During the 1990s to the early 2010s, it was believed that the brain-size IQ correlation among adults living in developed countries was about 0.4.  Then in 2015, a meta-analysis by Jakob Pietschnig, Lars Penke, Jelte M. Wicherts, Michael Zeiler, and Martin Voracek surfaced claiming the brain size-IQ correlation was only 0.24!  The paper argued that the 0.4ish figure that was typically cited was inflated by publication bias and these authors went out of their way to counter this.

But then in 2017, a meta-analysis by Gilles E. Gignac and Timothy C. Bates, published in the peer reviewed journal Intelligence showed once again that it was 0.4.  The authors reviewed the research cited by Pietschnig et al but corrected for range restriction, test quality, and sample quality and a 0.4 correlation was found.

Now, we have a massive new study to help settle the debate. The paper is called Are Bigger Brains Smarter? Evidence From a Large-Scale Preregistered Study by G. Nave et al.

Here’s the abstract:

A positive relationship between brain volume and intelligence has been suspected since the 19th century, and empirical studies seem to support this hypothesis. However, this claim is controversial because of concerns about publication bias and the lack of systematic control for critical confounding factors (e.g., height, population structure). We conducted a preregistered study of the relationship between brain volume and cognitive performance using a new sample of adults from the United Kingdom that is about 70% larger than the combined samples of all previous investigations on this subject (N = 13,608). Our analyses systematically controlled for sex, age, height, socioeconomic status, and population structure, and our analyses were free of publication bias. We found a robust association between total brain volume and fluid intelligence (r = .19), which is consistent with previous findings in the literature after controlling for measurement quality of intelligence in our data. We also found a positive relationship between total brain volume and educational attainment (r = .12). These relationships were mainly driven by gray matter (rather than white matter or fluid volume), and effect sizes were similar for both sexes and across age groups.

This study is important not just because of its colossal sample size, but the fact that it was preregistered, meaning they agreed to publish the results before they knew what said results were. Critics of this research have worried there’s a file drawer effect, where studies not finding the desired result get shelved, and mostly larger positive correlations get published.

Also interesting is that the correlation between intelligence and brain size remained even after controlling for sex, height, population structure and socioeconomic status. One could even argue this is an over-correction, since socioeconomic status is itself a crude measure of intelligence, and yet even after this over-correction, the correlation remained, suggesting the link between brain size and intelligence is causal and not just a byproduct of a shared correlation with body size (i.e. height) or the nutrition that comes from high SES.

However IQ enthusiasts might be disappointed by how low the correlation is. Only 0.19. And as the matrix below shows, even before they corrected for anything, the correlation was only 0.21 (only half the 0.4 correlation long cited by IQ enthusiasts and confirmed by Gilles and Bates, 2017)

Supplementary material from Are Bigger Brains Smarter? Evidence From a Large-Scale Preregistered Study by G. Nave et al.G

The most likely explanation for this low correlation is that in order to get such a huge sample, they needed a really short test. And indeed the Fluid IQ test used in the study is just the two minute Verbal Numerical Reasoning test sometimes administered on more than one occasion to improve reliability. But as we can see from the 0.31 correlation between Fluid IQ and Education Attainment, the test is not that g loaded.

Indeed on tests with near-perfect g loadings like the WAIS-III, IQ correlates 0.55 with education (though this is in America). If we divide the Fluid IQ test’s correlation with education by the WAIS-III’s, we might get a very crude estimate of the Fluid IQ test’s correlation with the WAIS-III and that correlation is 0.56. If we further divide brain size’s 0.21 correlation with the Fluid IQ test by 0.56, we might get a crude estimate of brain size’s correlation with the WAIS-III and that correlation is 0.38. Thus confirming Gilles and Bates, 2017.

Of course Pietschnig et al can counter that brain size in this massive study only correlates 0.14 with education. Dividing 0.14 by the WAIS-III’s 0.55 correlation perfectly confirms their 0.25 correlation between IQ and brain size.

Thus, the only solution is to average 0.38 and 0.25, which gives a 0.32 correlation between IQ and brain size. Still a moderate correlation, but the low end of the moderate range. Indeed long before MRIs, a correlation of 0.3 was considered the best estimate of IQ’s correlation with brain size. The figure was achieved by adjusting the weak correlation between head size and IQ for the fact that head size was only a rough proxy for brain size.

So even though IQ doesn’t correlate quite as well with brain size as IQ enthusiasts would have liked, the good news is that the correlation barely declines when you control (even over-control) for other variables, implying it’s overwhelmingly causal.