It was those little gray cells, Hercule Poirot always said, that made him
such a smart detective.
But Agatha Christie was not a neuroscientist, so her books never
explained why her heroes, such as Poirot and Miss Marple, were so much
smarter than the killers. Or why anybody is smarter than anyone else.
Of course, even neuroscientists aren't smart enough to explain all the
individual differences in intelligence. But the latest scientific
detective work has turned up some clues. Mental ability seems closely
related to the volume of those little gray cells in the brain. And
differences in that volume seem to be largely a result of heredity.
Discussions of genetic influence on intelligence always pose a danger of
degenerating into a dichotomous dispute about nature vs. nurture. And
even people who agree that genes and environment are both important will
then start arguing over whether there's such a single thing as
"intelligence" at all. People who are pretty smart at some realms of
life - say science and math - can be pretty dumb when it comes to
figuring out what birthday present to buy for their spouse.
Nevertheless, numerous studies suggest that people with superior mental
agility for some tasks are more likely than not to score high on other
mental tests as well. Psychologists refer to this "general" mental
ability as a mysterious property labeled "g."
"The concept of 'g' is controversial," note neuroscientists
Robert Plomin and Stephen Kosslyn. "Not all researchers are comfortable
with the idea that a single factor may influence all types of
intelligence." But while g is not the whole story of
intelligence, they say, "trying to tell the story without 'g
' loses the plot entirely."
The latest chapter in that story comes from a study published last month
in the journal Nature Neuroscience. A team of scientists from
Finland and UCLA reported that g is closely linked to the volume
of gray matter in the front part of the brain. Other studies have found
similar links. But the new study also shows that the gray-matter volume
is strongly dependent on heredity.
It is no surprise that smarter-than-average ancestors produce
smarter-than-average progeny. But are the children of smart parents
smart because of their genes or because of a favorable environment for
upbringing? The way to tell, the Finnish-UCLA team noted, is by
comparing identical twins with fraternal twins. Identical twins share
all the same genes; fraternal twins are no more genetically similar than
any two siblings, sharing about half the same genes. If only the
environment is important, it shouldn't matter whether the twins are
identical or fraternal.
Using sophisticated brain imaging techniques, the UCLA-Finnish team
tested 20 sets of twins (10 identical pairs, 10 fraternal) and produced
3-D brain maps revealing the amount of gray matter (made of nerve cell
bodies) in different brain regions. The researchers found that gray
matter volume in certain parts of the brain was more closely matched in
the identical twins than in twins who were less similar genetically.
Consequently, it appears that some parts of the brain's structure are
under genetic control.
Next the researchers subjected the twins to mental skill tests designed
to measure g. Better g scores corresponded to higher
gray-matter volume in the brain's frontal region.
"We found that differences in frontal gray matter were significantly
linked with differences in intellectual function," the researchers
reported.
Such findings do not suggest that intellectual ability is purely
genetic. For any individual, mental ability depends both on nature and
nurture - even aspects of genetic control depend on interactions with
the environment.
"Genes necessarily operate through the environment, particularly if they
concern susceptibilities to environmental stressors or hazards," the
researchers point out.
Nor is every part of the brain's structure predetermined by a genetic
blueprint. Some brain structures are more dependent on genes than others.
Furthermore, it is unlikely that g depends on frontal gray matter
alone, Drs. Plomin and Kosslyn point out in a Nature Neuroscience
commentary.
"Although it is possible that a single fundamental brain characteristic
such as frontal gray matter volume is responsible for g, it seems
more likely that many brain processes are involved," they write.
In any case, many genes are involved in shaping the structure of the
brain, and any notion of identifying a "smart" gene that determines who
is and isn't is nonsense.
Besides, it would be a serious mistake to apply the statistical features
of a population to an individual. Any individual may, for unknown
reasons, perform outside the range of expectations that genetic
statistics predict. Good environments can certainly enhance the
prospects of intellectual success, and a life of fear, hunger and abuse
can surely impair the expression of innate genius.
Still, denying or ignoring the role of genes in mental ability is no
smarter than disregarding the environment. The new research can aid the
understanding of how the brain works and may lead to ways of exploiting
the catalog of human genes for dealing more effectively with learning
disabilities and brain diseases.
