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LOS ANGELES (AP) — Scientists who hunt for “intelligence genes”
used to think there were fewer than half a dozen of
them.
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In recent years, they determined there may be at least 1,000 – each
with just a tiny effect on the differences in people’s IQ. A study
released Tuesday found new evidence that many genes play a role in
intelligence, but scientists still couldn’t pinpoint the specific
genes involved.
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“It’s been kind of a shock to the system that it hasn’t worked,”
said psychologist Eric Turkheimer at the University of Virginia,
who had no role in the study. “We can’t find the effects of any
individual genes that are large enough to seem worth worrying
about.”
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Previous work involving twins and adopted children has found that
genes have a significant influence on differences in IQ scores,
producing about half the difference between adults in general. The
influence of genes on IQ appears to grow from childhood to
adulthood.
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Scientists have come to realize that, as with height, differences
in intelligence come not from a few genes, but rather the overall
effect of many genes, each with only tiny influence. That makes
them hard to tease out.
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The new DNA study, reported online Tuesday in the journal Molecular
Psychiatry, came to similar conclusions. Many genes work together
to shape intelligence much like the different instruments of an
orchestra that play in sync. Unless there’s a soloist playing, it’s
often difficult to decipher the contributions of individual
instruments.
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As important as genes are in determining intelligence, they don’t
act alone and the role of one’s upbringing and experiences cannot
be ignored.
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So why do researchers care so much about the relationship between
genes and intelligence?
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Our memory, reasoning skills and thinking abilities tend to decline
as we age, some faster than others. Understanding the genetics of
intelligence may someday help researchers gain a better handle on
mind-robbing diseases such as Alzheimer’s.
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The new work was done by I.J. Deary of the University of Edinburgh
in Scotland and colleagues in several countries.
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The team wanted to find out “whether genetic differences that we
could test on people’s DNA could explain some of the reasons that
people have different intelligence test scores,” Deary said in an
email.
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Researchers didn’t ID any genes affecting IQ. But they estimated
that they found a genetic influence that accounts for at least 40
percent to 50 percent of the differences on intelligence test
scores in the 3,511 unrelated adults in their study who were tested
on knowledge and problem-solving skills.
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They focused on more than 500,000 places in the participants’ DNA,
looking for evidence that IQ-influencing genes lay close to those
places. They concluded that the overall effect was coming from many
scattered genetic differences, each of only small
influence.
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The latest work adds to evidence that even the most powerful of
these has only weak influence. Deary said that future studies will
probably need to involve millions of people to detect the genetic
effects.
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Robert Plomin of the Institute of Psychiatry in London, who’s
looked for intelligence-related genes for 15 years but didn’t
participate in the new study, isn’t surprised by the latest
findings.
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“We’ve got a century of twin and adoption studies,” such as those
comparing twins reared in different families, that support the
notion that about half of IQ differences come from DNA, he
said.
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Plomin said this doesn’t mean half of a person’s intelligence is
due to genes nor does such a genetic influence imply that a
person’s intelligence is fixed.
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Turkheimer, the Virginia psychologist, thinks other types of
research such as brain scans might have better luck in
understanding what intelligence is.
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Those methods are better than “pinning your hopes on adding
together a bunch” of small effects from individual genes, he
said.
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John Olsen, of Orange, Calif., who was adopted at birth, attributes
his brainpower to his genes. As a kid, he always wondered where his
inquisitiveness came from. School bored him and there were no
lively debates at the dinner table growing up.
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“I was a bit of a challenge,” he recalled. “I was very curious and
like a lot of intelligent people always asked, `Why?'”
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In his late 20s, Olsen took a genius test and scored high enough to
get accepted into Mensa, the high IQ group. A telephone call from a
long-lost aunt several years ago led to a reunion with his
biological mother.
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Olsen soon discovered his mother had the same curiosity and liked
to ask probing questions. He also learned his maternal grandmother
was fond of one-line comebacks and “was wickedly smart till the day
she died.”
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Online: Journal: href=”http://www.nature.com/mp/index.html” target=
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Ritter reported from New York.
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