lunging 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 structure."
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 cells.
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
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
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
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 neurons.
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.
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