What About Race, Brain Size and IQ?

Using the head circumference measures to calculate cranial capacity at birth, 4 months, 1 year, and 7 years, at each of these ages, the Asian American children averaged larger cranial volumes than did the White children, who averaged larger cranial volumes than did the Black children. Within each race, cranial capacity correlated with IQ scores. By age 7, the Asian American children averaged an IQ of 110; the Whitechildren, 102; and the Black children 90. _PDF_30 Years of Research on Race Differences_PDF

It happens that brain size and intelligence are correlated on many scales.

A study on twins (Thompson et al., 2001) showed that frontal gray matter volume was correlated with g and highly heritable. A related study has reported that the correlation between brain size (reported to have a heritability of 0.85) and g is 0.4, and that correlation is mediated entirely by genetic factors (Posthuma et al. 2002).


In a study of the head growth of 633 term-born children from the Avon Longitudinal Study of Parents and Children cohort, it was shown that prenatal growth and growth during infancy were associated with subsequent IQ. The study’s conclusion was that the brain volume a child achieves by the age of 1 year helps determine later intelligence. Growth in brain volume after infancy may not compensate for poorer earlier growth.[6] _Wikipedia

But the development of particular grey matter loci and white matter pathways are correlated with IQ even better than simple gross brain size.

Previous research had shown that larger brains are weakly related to higher IQ, but this study is the first to demonstrate that gray matter in specific regions in the brain is more related to IQ than is overall size. Multiple brain areas are related to IQ, the UCI and UNM researchers have found, and various combinations of these areas can similarly account for IQ scores. Therefore, it is likely that a person’s mental strengths and weaknesses depend in large part on the individual pattern of gray matter across his or her brain. _UCI

The neural determinants of IQ go far deeper than gross brain size and structural features of grey and white matter, however. The molecular functioning of neurons and synapses, as well as glial cells, depends upon the genetic complement provided. On all of these neural scales, IQ depends upon the genes. (see Heritability of IQ, Wiki)

One particular genetic polymorphism -- COMT Val158Met -- appears to play an interesting role in the speed of some pre-frontal functions. More here. It is easy to see why some particular genetic correlations to IQ might be difficult to replicate, even if the correlation were real. The size of the correlation of particular genes to IQ are individually small, and certain to vary, depending upon the rest of the genome in the population being studied.

In other words, genes work together in concert, so that certain genes tend to magnify (or block) the effect of certain other genes. Even more complicating is the fact that the same proteins may serve multiple functions in many tissues. Discovering "IQ genes" that can be manipulated to increase IQ -- without altering other body functions in an adverse way -- may be impossible in the near to intermediate time frame.

Summary: Brain size does correlate with genes, race, and IQ. But better correlations can be found between the morphology of certain grey matter centers and white matter pathways, and IQ. The speed of white matter neural information transmission is another part of the picture -- and is influenced by the genes.

By comparing brain maps of identical twins, which share the same genes, with fraternal twins, which share about half their genes, the team calculate that myelin integrity is genetically determined in many brain areas important for intelligence. This includes the corpus callosum, which integrates signals from the left and right sides of the body, and the parietal lobes, responsible for visual and spatial reasoning and logic (see above). Myelin quality in these areas was also correlated with scores on tests of abstract reasoning and overall intelligence (The Journal of Neuroscience, vol 29, p 2212).

At the molecular level of ion channels and neurotransmitter receptors (etc), the genes play an even greater role.

Denial of genetic influence on IQ is one of many cancers eating away at scientific integrity in academia. It is driven by ideology and has a devastating impact on young brains that are often too trusting in the integrity and disinterestedness of the professor.