NYT article on left-handedness (from which I copied list in previous post) (it will not copy on this "copy and paste")
May 16, 2000
On Left-Handedness, Its Causes and Costs
By DAVID E. ROSENBAUM
FREDERICK, Md. -- In medieval
times, right-handed warriors had a
distinct advantage in swordfights.
They held their shield with their left hand
-- over their heart -- and thus lived to
fight another day, and to reproduce, even
after they had been stabbed.
That, suggested Thomas Carlyle, the 19th
century British writer, is why so many
more people are right-handed. The lefties
never lived to have offspring.
It is a preposterous theory. Chances are,
Carlyle, a brilliant essayist and himself
left-handed, offered it tongue in cheek to
tweak his contemporary Charles Darwin,
whom he did not care for.
But the truth is, the question of what
causes people to be right-handed or
left-handed is nearly as much of a
mystery today as it was when Carlyle
addressed it in 1871.
The latest word comes from a geneticist
at the National Cancer Institute laboratory
here who has been working for years
with yeast and mutant mice and who has
developed a novel theory that he believes
will explain why 9 out of 10 people are
right-handed, why left-handed parents are
more likely to have left-handed children
and why identical twins often have
different handedness.
The geneticist, Dr. Amar J. S. Klar,
hypothesizes that most people have a
specific dominant gene that makes them
right-handed. But about 20 percent of
people, under this theory, lack the
right-handed gene, and these people
without the gene have a 50-50 possibility
-- a random chance -- of being
right-handed or left-handed.
Whether a person has or lacks this gene,
Dr. Klar supposes, is a function of
conventional genetics, just like eye color
or baldness.
The gene has yet to be identified --
assuming it exists. Dr. Klar is assembling
a group of about 100 families that he
plans to subject to genetic testing. If
everything goes right, he said in an
interview in his lab, he hopes to be able to
prove his theory by isolating a
right-handed gene, perhaps within three
years.
Across the continent, at the University of
British Columbia in Vancouver, Dr.
Stanley Coren, a psychologist, has been
working for years to document his view
that left-handedness has nothing to do
with genetic variables and almost
everything to do with prenatal traumas --
with some sort of stress that damages the
fetus.
Many researchers around the world do
not take the all-or-nothing stands of Dr.
Klar and Dr. Coren but hold that both
genetics and development are involved.
But what are the genetic elements? What
are the developmental ones? There is no
consensus.
The question of what causes handedness
is interesting as a matter of curiosity and
pure science. It is hard to think of another
behavioral difference so observable and so
fundamental that has not been explained.
But there is also a practical reason to want
to know the answer. Left-handedness is
closely identified with mental illnesses like
schizophrenia and language difficulties like
dyslexia and stuttering. If scientists knew
the connection, they might be better able
to deal with the problems.
"I'm not interested in handedness per se,"
Dr. Klar said. "I'm interested in how the
brain works, and handedness is the only
external feature we can monitor, because
we can't look at the brain directly."
Born and reared in India, Dr. Klar, who is
right-handed, came to this country to
attend graduate school at the University of
Wisconsin and did postdoctoral work at
the University of California at Berkeley.
Before going to the National Cancer
Institute, he worked under Dr. James D.
Watson (a lefty), the co-discoverer of the
structure of DNA, at the Cold Spring
Harbor laboratory on Long Island.
His studies of handedness, he said, began
with his interest in the fact that yeast,
single-cell organisms, have two distinct
sides. His theory about the right-handed
gene and the absence of this gene in
lefties, he said, grew out of studies of
mutant mice.
In most mice, the heart is on the left side.
But there is a mutant strain of mice that
have hearts on the right. When the
mutants mate, half the offspring have
hearts on the left and half on the right.
Last year, Dr. Klar said, geneticists
identified a gene that is present in normal
mice but totally missing in the mutants.
"If it can work in mice," Dr. Klar asked,
"why can't it work for handedness in
people?"
The beauty of the theory is that it explains
a phenomenon that has long baffled
geneticists: how can identical twins with
exactly the same genetic makeup have
different handedness, as 18 percent of
them do? Dr. Klar's explanation is that
these twins lack the right-handed gene,
and each one has an equal chance of
being right-handed or left-handed.
He dismisses as inconclusive studies that
militate against his genetic theory,
showing that more men than women are
left-handed and that left-handed mothers
have a disproportionate number of
left-handed sons. Possibly, he said, these
findings can be explained by cultural
factors like a tendency of left-handed
males to be more stubborn than
left-handed females in resisting efforts to
switch them to using their right hands.
Hoping to prove his hypothesis, Dr. Klar
plans to test blood samples or cheek
swabs from the members of 100 families
in which at least one parent is
right-handed and two of the children are
left-handed. "I'm looking for markers in
their DNA to see how often these
particular children inherit particular
markers," he said.
He has a rigid definition of what
constitutes right-handedness. He asks
people which hand they use to throw a
ball, use a spoon, saw, sew, shoot
marbles, bowl, cut with a knife, cut with
scissors, hammer and write. He does not
consider people right-handed who
routinely use their left hands for any of
these activities.
As a research biologist who spends his
life testing hypotheses, Dr. Klar said, he is
all too familiar with going down blind
alleys. But he said he was confident he
would be able to isolate the right-handed
gene.
Others experts wonder. One skeptic is
Dr. Coren, the psychologist in British
Columbia, who created a splash about a
decade ago with his book, "The Left-Handed Syndrome" (Free Press, 1992),
which maintains that lefties die younger and are more prone to accidents than
right-handers. (Dr. Coren's avocation, incidentally, is writing about dogs, and
he has published such page-turners as "How to Speak Dog" and "What Do
Dogs Know?")
Dr. Coren is convinced that humans are naturally right-handed and that
left-handedness is largely a consequence of what he calls "birth stress," by
which he means factors like an unnatural placement that causes damage to
the fetus in the womb.
Prenatal brain injuries explain why a larger percentage of left-handers have
psychological and emotional problems, he said. He believes left-handedness
runs in families because of a predisposition of the women in those families to
have difficult pregnancies. As for the paradox of identical twins, he said
twins are always subjected to birth stress because of crowding in the womb.
Two other scientists who have studied handedness -- Dr. Walter F.
McKeever, a psychologist at the University of Toledo in Ohio, and Dr. Daniel
H. Geschwind, a neurologist at the University of California at Los Angeles --
said they believed that handedness was heavily based in genetics. But they
said they doubted a single gene was responsible, as Dr. Klar suggests. And
they give more weight than Dr. Klar to developmental factors.
"Handedness is a complex behavior," Dr. Geschwind said, "and no complex
behavior has ever been shown to be due to only a single gene without any
environmental influence."
But Dr. Klar is undaunted. "What's good for yeast should be good for mice,
and what's good for mice should be good for men," he said. "Biology is
biology, and DNA is DNA."
Copyright 2000 The New York Times Company |
