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On average, the brains of African-Americans are 5% smaller than the brains of Whites and 6% smaller than East Asians, according to studies of brain weight at autopsy, endocranial volume of empty skulls, head size measurements by the U.S. military and NASA, and two dozen MRI volumetric studies.
That is a genetic trait because even malnourished Asians from poor countries have a larger brain on average than well fed blacks from western countries.
Sources:
Beals, K. L., Smith, C. L., & Dodd, S. M. (1984). Brain size, cranial morphology, climate, and time machines. Current Anthropology 25, 301–330.
Ho, K. C., Roessmann, U., Straumfjord, J. V., & Monroe, G. (1980). Analysis of brain weight: I and II. Archives of Pathology and Laboratory Medicine 104, 635–645.
Johnson F. W. & Jensen (1994). Race and sex differences in head size and IQ. Intelligence 18: 309–33
Rushton JP. (1997). Cranial size and IQ in Asian Americans from birth to age seven. Intelligence 25: 7–20.
Rushton JP (1991). Mongoloid-Caucasoid differences in brain size from military samples [and NASA]. Intelligence 15: 351–9.