In New Scientist: Picking our brains: Why are some people smarter?
One important factor [in intelligence] seems to be how well our neurons can talk to each other. 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.
So can we extrapolate that ``smarter'' cities have more efficient networks (in a sense, higher accessibility)? This may be the source of agglomeration economies that give value to cities over random space, the ability to connect.