THE contribution genes make to intelligence increases as children grow older. This runs counter to the notion that as we age, environmental influences gradually overpower the genetic legacy we are born with - and may have implications for education.
"People assume the genetic influence goes down with age because the environmental differences between people pile up in life," says Robert Plomin of King's College London. "What we found was quite amazing, and goes in the other direction."
Previous studies have shown that variations in intelligence are at least partly due to genetics. To find out whether this genetic contribution varies with age, Plomin's team pooled data from six separate studies carried out in the US, the UK, Australia and the Netherlands, involving a total of 11,000 pairs of twins.
In these studies, the researchers tested twins on reasoning, logic and arithmetic to measure a quantity called general cognitive ability, or "g". Each study also included both identical twins, who have the same genes, and fraternal twins, who share about half their genes, making it possible to disentangle the contributions of genes and environment to their g scores.
Plomin's team calculated that in childhood, genes account for about 41 per cent of the variation in intelligence (see graph). In adolescence, this rose to 55 per cent; by young adulthood, it was 66 per cent (Molecular Psychiatry, DOI: 10.1038/mp.2009.55).
No one knows why the input from genes should increase with age, but Plomin suggests that as children get older, they become better at exploiting and manipulating their environment to suit their genetic needs. "Kids with high g will use their environment to foster their cognitive ability and choose friends who are like-minded," says Plomin. Children with medium to low g may choose less challenging pastimes and activities, further accentuating their genetic legacy.
