Intelligence could be measured with a swab of saliva, or a drop of blood, after scientists showed for the first time that a person's IQ can be predicted just by studying their DNA.

In the largest study looking at the genetic basis for intelligence, researchers at the University of Edinburgh and Harvard University have discovered hundreds of new genes linked to brain power.

Previous studies have suggested that around 50 per cent of intelligence is inherited, and the rest comes through upbringing, friendship groups and education.

That figure was calculated by studying identical twins who share the same DNA, therefore any differences in IQ between them must be non-genetic.

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But nobody knew which were the "smart genes".

Now by studying the genetic data from more than 240,000 people, scientists have found 538 genes which are linked to intelligence.

Researchers were able to predict intelligence solely based on a person's DNA, a breakthrough which could potentially help doctors to diagnose impaired cognitive ability, or allow children to be given a tailor-made education based on their innate abilities.

Dr David Hill, of the University of Edinburgh's Centre for Cognitive Ageing and Cognitive Epidemiology, who led the research, said: "Our study identified a large number of genes linked to intelligence.

"We were also able to predict intelligence in another group using only their DNA."

The study also showed that the same genes which influence intelligence are also linked to other biological processes such as length of life.

The new research suggests that intelligent people are biologically fitter.

The research was published in the journal Molecular Psychiatry.