"Our study reveals there is a heritable component to intelligence,"
said neuroscientist Paul
Thompson of the School of Medicine at the University of
California at Los Angeles, who led the research.
"The finding is particularly surprising," he said, "because you
wouldn't think something as general as the volume of gray matter could
effect something as complex as intelligence."
The brain consists of two layers: gray matter and white matter. Gray
matter is a one-quarter-inch layer of brain cells surrounding a ball of
white matter. The white matter occupies the inner core of the brain and
contains the wiring that connects the brain cells. The gray matter is
thought to be the most important part of the brain for cognition and
emotion.
Thompson and colleagues in Finland set out to determine which brain
structures are controlled predominantly by genes. They used magnetic
resonance imaging (MRI), which can distinguish between white and gray
matter, to produce brain images of identical and fraternal twins.
A comparison of the MRI scans revealed that the volume of gray matter
in the frontal lobe, the area just behind the eyes, is highly heritable.
"But the volume of gray matter alone cannot be used to gauge an
individual's IQ," Thompson cautioned. The study found that differences in
the volume of gray matter account for only 10 to 15 percent of the
variation in intelligence.
"That's good news," said Thompson. "This shows how important the
'nurture' part is."
Science has long been divided about whether levels of intelligence are
shaped mainly by "nature" (genetics) or by "nurture," which refers to
non-genetic factors such as education, environment, diet, rest, and
overall health.
The brain is a highly modular structure, with different sections
handling discrete tasks—reading, speaking, risk assessment, and visual
processing, for example. Thompson and his colleagues sought to find out
whether the size of these modules was influenced by genetics.
The MRI scans indicated that two areas of gray matter that control
reading comprehension and speaking (known respectively as Wernicke's area
and Broca's area) were highly similar in size in identical twins, which
share an identical set of genes.
The Broca's and Wernicke's areas were also similar in non-identical
twins, who on average share about half of their genes. But these
differences were greater than in the comparison of identical twins, and
fewer than in two unrelated individuals.
The study shows that the more closely related two people are, the more
likely they are to share similar brain structure in regions heavily
controlled by genetics. They are also more likely to share vulnerabilities
to specific diseases affecting these areas.
While these ideas are not new, Thompson's work is the first detailed
study showing how strongly brain structure is determined by genes and
inheritance.
The results are described in the November 2001 issue of the journal
Nature Neuroscience.
Thompson's study is part of a much broader effort to understand which
regions of the brain are associated with brain diseases such as
Alzheimer's, schizophrenia, and Parkinson's.
