I want to add what I believe the science currently tells us about differences in group intelligence.
- Differences in demographic distribution. Rather than state that the races possess terribly meaningful differences, the data suggests pretty strongly that certain groups have expanded their middle and upper classes under agrarianism, and some have expanded their lower and subclasses under pastoralism.
- Success at Neoteny/Pedomorphism in other words, ‘self domestication’ of certain groups was more effective largely because of geography. This is why intensity and rate sexual development is the inverse of intelligence in all groups. And this is measurable by testosterone in each race and subrace. Africans a lot, steppe/desert people a bit less, caucasians quite a bit less, and east asians a lot less. Why? Early maturity is absolutely necessary in Africa if for no other reason than the disease gradient. Late maturity or reduced depth of maturity (see body odor differences between races and ages), is beneficial in the rigours of above 45th agrarianism, and the genetic underclass cannot survive the seasonal cycles.
- Balance of Dimorphism. Male and female brains differ in structure and chemistry. Verbal and spatial specializations (biases really) follow these differences. Some groups demonstrate both bias to the female reproductive strategy and verbal superiority, some balance, and some demonstrate the bias to male reproductive strategy and spatial superiority.
As far as I know all substantive differences in Homo Sapiens Sapiens are explicable by minor variations in endocrine expression both in utero and in early development.
And that our racial differences are largely due to (a) the generation we left africa or remained there, and (b) the degree of neoteny and balance we achieved during self-domestication, and (c) the number and diversity of competitors we faced in our geography.
And so far the data agrees.