杏吧原创

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 the fact that intelligence is inherited to some degree. Researchers found that the IQ of children adopted at birth bore , but strongly correlated with that of their biological parents. What鈥檚 more, this association became stronger as the children grew older.

鈥淭hat鈥檚 counter-intuitive for most people,鈥� says Robert Plomin at King鈥檚 College London, who led the study. 鈥淭hey think as you go through life, the slings and arrows of outrageous fortune build up and environmental differences become cumulatively more important, because they think that genes only influence what happens at the moment of conception.鈥� That鈥檚 not true, of course.

In fact, hundreds of studies all point in the same direction. 鈥淎bout 50 per cent of the difference in intelligence between people is due to genetics,鈥� he says.

But genes are not destiny

For many years, the search for specific intelligence genes proved unfruitful. 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. 鈥淲e are still a long way from accounting for all the heritability,鈥� says Plomin, 鈥渂ut 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.鈥�

So genes matter, but they are certainly not destiny. 鈥淕enetics gives us a blueprint 鈥� it sets the limits. But it is the environment that determines where within those limits a person develops,鈥� says psychologist Russell Warne at Utah Valley University.

鈥淎bout 50 per cent of the difference in intelligence between people is due to genetics鈥�

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 on cognitive ability. Plomin therefore suspects that intelligence has less to do with parenting style than chance. 鈥淚t鈥檚 idiosyncratic factors that make a difference,鈥� he says, 鈥渓ike the kid becomes ill or something like that 鈥� but even then, children tend to bounce back to their genetic trajectory.鈥�

This article appeared in print under the headline 鈥淲hat makes one person smarter than another?鈥�