Brain size, IQ

Bryant (
25 Aug 1996 19:33:03 -0600

There's been some discussion of IQ on sci.anthropology lately, and I'd
appreciate any feedback from the interested and informed parties about my
questions below. Thanks.

As Dawkins points out in his _Extended Phenotype_, the association of
larger cranial capacity and apparently increased intelligence in the Homo
lineage suggests (demands) heritability for brain size in past human and
proto-human populations. This is, hopefully, not controversial.

But considerable controversy has surrounded the suggestion that this is
still true--that human IQ is still heritable, and that it is related to
gross brain size. One problem for those who would propose a simplistic
brain size/IQ (causal) relationship is that white women's smaller brains
score higher, on average, than black men's larger brains during IQ tests.

This is, to my knowledge, the only exception to the IQ/brain size rule to be
found in demographic data (as opposed to occassional anecdotal stories about
Einstein's small brain and the like). I found it described in a recent
paper by Rushton, an advocate of the IQ/brain size connection. He
described this exception to the rule as a "paradox."

I do not subscribe to the notion that there are inherent differences in
the cognitive potentials of people from different races/populations (this in
fact contradicts my day-to-day experiences with other folks)... but I find the
issue of brain size and intelligence itself to be rather fascinating.

Could sexually dimorphic neuron packing densities account for white women's
apparently higher 'IQ to brain-unit-area' ratio? How sensitive are neuron
densities to malnutrition during childhood?

Are there differences in the degree of cortical folding in men brains and
women brains, so that white women might "make up" for smaller brains with
equal cortical surface areas? How canalized ("genetically 'determined'")
are cortical folding patterns?

Your thoughts and references will sure be appreciated. Thanks!