Summary: There are four methods of measuring brain size, and studies utilizing all four methods have found that Whites have larger brains than Asians who have larger brains than Blacks. If you adjust for body size, the position of Whites and Asians flips. There are at least six lines of evidence to suggest that genes explain part of these racial differences: the differences are present at birth, around the world, the Black/White brain size gap is not smaller today than it was 100 years ago, mixed race individuals have brain sizes in-between their parent’s races, and, finally, traits that typically co-evolve with brain size differ racially in a way that mirrors brain size differences. Moreover, there is reason to think that climate was an important evolutionary factor driving these changes. Finally, racial brain size differences can account for some, but not all, of the racial IQ gaps.

Four Methods; One Finding

In 1994 Harvey et al preformed the first study comparing the brain size of different racial groups using MRI technology to measure brain size. They confirmed previous findings: Blacks have smaller brains than Whites. The same finding was reproduced by Jones et al (1994), though the difference wasn’t statistically significant. Chee et al (2010) found that Whites have larger brains than East Asians. Finally, Tang et al (2010) found that Whites have longer brains than East Asians while East Asians have wider and taller brains than Whites. Unfortunately, all of these studies have very limited sample sizes and, with the exception of Tang et al, failed to control for differences in the sexual composition of racial samples. Thus, if, for instance, the Black samples had a higher female to male ratio than the White samples then, because men have larger brains than women, Black’s brain size will be depressed relative to Whites.

These findings correspond well with what older studies measuring endocranial volume found. Endocranial volume is measured by filling the skull of a deceased person with some substance, often mustard seed, and then measuring the amount of said substance that fit into the skull. Beals (1984) aggregated data on roughly 20,000 subjects from past studies which showed East Asians having the largest skulls followed by Whites followed by Blacks. LKLKLI

A famous critique of this line of work was launched by Gould (1981) who argued that researchers involved in this work, most famously the 19th century anthropologist Samuel Morton, unconsciously allowed their racist views to cause them to back material more tightly into White skulls than into Black skulls and thus inflate the racial difference. Gould also accused Morton of excluding data from his tables that increased the racial disparity in brain size in favor of Whites. Both of these accusations have been shown to be false. Morton’s skulls were re-examined by modern researchers who reproduced Morton’s results, and the data that Gould accused Morton of omitting was actually included by Morton in the same book that Gould cited, just on different pages (Lewis et al. 2011). Gould was thus exposed as nothing more than a petty propagandist who believed that his clout in academia would prevent people from noticing his deceit.

Of course, a third way to measure brain size is to rip a brain out of a skull during an autopsy and measure its volume. Autopsy work done in the late 2000’s found that Whites averaged the largest brains, followed by, East Asians, followed by Blacks (Rushton and Ankney, 2009). This same method has been used to confirm the Back/White/Asian brain size disparity for well over a century:

Source: Rushton (1995)

Tobias (1970) offered a highly influential critique of this literature which in turn was cited and popularized by Gould. In it, Tobias argued that comparisons of the brain size of racial groups based on autopsies were invalid because they failed to control for a wide variety of variables that could impact brain size. Said variables included, but were not limited to, age of death, nutritional intake early in life, occupational status, cause of death, time of death, temperature the brain was kept in after death, and the exact place the brain was cut from the spinal cord.

Some of these controls would obviously be misleading because of the correlation between things Tobias wanted held constant and intelligence (occupational status for instance) and so holding them constant would involve holding a part of the racial IQ gap constant. That being said, of these criticisms are valid. However, there is no reason to think that any of these problems would bias the results in favor of one race or another. And random error introduced can be overcome via aggregation. Thus, Rushton (1995) combined all the data from the studies Tobias critiqued and found the now familiar pattern of Whites having the largest brains, followed by Asians, followed by Blacks.

The fourth way to measure brain size is to estimate it based on external head size. The major advantage to this study is that researchers can cheaply measure the brain sizes of large and representative samples of living people. One of the largest of these studies was Rushton (1994) which had a sample in the tens of thousands and found that Whites had the largest brains, followed by East Asians, followed by Blacks. Similar results were found by Rushton (1992) in an analysis of 6,325 military personnel, and by Rushton (1997) in an analysis of nearly 40,000 Americans.

As we’ve seen, some of these methods are controversial. However, many of these criticisms are invalid and the criticisms which are valid would not bias the results with respect to race. Moreover, all four methods result in the same findings regarding the racial ordering of brain size. Given this, we can take it as well established that Whites have larger brains than Asians who in turn have larger brains than Blacks.

Brain Size and Body Size

The racial ordering by brain size changes if you adjust for racial differences in body size. When done, East Asians have the largest brains, followed by Whites, followed by Blacks (For instance: Rushton, 1992 and Rushton, 1994). Adjustments for body size became much more common in the late 20th century, causing the racial ordering reported in papers to change. Lieberman (2001) invented some creative science fiction about how this change reflected Japan’s economic success during this time period which caused the evil racist professors, who want to justify whatever the economic statues quo is, to change the racial brain size ordering. But body size adjustments are what actually happened.

This brings up an interesting question: which matters more for intelligence absolute brain size or brain size relative to body size? It is plausible to think that larger bodies require more brains to “manage”. But how strong is that relationship? Do organisms with bodies 10% larger need brains that are 10% larger, 5% larger, or 1 % larger, in-order to manage their extra body size? The fact is that we don’t know, and, because of this, we don’t really know whether East Asians or Whites get a larger boost to their IQ as a result of their brain size. (We do, however, know that both groups have larger brains than Blacks.)

The Cause of Racial Brain Size Differences

So, the races differ in brain size. But why? Well, there are at least six lines of evidence suggesting that genes explain at least some, though not necessarily all, of the racial brain size disparities.

The first thing to note is that brain size in the general population has a heritability of 87%. Thus, almost all the differences between individuals in brain size are caused by genes.

Secondly, we should note that racial brain size differences are present at, and even before, birth. Thus, Schultz (1922) found that White fetuses had larger brain cases than Black fetuses in a sample of 623 fetuses. Similarly, Rushton (1997) found that, at birth, Asians had larger brains than White who in turn had larger brains than Blacks in a sample of 35,859 American infants. Lastly, Ho et al (1980) found that White newborns had larger brains than Black newborns in a sample of 782 infants. The use of Ho et al’s data is controversial, because the racial difference in this sample is due to more Blacks than Whites being born prematurely. It isn’t clear what we should make of this fact, because the norms for what a healthy length of gestation is are based on Whites who physically mature more slowly than Blacks.

The third piece of evidence is that the racial gaps have existed for a long time. In fact, based on the data from the tables above, the White/Black brain size gap doesn’t seem to have gotten any smaller during the 20th century.

The Asian/White gap has, suggesting that it may be more environmental in origin. (The female data paints the same picture, but there wasn’t enough data to meaningfully break it down into 20 year periods.)

This is in-spite of the fact that the environmental differences between Blacks and Whites have significantly lessened. For instance, racial differences in education and income have fallen, and explicit racism against Blacks is now rare.

Fourthly, racial brain size differences have been found all around the world. For instance, the Beals and Smith data set previously referred to features skulls from over 100 populations world wide. Many of the autopsy studies previously cited were done in East Asia, as were two of the MRI studies. This makes any gene independent cultural explanation less likely.

Fifthly, several studies have shown that mulattoes have an average brain size in between that of Blacks and Whites (Pearl, 1934; Bean, 1906). This finding has been established on multiple occasions and is what a hereditarian hypothesis would predict since mulattoes are half White and half Black genetically speaking.

Sixth, many traits which tend to co-evolve with larger brains also differ racially in a way that mirrors the body size adjusted brain size pattern. Rushton and Rushton (2003) looked at 37 anatomical features which 3 textbooks on human evolution identified as tending to co-evolve in the hominid line with larger brains. For instance, larger pelvic size tends to co-evolve with brain size so that mothers can give birth to larger brained infants. Rushton then utilized 5 forensic anthropology textbooks to look at racial differences in these traits. These traits followed the East Asian>White>Black pattern in 25 out of 31 cases. The probability of this happening at random is .000000001.

Similarly, Rushton (2004) showed that, across 234 mammalian species, brain size correlates with longevity, gestation time, birth weight, litter size, age of first mating, body weight, and body length. Various studies have shown that each of these variables also differ between the races in a way that, based on what we find across the animal kingdom, would predict the body size corrected brain size differences we observe (Rushton, 1995; Templer 2006; Rushton and Templer, 2009;).

Thus, we have six lines of evidence all of which would be predicted by a hereditarian view on racial differences in brain size. While each line of evidence on its own may not be compelling, the combination of all six seems to strongly imply that racial brain size gaps are partly heritable.

It is worth noting that the racial brain size gaps are probably not entirely attributable to genes. Some authors, including Richard Nisbett, have plausibly argued that nutrition also plays a role. However, there explanations are not mutually exclusive, no environmental variables has been shown to account for the majority of the gap, and, as we have seen, there are many separate lines of evidence indicating that genes also play a role

The Evolution of Brain Size Differences

There is good evidence that the races evolved different brain sizes in response to climate. Specifically, various studies have found that a population’s brain size correlates with climate related variables. For instance, Pearce and Dunbar (2011) ‘s data set produces a correlation of .74 between a population’s brain size and its latitude. Similarly, Ash and Gallup (2007) found a correlation of .48 between the size of 109 fossilized human skulls and the latitude at which they were found. Further still, Bailey and Geary (2009) analyzed 175 skulls ranging in age from 10,000 years old to 1.9 million years old and found a correlation of -.41 between brain size and winter temperature and -.61 between size and latitude (larger brains were found in areas more distant from the equator).

This also, by the way, constitutes a seventh arguments for a genetic explanation of racial brain size differences. After all, this data shows that, broadly speaking, the same geographic variables that predict brain size today also did so 10,000+ years ago, suggesting that population brain size differences have been with us for a very long time. This data also gives us reason to think that distance from the equator, and the cold climates that come with it, had something to do with the evolutionary pressures that lead to racial differences in brain size.

Race, IQ, and Brain Size

To end this post, let’s look at how much of the racial IQ gaps can be accounted for by racial brain size differences.

Lynn (2015) used Smith and Beals data set of 20,000 skulls from 122 populations to estimate that roughly 30% of the African-European IQ gap can be statistically accounted for by brain size differences. By contrast, brain size differences would actually predict an Asian-European IQ gap 35% larger than the one that actually exists. Thus, brain size is probably one of many factors, both biological and environmental, which account for racial intelligence differences.