One reason people may find discussing intelligence uncomfortable is the belief that it is something you are born with and so you can do nothing to influence it. This undercuts social equality and feeds into the link between intelligence testing and eugenics, which still looms large for many.
However, there is no escaping substance that intelligence is inherited to some degree. Researchers found that the IQ of children adopted at birth bore little correlation with that of their adoptive parents but strongly correlated with that of their biological parents. What’s more, this association became stronger as the children grew, older.
But genes are not destiny
For many years, the search for specific intelligence genes proved unfruitfully. Recently, however, genetic studies have grown big and powerful enough to identify at least some of the genetic underpinnings of IQ. Although each gene associated with intelligence has only a minuscule effect in isolation, the combined effect of the 500-odd genes identified so far is quite substantial. “We are still a long way from accounting for all the heritability,” says Plomin, “but just in the last year we have gone from being able to account for about 1 per cent of the variance to maybe 10 per cent.”
So genes matter, but they are certainly not destiny. “Genetics gives us a blueprint – it sets the limits. But it is the environment that determines where within those limits a person develops,” says psychologist Russell Warne at Utah Valley University.
“About 50 per cent of the difference in intelligence between people is due to genetics”
Consider height, another highly heritable trait. Children will grow taller if they eat a nutritious diet than if they eat a less nutritious one because a good diet helps them achieve their full genetic potential. Likewise with intelligence. Iodine deficiency during childhood is associated with lower IQ and addressing this in developing countries has boosted cognitive skills. So too has treating parasitic worms and removing lead from petrol.
Other environmental influences on IQ are not as obvious. Cases of abuse and neglect aside, twin studies reveal that the shared family environment has only a very small effect on cognitive ability. Plomin, therefore, suspects that intelligence has less to do with parenting style than chance. “It’s idiosyncratic factors that make a difference,” he says, “like the kid becomes ill or something like that – but even then, children tend to bounce back to their genetic trajectory.
Credits: Nivethithaa
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