It’s Not All Genes Prenatal Care Plays Vital Role In Shaping Human Intelligence, Study Indicates

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 concluded 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.

Devlin’s study, conducted with Michael Daniels and Kathryn Roeder of Pittsburgh’s Carnegie Mellon 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 Thursday’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 the researchers 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.