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Study: Nongenetic factors play
biggest role in high IQ

By Matt Crenson
Associated Press

NEW YORK- Nurture edges out nature in determining a person’s IQ, according a new study that also finds a surprisingly important role for prenatal development in shaping human intelligence.

In an analysis combining more than 200 earlier studies, statisticians conclude that genes account for 48 percent of the factors that determine IQ.

That’s less than most psychologists would estimate, said study author Bernie Devlin, and far enough below the figure cited by the controversial 1994 book “The Bell Curve” to undercut its authors’ main conclusions. “That number is way too small for their arguments to be of any great consequence,” said Devlin, a professor of psychiatry at the University of Pittsburgh School of Medicine.

His study, conducted with Michael Daniels and Kathryn Roeder of Pittsburgh’s Carnegie Melton University, also found that conditions during prenatal development significantly affect a person’s intelligence.

That suggests that inadequate prenatal care may explain why poorer people and
blacks generally score lower on IQ tests.

“Our study gives credence to that idea, and that, in fact is something that people
should be concerned about,” Devlin said. “Poor prenatal care may have a negative impact on IQ.”

He and his colleagues report their findings in today’s issue of the journal Nature.
The statisticians combined the results of 212 earlier studies that compared the IQs of twins, siblings or parents and their children.

Then they constructed a set of statistical models, or predictions, to determine which one best fit the accumulated data.

The best-fitting model was one that included genetic effects, environmental effects such as being raised in the same or different households and prenatal conditions.

Many of the 212 earlier studies had found a strong genetic component to
intelligence by showing that identical twins raised apart have remarkably similar IQs.

But the new study weakens that conclusion somewhat by showing that fraternal twins - who are different genetically - also have more similar IQs than siblings.

The mere fact that they occupy the same womb at the same time accounts for 20
percent of the similarity between twins, decreasing the relative significance of genes.

Even for siblings who aren’t twins, being carried by the same mother at different
times explains 5 percent of the similarity in IQ.

“The implication would be that the in-utero environment has a profound effect on IQ
in the general population,” said University of Minnesota psychologist Matt McGue.

“It will stimulate people to think about prenatal factors in a way they hadn’t before.”

It is already well known that drinking or smoking during pregnancy can cause decreased IQ in children, and that exposure to lead in the womb can also lead to lower intelligence.There may be other prenatal factors that are important as well, Devlin said.

McGue said the study casts doubt on the main hypothesis of “The Bell Curve.”

The book, by the late Harvard University professor Richard Herrnstein and political
analyst Charles Murray, argues that the continued intermarriage of highly intelligent, well-educated people will lead to the development of a two-tiered American society -
one rich and smart, the other poor and dumb.

But that can’t come to pass, Devlin and his colleagues argue, because intelligence depends too little on genes.