Human brain size

Human brain size is a line of "evidence" often brought up by racialists and sexists, who attempt to prove that one race or gender is superior to another by advocating long-discredited 19th-century theories that brain size predicts intelligence.

Studies in physical anthropology and neuroimaging have found that differences in average brain volume exist between various racial or ethnic groups,  but this variation has no relation to any difference in psychological traits or abilities. Despite having been repeatedly debunked, claims of a relationship between brain size and intelligence continue to appear in academic publications.

Arguments for racism
In the 1840s, Samuel George Morton attempted to prove that differences in average brain size demonstrated a racial hierarchy. Morton faked his results in order to support his racial prejudices. Morton's methods and conclusions were imitated by various other scientific racists of the early twentieth century. Papers by these authors are laden with the virulent bigotry that was commonplace in the Jim Crow era.

In the late 20th century, arguments for a racial hierarchy based on brain volume were resurrected by Pioneer Fund grantees such as Arthur Jensen, J. Philippe Rushton and Richard Lynn. These studies use modern IQ testing to argue for the same tired conclusions that were advocated by scientific racists of the Jim Crow era. An illustrative modern example is a 1990s paper by Arthur Jensen and Fred Johnson claiming that races differ in both average brain size and average IQ, but that when children of different races are matched for IQ, the differences in average brain volume disappear. Like the "research" from the Jim Crow era, these claims are transparently advocating the pseudoscientific idea that race differences in average brain volume produce biologically innate differences in intelligence.

Arguments for sexism
A 2017 paper by Dimitri van der Linden, Curtis S. Dunkel and Guy Madison claims that larger average brain size among men makes men innately more intelligent than women. All three of these authors are men. (You would never see a woman advocating such an idea, would you?) Like the racial claims about brain volume and IQ, any attempt to link brain size to differences in intelligence between genders is inherently ridiculous, because it relies on the discredited assumption that brain size predicts intelligence.

Notable claims
Outside of abnormalities, such as, brain size is not statistically related to any measure of intelligence. This should be obvious, because if such a relationship did exist, sperm whales would be more intelligent than humans! Furthermore, having an abnormally large amount of brain tissue,, is pathological and strongly associated with several conditions, especially autism and severe seizure disorders. Nevertheless, various authors have made pseudoscientific claims of such a relationship.

While not overtly racial in nature, these claims are closely linked to modern scientific racism. Publications alleging such a relationship typically contain citations to hardcore racialists, who in turn cite these publications to advance their own theories. Notable examples of publications alleging relationship between brain volume and intelligence include the following:


 * A 2002 study by Daniëlle Posthuma, Eco J. C. De Geus, Wim F. C. Baaré, Hilleke E. Hulshoff Pol, René S. Kahn and Dorret I. Boomsma claimed that the relationship between brain volume and IQ is entirely mediated by genetics. This is among the most egregious of such papers, because while it assiduously avoids mentioning race, aside from that omission it is nearly indistinguishable from the racialist arguments about brain volume and intelligence. It is unclear whether Posthuma et al. were oblivious to their claim's ethical implications, or whether the claim was deliberately malicious.
 * A 2010 literature review by Ian Deary, Lars Penke and Wendy Johnson claimed that intelligence is "modest[ly]" correlated with both head size and intracranial volume. However, to quote, "of course, finding correlations does not explain how one variable affects another, and explaining such correlations is considerably more difficult than identifying them." The review includes a section where biases against race, gender, etc. are discussed, and admits "The tools that are currently available to address these issues... are inadequate to resolve them. This is because, at present, we can only measure intelligent performance, which develops over time. Its development in an individual is therefore embedded in the individual’s environment of origin."
 * A 2015 meta-analysis by Jakob Pietschnig, Lars Penke, Jelte Wicherts, Michael Zeiler, and Martin Voracek claimed that intelligence correlates with brain volume. It also claimed that men have larger brains than women, but stopped short of saying that this makes men more intelligent. This paper cites four(!) publications by despised and discredited white nationalist J. Philippe Rushton.
 * The 2015 textbook Experimental Psychology claimed that brain volume correlates with IQ, citing "research" by pseudoscientist Richard Haier.
 * In Stuart Ritchie's 2015 book Intelligence: All that Matters, Ritchie says "There's little reason to doubt that larger brains mean better cognition" and tries to cover his tracks by also saying, "(T)hose with bigger brains have higher IQs. This finding has been highly controversial, but it is now so well established and well replicated that it's pointless to deny it."
 * A 2017 meta-analysis by Gilles Gignac and Timothy Bates claimed that brain volume and IQ have the preposterously high correlation of .40.
 * A 2020 study by Philip R. Jansen, Mats Nagel, Kyoko Watanabe, Yongbin Wei, Jeanne E. Savage, Christiaan A. de Leeuw, Martijn P. van den Heuvel, Sophie van der Sluis and Danielle Posthuma claims to have used a method called genome wide association analysis (GWAS) claims to have found a correlation of 0.24 between polygenic scores for brain volume and intelligence. The authors state in the abstract "These results provide information on the genetics of BV and provide biological insight into BV’s shared genetic etiology with intelligence."
 * A 2019 study by James J. Lee, Matt McGue, William G. Iacono, Andrew M. Michael, and Christopher F. Chabris boldly claims to have found causal evidence of an effect of brain size on IQ. They use within family phenotypic correlations and genome wide association to make these claims. They claim to have found within family spearman rank correlations of about 0.2 between both head circumference and intelligence and brain volume and intelligence. Arthur Jensen is referenced four times in this paper.