In a totally impossible, unethical, experiment (lol) it'd be interesting to go back in time before mass colonization, and take 10,000 babies from every culture and disperse them throughout every other culture, assuming they all had ideal upbringings in their respective adoptive cultures, it would be interesting to test the 'implanted' babies against babies of the same 'race' who were left in their original culture, and also to test the 'implanted' babies against other children in their new, assimilated culture. I think that, or something like that, would be the only real way to test if intelligence or had slanted in favour or in detriment to specific phenotype's of humans, in different geographical areas.
I don't see how it would be impossible for intelligence, or at least certain forms of intelligence, to be favoured in certain areas. It makes me think of this quote;
“Everybody is a genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is stupid.”
― Albert Einstein