Most of the genes thought to be linked to human intelligence have nothing to do with it at all, according to Harvard researchers.
The team examined a dozen genes using both the results of intelligence tests and genetic data - and found that in nearly every case the hypothesized genetic pathway failed to replicate.
"In all of our tests we only found one gene that appeared to be associated with intelligence, and it was a very small effect," says professor Christopher F Chabris.
"This does not mean intelligence does not have a genetic component, it means it's a lot harder to find the particular genes, or the particular genetic variants, that influence the differences in intelligence."
The team drew information from a massive study of Wisconsin high school graduates that began in the 1950s, the Framingham Heart Study, and an ongoing survey of all twins born in Sweden, to give a far larger data set than has been possible in the past.
Previous studies identified genes that were already linked with a known biological function, such as Alzheimer's disease or the production of a specific neurotransmitter, and examined whether people carrying a particular variant scored differently on IQ tests. If high scorersshared a particular variant of that gene, , they hypothesized, that demonstrated the gene's role in intelligence.
"These were reasonable hypotheses," says Daniel J Benjamin of Cornell University. "But in retrospect, either the findings were false positives or the effects of the genes are much, much smaller than anyone had anticipated."
This, though, doesn't mean there's no link at all.
"As is the case with other traits, like height, there are probably thousands of genes and their variants that are associated with intelligence," says Chabris.
"And there may be other genetic effects beyond the single gene effects - there could be interactions between genes, there could be interactions between genes and the environment."