A study of almost 80,000 people has identified 40 new genes linked to intelligence.
The findings bring the number of known genes associated with intelligence to 52, which the researchers say can account for 20 percent of difference in IQ tests.
'We want to understand how the brain works and learn what are the biological underpinnings of intelligence,' said Danielle Posthuma, a professor at the Vrije Universiteit Amsterdam and principal investigator of the study, which was carried out by an international team of 30 scientists.
The researchers used genotype and intelligence test data (based on IQ test scores) from 13 earlier studies, involving nearly 20,000 children and 60,000 adults of European descent.
The majority of the SNPs were found in noncoding regions of DNA, making it difficult to interpret which genes they were associated with. To overcome this, the researchers carried out a genome-wide gene association analysis (GWGAS) which calculated the effect of multiple SNPs within genes and identified associated genes.
The researchers found the most significant genes were involved in regulating the nervous system.
'The genes we detect are involved in the regulation of cell development and are specifically important in synapse formation, axon guidance, and neuronal differentiation,' said Professor Posthuma. 'These findings for the first time provide clear clues towards the underlying biological mechanisms of intelligence.'
The most significant SNPs were found in FOXO3, a gene involved in nerve cell apoptosis, and in CSE1L, also involved in apoptosis and cell proliferation.
Genes linked to high IQ also correlated with other factors, including more years spent in school, a larger head size at birth, success in quitting smoking, and surprisingly, autism. Individuals with schizophrenia or obesity were less likely to have certain high IQ genes.
Altogether, the new genes account for 5 percent of IQ variation. Professor Posthuma said thousands of genes could be linked to intelligence, but that scientists would have to scan millions of genomes to find them all.
However, other factors beyond genetics influence intelligence, including conditions in the womb, nutrition, pollution and social environment.
'We are looking at all these genetic effects in isolation,' said Professor Posthuma. 'Maybe it's a certain pattern of genetic variants that makes you more intelligent.'
She suggested genetic information could be used to personalise teaching for individuals. 'Maybe one day we can say that based on your genetic makeup, it could be easier for you to use this strategy rather than that one to learn this task. But that's still very far off. I don't think what's written in our genes determines our lives.'
The study was published in Nature Genetics.