Wiring the Brain

What if we’ve been thinking about the genetics of intelligence from completely the wrong angle?  Intelligence (as indexed by IQ or the general intelligence factor “g”) is clearly highly heritable in humans – people who are more genetically similar are also more similar in this factor. (Genetic variance has been estimated as explaining ~75% of variance in g, depending on age and other factors).  There must therefore be genetic variants in the population that affect intelligence – so far, so good.  But the search for such variants has, at its heart, an implicit assumption: that these variants affect intelligence in a fairly specific way – that they will occur in genes “for intelligence”. 
An implication of that phrase is that mutations in those genes were positively selected for at some stage in humanity’s descent from our common ancestor with apes, on the basis of conferring increased intelligence.  This seems a fairly reasonable leap to make – such genes must exist and, if variation i…