美國英語聽力80篇2 Lesson38

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Left-handed

Why are some people left-handed?
Scientists have been trying to answer that question for many years.
Research has shown that ninety percent of people naturally use their right hands for most tasks.
But hundreds of millions of people use their left hands.
A study done in 1992 found that men are more likely to be left-handed than women.
It also found that Asian or Hispanic people are less likely to be left-handed than white people,
black people or North American Indians.
Some cultures accept people who do things mostly with their left hands.
Others do not.
For example, in some countries eating with the left hand is considered an insult to others.
In the past, some parents and teachers forced lefthanded children to use their right hands instead.
Scientists want to know the reason for left-handedness
because it is closely linked to mental problems like schizophrenia and language difficulties.
One idea about the cause of left-handedness is the genetic theory.
It says that people are right- or left-handed because of genes passed to them by their parents.
For example, it has been shown that the handedness of adopted children
is more likely to follow that of their birth parents than their adopted parents.
Other evidence of genetic involvement can be found in some families.
One famous example is the left-handed members of the present British Royal Family.
These include the Queen Mother, her daughter Queen Elizabeth the Second,
the Queen's son Prince Charles and his son Prince William.
Amar Klar at the National Cancer Institute in Washington D.C. believes right-handed people are born with a gene for it.
But, he says, about twenty percent of people do not have the right-handed gene.
These people could be either left- or right-handed.
Doctor Klar wants to find this gene.
He plans to test the genes of members of 100 families
in which at least one parent is right-handed and two of the children are left-handed.
He will look for markers in the DNA to see how often the children have the same markers as the parents.
Stanley Coren of the University of British Columbia in Canada thinks the reason for left-handedness is linked to fetal development.
This idea says left-handedness results from damage to a fetus during or before birth.
One cause of such damage could be an unusual position of the fetus in the mother's uterus.
Another cause could be unusual levels of chemicals in the uterus.
Doctor Coren says this damage could explain why a larger percentage of left-handed people have mental and health problems.
Both these ideas have been offered to explain a situation that has been,a mystery to scientists for a long time:
how can two babies who developed from the same egg and have the same genes use different hands?
In 18% of identical twins,one twin is right-handec and the other is left-handed.
One possible answer is that both twins lack the right-handed gene,
so each has a chance to be either right-or left-handed.
Another possible answer is that the difference is the result of damage caused by both babies being in a crowded uterus.
Still other scientists say handedness could have more than one cause which could include both genetics and development.
Experts also link left-handedness to artistic and athletic ability as well as extremely high intelligence.
They say a great number of left-handed people were famous political leaders, artists, scientists or athletes.
These include the political leaders Alexander the Great and Napoleon Bonaparte. The musicians Sergei Rachmanineff and Ludwig Van Beethoven.
The artists Michelangelo,Leonardo da Vinci and Pablo Picasso.
The scientist Albert Einstein.
And the baseball player Babe Ruth.
Other experts say every American president in the last twenty-five years has been at least partly left-handed,except for Jimmy Carter.
They are Gerald Ford, Ronald Reagan,George Bush and Bill Clinton.
However, some left-handed people were famous criminals.
They include Billy the Kid, Jack the Ripper and the Boston Strangler.
Much interesting information about left-handedness can be found on the Internet computer network.