The causal influence of brain size on human intelligence: Evidence from within-family phenotypic associations and GWAS modeling - PMC
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Intelligence<br>. Author manuscript; available in PMC: 2020 Aug 20.
Published in final edited form as: Intelligence. 2019 May 7;75:48–58. doi: 10.1016/j.intell.2019.01.011
The causal influence of brain size on human intelligence: Evidence from within-family phenotypic associations and GWAS modeling
James J Lee<br>James J Lee
aDepartment of Psychology, University of Minnesota Twin Cities, 75 East River Parkway, Minneapolis, MN 55455, USA
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a,*, Matt McGue<br>Matt McGue
aDepartment of Psychology, University of Minnesota Twin Cities, 75 East River Parkway, Minneapolis, MN 55455, USA
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a, William G Iacono<br>William G Iacono
aDepartment of Psychology, University of Minnesota Twin Cities, 75 East River Parkway, Minneapolis, MN 55455, USA
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a, Andrew M Michael<br>Andrew M Michael
bGeisinger Health System, 120 Hamm Drive Suite 2A, Lewisburg, PA 17837, USA
cDuke Institute for Brain Sciences, Duke University, 308 Research Drive, LSRC M051, Durham, NC 27708, USA
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b,c, Christopher F Chabris<br>Christopher F Chabris
bGeisinger Health System, 120 Hamm Drive Suite 2A, Lewisburg, PA 17837, USA
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aDepartment of Psychology, University of Minnesota Twin Cities, 75 East River Parkway, Minneapolis, MN 55455, USA
bGeisinger Health System, 120 Hamm Drive Suite 2A, Lewisburg, PA 17837, USA
cDuke Institute for Brain Sciences, Duke University, 308 Research Drive, LSRC M051, Durham, NC 27708, USA
*To whom correspondence should be addressed; leex2293@umn.edu
Issue date 2019 Jul-Aug.
PMC Copyright notice
PMCID: PMC7440690 NIHMSID: NIHMS1528848 PMID: 32831433
The publisher's version of this article is available at Intelligence
Abstract
There exists a moderate correlation between MRI-measured brain size and the general factor of IQ performance (g), but the question of whether the association reflects a theoretically important causal relationship or spurious confounding remains somewhat open. Previous small studies (n n = 1,022) and found a highly significant correlation (disattenuated ρ = 0.18, p n = 2,698), finding a highly significant within-family correlation between head circumference and intelligence (disattenuated ρ = 0.19, p n ≈ 10,000) and measures of cognition (257,000 n EduYears) of 0.41 (p p EduYears but many genetic variants associated with EduYears not being associated with ICV. This is the pattern of genetic results expected from a causal effect of brain size on intelligence. These findings give reason to take up the hypothesis that the dramatic increase in brain volume over the course of human evolution has been the result of natural selection favoring general intelligence.
Keywords: Intelligence, Brain size, Cognitive evolution, Statistical genetics, Causal inference
0. General Introduction
People with bigger brains tend to be smarter. Furthermore, overall brain size is the only proxy for neural complexity that can be reliably obtained from fossils across a wide range of species and times, specifically from skull endocasts (Jerison, 1973). For these reasons overall brain size is perhaps the anatomical correlate of g that most interests researchers contemplating the long-term evolution of human intelligence (Burkart, Schubiger, & van Schaik, 2017). Among all such correlates, none that can be simply measured shows a consistently replicated correlation with IQ exceeding the ∼0.25 shown by brain size (Chabris, 2007; Pietschnig, Penke, Wicherts, Zeiler, & Voracek, 2015; Haier, 2017; Gignac & Bates, 2017). Furthermore, since brain volume has tripled during the most recent 3 million years of our evolutionary history (Holloway, 2008; Herculano-Houzel, 2016), the study of its relationship with g promises to shed light on the nature of the selective advantage that has propelled this remarkable change.
There continues to be some doubt, however, about whether it is justified to interpret this correlation in terms of brain size affecting intelligence (Nisbett et al., 2012). One powerful method of testing...