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Brain Size and Cognitive Ability

2015-12-24

Studies have been done on large populations of men, women, Europeans, Africans, Asians, adjusted by body size, and correlated, and have found that Asians have the largest brains on average (say, 1350 cc), Europeans in the middle (say, 1300 cc), and Africans the smallest(say, 1250 cc); while men have larger (say, 1300 cc) brains than women (say, 1200 cc.)  Intelligence tests, again of large populations, show Asians to have the highest IQs (115), Europeans the middle (100), Africans the lowest (85); whereas women have, possibly, slightly lower (96) IQs than men (100).

If brain size is so closely correlated with intelligence, why do women have higher IQs than Africans, who have just as large brains?  Remember, that supposedly close correlation is not 1 to 1, it is 0.44; that is less than half the variance.

For comparison, the brain size of a gorilla is roughly 500 cc, and a chimpanzee, 350 to 400 cc.  Another comparison: bottlenose dolphins have brains about 1500 cc in size, with body weights not much bigger than human, say 110 kg.  Finally, the Neanderthal pre-human had an average brain size of 1500-1600 cc.

One answer is the degree of cerebral folding in each brain; a more tightly folded cerebrum can accommodate more surface area, and thus more neurons than a less tightly folded cerebrum.  With a cortical thickness of 2 mm in the cerebral column but more than 2 cm including the subjacent white matter containing the necessary connections to and from the cerebral column,  there is a limit to the amount of folding that can be obtained in a given volume of skull.  The brain stem and midbrain are probably at least a minimum size, so tightly folding the cortex is an advantage.

The studies on brain size are quoted by racists as indicating that Africans are really less intelligent, on average, than whites; and Asians are the smartest of all.  The intelligence quotient studies also back this up.  However, there is the curious fact that despite having smaller brains, women test out as nearly as intelligent as men; in fact, women’s brains are, perhaps, slightly smaller than African’s brains, yet they are clearly more intelligent on the tests.  This suggests that intelligence is not that closely related to brain size, or else the tests really are biased against Africans.

There are also other factors that contribute to brain size other than intelligence such as relative myelinization, density of neural packing, size of neurons, number of glia relative to neurons, and so on.  In addition, the differences in average size are rather small and really don’t account for the fairly dramatic apparent differences in average intelligence.  That is to say, there is about one standard deviation’s worth of difference in intelligence test results between each of the three “races”– but there is only a five to ten percent difference in total brain size.

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