Martijn van den Heuvel, a neuroscientist at Utrecht University Medical Center in the Netherlands, found that smarter brains seem to have more efficient networks between neurons.
This means that it takes fewer steps to relay a message between different regions of the brain.
That could explain about a third of the variation in a population's IQ, he said.
Another key factor is the insulating fatty sheath encasing neuron fibres, which affects the speed of electrical signals.
Paul Thompson at the University of California, Los Angeles, has found a correlation between IQ and the quality of the sheaths.
May studies have estimated that genes contribute around 40 to 80 per cent to intelligence.
This wide range of estimates might have arisen because genes contribute more to IQ as we get older, according to a study published last year.
By comparing the intelligence of 11,000 pairs of twins, Robert Plomin of King's College London found that at age 9, genes explain 40 per cent of the variation, but by 17 they account for roughly two-thirds.
This could perhaps be because genes affect how our brain rewires itself as we mature.
On the other hand, they may dictate whether someone is likely to seek out stimulating experiences to help their brain grow and develop.
"If we are predisposed to have a talent, we may actively seek out an environment to suit it," New Scientist quoted Thompson as saying.
The study has been published in The Journal of Neuroscience. (ANI)