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Study Finds Genetic Link Between Intelligence and Size of Some Regions of
November 5, 2001
By NICHOLAS WADE
Plunging into the roiled waters of human intelligence and its
heritability, brain scientists say they have found that the size of
certain regions of the brain is under tight genetic control and that the
larger these regions are the higher is intelligence.
The finding is true only on average and cannot be used to assess an
individual's intelligence, said Dr. Paul M. Thompson, the leader of the
research team and a pioneer in mapping the structure of the brain.
The measurement of intelligence has long been a controversial issue, and
even more so the efforts to tease out the relative contributions of
heredity and environment.
Dr. Bruce L. Miller, a neurologist at the University of California at San
Francisco and an expert on brain changes in Alzheimer's disease, said Dr.
Thompson's work was "an exciting study that starts to show there are some
brain areas in which there are very significant genetic influences on
And Dr. Robert Plomin, a psychologist who studies intelligence at the
Institute of Psychiatry in London, said the high correlation found
between the size of certain areas of the brain and general intelligence
"does make it harder to dismiss intelligence as some meaningless
construct, as some want to do."
Dr. Thompson, who is at the University of California at Los Angeles, uses
a type of brain scanning called magnetic resonance imaging, which can
show the difference between gray matter and white matter in the living
brain. The gray matter consists of brain cells, while the white matter
comprises the bundles of wiring with which the cells communicate with one
another. The amount of gray matter is a measure of the number of brain
The human brain seems to be divided into modules that perform separate
tasks. The frontal lobes are involved in planning and risk assessment,
while regions at the back of the brain handle visual processing. Dr.
Thompson has tried to discover if the relative size of the brain's
modules is under genetic control by studying how their size varies in
With the help of colleagues in Finland, where a national registry of
twins is maintained, he scanned the brains of identical and fraternal
pairs of twins and measured the size of the brain modules. Qualities that
are under genetic control show a characteristic pattern of varying hardly
at all between identical twins, who have the same genes; quite a lot
between fraternal twins, who share about half their genes; and a great
deal between unrelated individuals.
The researchers had their computer draw three-dimensional maps of each
subject's brain, and then color coded the modules' degree of
heritability. In an article published in today's issue of Nature
Neuroscience, they report that the quantity of gray matter in the frontal
lobes was under particularly tight genetic control, as was a region at
the side of the left hemisphere known as Wernicke's area, which is
central to language.
Dr. Thompson's reason for probing the genetic control of brain structure
was to uncover genes that might be involved in mental diseases that can
be inherited, like schizophrenia and autism. But he and his colleagues
also wished to understand the role of brain modules in healthy
individuals, so they gave their subjects intelligence tests and found
that intelligence was significantly linked with the amount of gray matter
in the subjects' frontal lobes.
Dr. Thompson said the findings were "the first maps of the degree to
which the genes control brain structure." There were only 40 subjects in
his study - 10 pairs of identical twins and 10 pairs of fraternal twins -
but the results gave "enough statistical power to identify the key brain
systems," he said.
He expressed surprise that the amount of gray matter in the frontal lobes
turned out to be correlated with intelligence in his study "because you
wouldn't think something as simple as gray matter would affect something
as complicated as intelligence." But the amount of gray matter, which is
related to the number of brain cells, perhaps reflects something that
bears more directly on intelligence, like the number of cell- to-cell
connections, he said.
Dr. Plomin, who wrote a commentary on the study in the journal, said the
larger volume of gray matter could be the cause of higher intelligence,
or it could be the other way around - people with a stronger motivation,
say, might exercise their brains harder and develop a higher density of
As brain-scanning studies like Dr. Thompson's become more refined, they
raise the possibility that a scan could be used to gauge various elements
of personality or behavior.
Dr. Thompson said he believed that as brain scans become increasingly
informative they will raise issues of personal privacy just as genetic
testing has done, and should be protected with similar safeguards.
The size of gray matter in the frontal lobes cannot be used to measure an
individual's intelligence, he said. Some potential uses, such as scanning
to compare the intelligence of different groups, would be unethical, he
added. "It would be remiss to use technology developed for disease for
those types of goals," he said.
Copyright 2001 The New York Times Company
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