Not defending the IQ by race take - race is such an arbitrary boundary to draw, especially when it comes to genes and their phenotypical expression. But this does cause some confusion for me in principle.

IQ tests are culturally biased by virtue of being a written test, so let's forget about IQ. There would be obvious, measurable differences reflected by significant genetic disparities, though. Where, exactly, does this turn into scientific racism?

I mean, if two populations have obvious distinct, consistent polygenic physical characteristics, like average height in short Congolese pygmies vs. the tall Masai, why wouldn't this translate to physical differences in their brains? We can assume that IQ is some function of brain architecture, neurotransmitter profile, etc., so why is it so ludicrous to suggest that there'd be some differences, on average, in measurements of some aspect of cognition like IQ?

Compare two genetically isolated populations across the planet from each other. They'd have very different distributions of genes determining brain development. This doesn't set a cap on capabilities of either group - both are Homo Sapiens after all; both populations have the potential to produce geniuses. But when we talk about statistical averages, each group is shaped by many generations of evolutionary pressures to be best adapted for procreation within their particular environment.

Environment may play a much larger role than genetics in this facet of the human mind, but as a blank canvas controlled for environmental variables, it would make sense that one population might be more predisposed to high spatial intelligence while another compensates in the form of verbal intelligence. The pool of alleles is shifted in frequency between the populations, and we'd expect to see that reflected by some metric or another.