I believe that the human mind was built by two rather opposing selection pressures. 1) cold climate which selects for bigger brains, either directly because of thermoregulation, or indirectly because it takes intelligence to survive the cold, and 2) self-domestication which selects for smaller brains either directly by shrinking the aggressive part of the brain, or indirectly by shrinking the body in general.

For the vast majority of our prehistory, climate related pressure to have bigger brains overpowered domestication pressure to have smaller brains. Only in the last 3000 years when both the ice age ended and civilization began did we see a reverse of this trend. And while most of our declining brain size can be explained by the poor living conditions of humans when they stopped hunting and gathering, even in the developed World, our brains are still somewhat smaller than they were at the end of Last Glacial Maximum.

Because civilization came to East Asian before it came to Europe before it came to sub-Saharan Africa, we’d expect black Africans to have the biggest (least domesticated) brains, followed by whites, followed by East Asians. But at least within the United States, we see the exact opposite pattern. In a stratified sample of 6,325 military personnel measured in the 1980s, Rushton found East Asian Americans averaged 1416 cc, white Americans averaged 1380 cc, and African Americans averaged 1,359 cc.

This tells us that the extreme cold that East Americans experienced in Northern Siberia during the Last Glacial Maximum, was so powerful, that not even several thousand years more domestication could shrink their brains to White levels (note: commenter Mug of Pee has gathered data showing worldwide, whites have bigger brains than East Asians but he is comparing different studies unlike the Rushton study that compared the races in a single apples to apples study).

Meanwhile South Asian Indians, despite being more intelligent than Aficans, have perhaps the smallest brains of any large ethnic group. This is a combination of small body size (they mated with Andaman island types) lack of exposure to the ice age, and the fact that human civilization started in Southern Asia making them even more domesticated than East Asians (I realize civilization did not specifically start in India but those who started civilization would later migrate to India and those who remained in the Near East would be undomesticated by Islam). Even James Watson noted that Indians tend to be servile. This is part of the reason they were brought to Africa by the Europeans to work on the railways (the less domesticated Native Africans refused to work)

One of the reasons the correlation between IQ and brain size is not stronger is the smartest members of a macro-race figured out how to create civilization which ironically shrunk their brains but not necessarily their intelligence. And so we have the paradox of the undomesticated Mongolians being bigger brained than the more intelligent Japanese.

This also explains why East Asians, despite being slightly more intelligent than whites, do not rule the World. Domestication keeps them humble.