In 2006 there was a study that found a near perfect NEGATIVE correlation between national IQ and national skin colour. National skin color is a good proxy for ancestral climate because the colder the climate, the lighter the skin, generally speaking.
Growing up in the freezing cold of Canada, I know how much brains it would have taken archaic humans to colonize cold climates by solving the novel challenges of building warm sturdy well insulated shelter, creating fires, creating stone tools to hunt with, creating warm clothing, learning to sew warm insulated boots etc.
Even still, I’m surprised at just how strong the correlation between national skin colour and IQ is because there are other factors besides just ancestral climate that determines a populations IQ. For example arctic people and native americans have IQs nearly 1 standard deviation lower than most “mongoloids”. According to scholar Richard Lynn, this is probably because they evolved in remote low population density regions of the world where they didn’t have access to novel high IQ mutations that large populations produced through random chance. But there was still enormous selection for high IQ among arctic people but because they probably didn’t have these mutations from which to select, IQ could only be increased via brain size (which is very large in arctic people). By contrast large population East Asian IQ got much higher perhaps because natural selection could increase brain size in combination with novel mutations which enhanced brain efficiency.
Analogously, Australian aboriginals, bushmen, and pygmies, all have lower mean IQs than mainstream “black people”. These groups too had low ancestral population and/or were geographically isolated from the genetic mainstream, and thus also did not have access to novel mutations.
Richard Lynn created an interesting chart comparing the the IQs and brain size of ten human populations with ancestral climate. It should be noted that the group differences he reports are not corrected for nutrition, so people in third world countries will score way below their genetic potential. While correcting for nutrition does greatly diminish the differences, is does not seem to change the rank order:
One anomaly is the chart is that Pacific Islanders/Southeast Asians seem to have relatively high IQ/brain size for such warm ancestral climates, but this is either because they were geographically isolated, had to cross through cold climates to get to their destination, and/or mixed with Northern adapted East Asians.