Meta-analysis of associations between human brain volume and intelligence differences: How strong are they and what do they mean?☆
Introduction
Associations between brain size and intelligence have been subject to investigation for more than a century. In 1836, Friedrich Tiedemann, a German anatomist and physiologist wrote: “There is undoubtedly a very close connexion between the absolute size of the brain and the intellectual powers and functions of the mind” (Tiedemann, 1836, p. 502). Thereafter, this assertion has been entertained by influential minds (e.g., Darwin, 1871, Lombroso, 1864; Broca, 1861, as cited in Rushton and Ankney, 2009) until today. It is extensively reflected in the literature (Deary et al., 2010, Jensen, 1982, McDaniel, 2005, Rushton and Ankney, 2009; Van Valen, 1974) and indeed in lay psychology and the common language, as in the proverbial “big brained” as a synonym for being smart. However, this alleged association has also been subjected to intense debate and controversy (e.g., Deary et al., 2010, Gould, 1981, Jensen, 1982) about its meaning and strength. In this article we will review the evidence on the strength of the linear association between brain size and measures of intelligence, with a particular focus on the most comprehensive and detailed meta-analysis of the relationship between human in vivo brain volume and IQ. We will then critically discuss how this association can be interpreted and how it relates to brain size differences between species.
Section snippets
Surrogate measures of brain volume and intelligence
Even though an association between brain volume and intelligence had been hypothesized early on, for long there was a lack of good in vivo measures of brain volume. As a first attempt to quantify the association between brain volume and intelligence, Galton (1888) used linear external head measures (height, breadth, depth) as a proxy for brain size and achievements at universities as a measure for cognitive abilities. The introduction of intelligence tests allowed assessment of cognitive
Meta-analysis of the association between in vivo brain volume and IQ
In the first study using MRI, Willerman et al. (1991) reported an association of r = .51 between brain volume and full-scale IQ of the revised Wechsler Adult Intelligence Scale (WAIS-R; Wechsler, 1981) in a sample of 40 healthy college students. Subsequently, as the use of MRI became more common, several replication attempts of this strong effect of MRI-based studies (a previous review of head circumference and IQ had found overall effects of only about r = .30; Van Valen, 1974) were published, but
Why is brain size associated with intelligence?
In vivo MRI studies clearly confirm earlier findings of an association between intelligence and brain size based on surrogate measures such as head dimensions or inner skull volume. Interestingly, the association generalizes across sex despite marked sexual dimorphism in brain volume, and the relationship with intelligence even seems to hold when correcting for height and body mass (Rushton and Ankney, 2009). Furthermore, post-mortem studies of brain weight also show an association with
Conclusion
In conclusion, we could show a robust, albeit modest, association between IQ and brain size in humans. Surprisingly, this association remains robust across age, intelligence domain, and sex of participants and even holds when accounting for effect inflation due to publication bias.
However, invoking the literature on cross-species comparisons and primate cognitive evolution to argue for brain size as an isomorphic proxy for human intelligence differences is not warranted. Such assumptions are
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The authors would like to thank T. Haubner and S. Pavlovic for their help regarding literature acquisition.