypothesized that no single gene determines handedness. Rather, during

fetal

development,

a

certain

molecular

factor

helps

to

strengthen

the

brain’s

left

hemisphere,

which increase the probability that the right hand will be dominant, because the left side of the

brain controls the right side of the body, and vice versa. Among the minority of people who lack

this factor, handedness develops entirely by chance. Research conducted on twins complicates

the

theory,

however.

One

in

five

sets

of

identical

twins

involves

one

right-handed

and

one

left- handed person, despite the fact that their genetic material is the same. Gene, therefore, are

not solely responsible for handedness.

B.

Genetic

theory

is

also

undermined

by

results

from

Peter

Hepper

and

his

team

at

Queen’s University in Belfast, Ireland. In 2004 the psychologists used ultra sound to show that b

y

the 15th week of pregnancy, fetuses already have a preference as to which thumb they suck. In

most cases, the preference continued after birth. At 15 weeks, though, the brain does not yet

have control over the body’s limbs. Hepper speculates that fetuses

tend to prefer whichever side

of

the

body

is

developing

quicker

and

that

their

movements,

in

turn,

influence

the

brain’s

development. Whether this early preference is temporary or holds up throughout development

and

infancy

is

unknown.

Genetic

predetermination

is

also

contradicted

by

the

widespread

observation

that

children

do

not

settle

on

either

their

right

or

left

hand

until

they

are

two

or

three years old.

C.

But

even

if

these

correlations

were

true,

they

did

not

explain

what

actually

causes

among animals. Cats will favor one paw over another when fishing toys out from under the couch.

Horses stomp more frequently with one hoof than the other. Certain crabs motion predominantly

with

the

left

or

right

claw.

In

evolutionary

terms,

focusing

power

and

dexterity

in

one

limb

is

more efficient than having to train two, four or even eight limbs equally. Yet for most animals, the

preference for one side or the other is seemingly random. The overwhelming dominance of the

right

hand

is

associated

only

with

hu

mans.

That

fact

directs

attention

toward

the

brain’s

two

hemispheres and perhaps toward language.

D.

Interest in hemispheres dates back to at least year, at a medical conference,

French physician Marc Dax reported on an unusual commonality among his patients. During his

many years as a country doctor, Dax had encountered more than 40 men and women for whom

speech was difficult, the result of some kind of brain damage. What was unique was that every