Thread by @iointelresearch

[THREAD]

A VERY BASIC INTRODUCTION TO THE HEREDITARIAN POSITION ON RACE AND IQ.

PART 12: The Hereditarian Explanation –- Brain Studies

Hereditarians are quick to point out that one of the most reproducible findings in the science of human intelligence, across dozens of studies, is the association between brain size and intelligence.

Bigger brains (or larger cranial capacity or volume of the interior of the cranium) associate with higher intelligence, on average.

Larger brains contain more neurons and synapses and process information faster.

This finding is important in the race and IQ debate because average brain size has been found to vary by race — northeast Asians have the largest brains, followed by Europeans, and then Sub-Saharan Africans. The mean brain size of races correlates with their average IQs.

A massive 2018 study using UK Biobank data from 13,608 adults, which was controlled for sex, age, height, socioeconomic status and race found a “robust association between total brain volume and fluid intelligence... consistent with previous findings.”

Another even larger (29,004 individuals) and more recent age- and sex-corrected study from 2019 also found an association between total brain volume and general intelligence.

These are just two of the more recent large well-controlled brain size studies. There are many others.

The correlation between brain size and intelligence in MRI studies is typically a “moderate” one between .30 and .40.

It's important to note that modern brain studies use advanced imaging and scanning technologies to accurately measure brain size, and study sample sizes can be dozens or even hundreds of times larger than in studies conducted only decades ago.

Many of the specific genes associated with brain size have now been identified in a large genome-wide association study (GWAS). Others will likely be shortly identified.

Even more intriguing, a 2019 GWAS meta-analysis, one of the largest ever conducted (involving over 50,000 individuals), identified 346 new genes associated in the overlap between brain volume and intelligence.

Estimates from twin studies indicate that genes contribute from 50% to 90% of the variance to both cranial capacities (based on external head size measures) and to brain volume measured by MRI.

“[M]any specific brain structures [are] strongly heritable (i.e., 80% or more)... [there is] a strong heritability for brain volume (greater than 80%).”

Significantly, recent studies, drawing upon multiple data sets, find that the correlation between bigger brains and higher intelligence is a *causal* one.

As previously mentioned, these findings are significant because average brain size appears to vary by race, with most studies finding NE Asians having the largest cranial volume, followed by whites, and then sub-Saharan Africans -– the same order as in their average IQs.

Brain size differences between races are present at birth, which “implies some prenatal and biological mediation.”

One fairly recent study found pre-natal racial differences in head circumference “evident at 21 weeks of gestation.”

Brain size differences between races cannot be adequately explained by poverty and nutrition because they were found even when these variables were dramatically worse in China and Korea than in Europe and the US (which demonstrated smaller brain sizes than NE Asians).

Not only do whites have larger brains, on average, than blacks, but also different regions of the brain vary in size by race — for example, “the OFC and amygdala appear to occupy a significantly greater proportion of the total cerebral volume” of blacks.

Brain structure has also been found to be different between whites and northeast Asians.

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Tang Y, Hojatkashani C, Dinov ID, Sun B, Fan L, et al. (2010) The construction of a Chinese MRI brain atlas: a morphometric comparison study between Chinese and Caucasian cohorts.