A number of studies have reported a moderate statistical correlation between differences in IQ and brain size between individuals in the same group.
[143] [144] And some scholars have reported differences in average brain sizes between Africans, Europeans and Asians.
[145] J. P. Rushton has argued that Africans on average have smaller brain cases and brains than Europeans, and that Europeans have smaller brains than East Asians, and that this is evidence that the gap is biological in nature. Critics of Rushton have argued that Rushton's arguments rest on outdated data collected by unsound methods and should be considered invalid.
[146] Recent reviews by
Nisbett et al. (2012b) and
Mackintosh (2011) consider that current data does show an average difference in brain size and head-circumference between American Blacks and Whites, but question whether this has any relevance for the IQ gap. Nesbitt et al. argue that crude brain size is unlikely to be a good measure of IQ; for example, brain size also differs between men and women, but without well documented differences in IQ. At the same time newborn Black children have the same average brain size as Whites, suggesting that the difference in average size could be accounted for by differences in postnatal environment. Several factors that reduce brain size have been demonstrated to disproportionately affect Black children.
[63]
Earl Hunt states that brain size is found to have a correlation of about .35 with intelligence among whites and cites studies showing that genes may account for as much as 90% of individual variation in brain size. According to Hunt, race differences in average brain size could potentially be an important argument for a possible genetic contribution to racial IQ gaps. Nonetheless, Hunt notes that Rushton's head size data would account for a difference of .09 standard deviations between Black and White average test scores, less than a tenth of the 1.0 standard deviation gap in average scores that is observed.
[121][147
