[danieltillett]

We know that eugenics efforts cannot produce any remarkable or decisive selection for increased cognitive abilities. We know this because eugenics is a sort of hobby-horse form of evolution, across handfuls of generations, and because evolution cannot, taken over thousands of generations, select any more strongly for increased cognitive abilities than it already has. If it could, it would; thus we must conclude that it can’t.

If this argument were true then it would not have been possible for the evolution of human intelligence to have occurred.

You have assumed that having a higher gca did not have a cost higher in the past than it does now. The environment we live in today is very different to that of even our recent ancestors.