Kevin Mitchell has a very interesting take on intelligence here:
http://wiringthebrain.blogspot.com/2012/07/genetics-of-stupi...Basically he suggests thinking about the formation of intelligence to be something like trying to run an obstacle course while holding an armful of things and trying to drop as little as possible. Essentially, he is arguing that there is some optimal or standard brain and since any mutation introduced into the brain is more likely to be harmful than beneficial, more intelligent individuals are not those that inherited some specific genes for intelligence but are most likely those who avoided the most damaging non-specific mutations.
That’s [runaway selection of intelligence boosting genes] all nice, though admittedly speculative, but those mutations are the ones that we would expect to not vary in human populations – they would now be fixed. In particular, there is little reason to expect that there would exist new mutations in such genes, present in some but not all humans, which act to further increase intelligence.
To test this he suggests using symmetry as a proxy for robustness in the individual. This fits well with the fact that all attempts thus far to find a common genetic basis for intelligence have failed, the correlation between intelligence and immune function, ideas that intelligence was indirectly selected when optimizing for healthy mates and findings that Nootropic like substances tend to have the least effect on already intelligent minds. In the last case, it is interesting to note nootropics should then be viewed not cheating but more leveling of the playing field. Taking this further, genetics are not the only obstacle course - there is still variation in the womb environment and early childhood. The interplay of genes and the stability of process in the growing brain; affected by a lack of stimulation, poor nutrition and a stressful womb environ would likely have more impactful effects on intelligence than pure genetics.