Scientists have identified 52 genes behind human intelligence, of which 40 are completely new discoveries.
Most of these genes are predominantly expressed in brain tissue, researchers said.

“These findings for the first time provide clear clues towards the underlying biological mechanisms of intelligence, Danielle Posthuma from Vrije Universiteit Amsterdam in the Netherlands.

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Intelligence is one of the most investigated traits in humans and higher intelligence is associated with important economic and health-related life outcomes.

Despite high heritability estimates of 45 per cent in childhood and 80 per cent in adulthood, only a handful of genes had previously been associated with intelligence and for most of these genes the findings were not reliable, researchers said.

“These results are very exciting as they provide very robust associations with intelligence. The genes we detect are involved in the regulation of cell development, and are specifically important in synapse formation, axon guidance and neuronal differentiation,” Posthuma said.

Researchers used a dataset of more than 78,000 individuals with information on DNA genotypes and intelligence scores and discovered novel genes and biological routes for intelligence.

The study also showed that the genetic influences on intelligence are highly correlated with those on educational attainment, and also, less strongly, with smoking cessation, intracranial volume, head circumference in infancy, autism spectrum disorder and height.

Inverse genetic correlations were reported with Alzheimer's disease, depressive symptoms, smoking history, schizophrenia, waist-to-hip ratio, body mass index, and waist circumference, researchers said.
Seven genes for intelligence that are also associated with schizophrenia, nine genes with body mass index, and four genes associated with obesity were found.

“These genetic correlations shed light on common biological pathways for intelligence and other traits,” said Suzanne Sniekers, a postdoc in the Posthuma's lab.

“These three traits show a negative correlation with intelligence. So, a variant of gene with a positive effect on intelligence, has a negative effect on schizophrenia, body mass index or obesity,” Sniekers said.

“The current genetic results explain up to five per cent of the total variance in intelligence,” Posthuma said.

Although this is quite a large amount of variance for a trait as intelligence, there is still a long road to go: given the high heritability of intelligence, many more genetic effects are expected to be important, and these can only be detected in even larger samples,” she added.