During the 1990s to the early 2010s, it had become well documented the brain-size IQ correlation among adults living in developed countries was about 0.4. Then in 2015, a meta-analysis by Jakob Pietschnig, Lars Penke, Jelte M. Wicherts, Michael Zeiler, and Martin Voracek surfaced claiming the brain size-IQ correlation was only 0.24! The paper argued that the 0.4ish figure that was typically cited was inflated by publication bias and these authors went out of their way to counter this.
While much of the HBD-o-sphere and academic community uncritically accepted the results of this meta-analysis and routinely cited it in their articles, I was immediately suspicious and argued that failure to correct to for range restriction and other methodological problems had spuriously deflated the correlation and that the true correlation was much closer to the traditional 0.4 than to the 0.24 Pietschnig et al had reported.
Now a brand new meta-analysis by Gilles E. Gignac and Timothy C. Bates is being published in the peer reviewed journal Intelligence showing once again Pumpkin Person was right! The authors reviewed the research cited by Pietschnig et al but corrected for range restriction, test quality, and sample quality and a 0.4 correlation was found.
However even 0.4 might be an underestimate of the within sex correlation between brain size and IQ because no correction was made for the fact that some samples combined men and women which lowers the correlation because men have substantially larger brains than women, yet virtually the same IQs. The within sex correlation might be closer to 0.45.
It seems the brain-size IQ correlation is very similar to the height-weight correlation. In a sample of male university students, the heigh-weight correlation was about 0.4. Arguably brain size is to IQ as height is to weight. A big brain helps make you smarter just as a tall height helps make you bigger but just as large brains are only one cause of IQ, greater height is only one cause of greater size, and it’s possible for small brained people to be brilliant just as it’s possible for very short people to be huge, and vice versa.
A genetic basis for IQ?
The brain-size IQ correlation is controversial because it suggests IQ is a biological variable, which in turn suggests it’s genetic. While IQ skeptics have been cheering the failure of genome-wide association studies to identify many genetic variants associated with IQ, they would be wise to not get their hopes up. Davies et al (2011) genotyped 3511 unrelated adults and found heritabilities of 0.44 for crystallized intelligence (acquired knowledge) and 0.51 for fluid intelligence (abstract reasoning). Taking the square root of these heritabilities suggests the IQ phenotype-genotype correlation may exceed 0.7! It should be noted that unlike traditional twin studies which yielded even higher numbers, genome-wide complex trait analysis only quantifies the additive portion of heritability, so the full heritability may be higher still.
Of course as commenter “Mugabe” notes, research is needed across a much wider range of environments to determine whether these are independent genetic effects.