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 - in other words, 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 says.People sometimes mistakenly assume that if a trait is genetic, then it is measurable at an early age.
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 (The Journal of Neuroscience, vol 29, p 2212).
We still don't know exactly how much genes contribute to intelligence, with various studies coming up with estimates ranging from 40 to 80 per cent. 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 (Molecular Psychiatry, DOI: 10.1038/mp.2009.55).
I mentioned below a study that correlates aggressive behavior in kids aged 3 to 5. It assumed that if a kid has an innate inclination towards aggressive behavior, then such an inclination would not change between 3 and 5. So if the kid became more aggressive, then it must be caused by something that the mom was doing. But it seems likely to me that, like IQ, genes explain a great portion of the variation in aggression as we get older.