I previously described a study published in the January 2021 volume of the journal Cerebral Cortex, one entitled "Is There a Correlation Between the Number of Brain Cells and IQ?" The authors (Nicharatch Songthawornpong, Thomas W Teasdale, Mikkel V Olesen, and Bente Pakkenberg) examined 50 brains of Danish males who had died for reasons other than brain disease. It was possible to reliably estimate the IQ of these Danish males because they all had taken a military mental performance test that very highly correlates with IQ, and is essentially an intelligence test.
The paper very clearly states its results:
""In our sample of 50 male brains, IQ scores did not correlate significantly with the total number of neurons (Fig. 1A), oligodendrocytes (Fig. 1B), astrocytes (Fig. 1C) or microglia (Fig. 1D) in the neocortex, nor with the cortical volume (Fig. 2A), surface area (Fig. 2B) and thickness (Fig. 2C). This also applied to estimates of the four separate lobes (frontal-, temporal-, parietal-, and occipital cortices; see Supplementary Material). Neither did IQ score correlate significantly with the volumes of white matter (Fig. 2D), central gray matter (Fig. 2E) or lateral ventricles (Fig. 2F), nor with the brain weight (Fig. 3A), or body height (Fig. 3B). All of these correlation coefficients were less than 0.2."
What this means is that the authors found:
- It is not at all true that the more brain cells you have, the more likely you are to be smart.
- It is not at all true that the more gray matter in your brain, the more likely you are to be smart.
- It is not at all true that the more white matter in your brain, the more likely you are to be smart.
- It is not all true that the heavier your brain, the more likely you are to be smart.
Although such results do not by themselves show that your brain is not the source of your mind, such results are quite compatible with the hypothesis that your brain is not the source of your mind. But the message coming from the study is not as loud as it might be, given the rather small sample size of only 50 brains. Much larger studies have been done using scans of thousands of brains.
The brain scans were done as part of the UK Biobank project. In that project about 29,000 subjects had their brains scanned, and about 7,000 of them also performed four cognitive tests. The study "Structural brain imaging correlates of general intelligence in UK Biobank" (which you can read here) analyzed performance on such tests, and estimated a general intelligence (which it called g) for each of several thousand people taking the tests, all of whom had their brains scanned. The study then estimated what the correlation was between things such as intelligence and brain volume. The study found a correlation of only .276 between brain volume and intelligence, and a correlation of only .281 between gray matter volume and intelligence.
How high a correlation is that? In the scientific paper entitled, “A guide to appropriate use of Correlation coefficient in medical research,” we have a Table 1 which has the heading of "Rule of Thumb for Interpreting the Size of a Correlation Coefficient." Here is that table:
| Size of Correlation | Interpretation |
| .90 to 1.00 (−.90 to −1.00) | Very high positive (negative) correlation |
| .70 to .90 (−.70 to −.90) | High positive (negative) correlation |
| .50 to .70 (−.50 to −.70) | Moderate positive (negative) correlation |
| .30 to .50 (−.30 to −.50) | Low positive (negative) correlation |
| .00 to .30 (.00 to −.30) | negligible correlation |
So by finding a correlation of only .276 between brain volume and intelligence, the study with a very big sample size of about 15,000 subjects has found only a negligible correlation between brain size and intelligence. And by finding a correlation of only .281 between gray matter volume and intelligence, the study with a very big sample size of about 15,000 subjects has found only a negligible correlation between gray matter volume and intelligence.
We have a result here quite compatible with the idea that your brain is not the source of your mind. Similarly, the 2019 study discussed here studied the brains of 324 people by brain scanning, and found that neither knowledge nor intelligence had any clear relation to brain parameters.
The authors of the study "Structural brain imaging correlates of general intelligence in UK Biobank" (which you can read here) seem to have made some arbitrary choices about how to analyze the data they had. In the total UK Biobank data there were more than 10,000 subjects who had their brains scanned and who also did a Verbal Numerical Reasoning test. But the study authors chose to use only about 7000 of those subjects, only those who had taken four cognitive tests. They also made the arbitrary decision to use only Part B of a Trail Making Test (a type of cognitive test) rather than the full data gathered (including a Part A and Part B). We can assume that the authors were trying to do an analysis that would show as high a correlation between brain volume and intelligence as they could get; but they have still only reported a negligible correction of .276. A different analysis of the same UK Biobank would have shown an even lower correlation between brain size and intelligence. This is proven by the quote below in the paper:
"Using an earlier data release (Ritchie et al., 2018), we previously estimated the correlation between brain size and one of those tests, 'Fluid Intelligence' (which we refer to as Verbal-Numerical Reasoning) to be r = 0.177. We found that the correlation did not differ by sex. Another study using an earlier release of UK Biobank imaging data examined the association between Verbal-Numerical Reasoning and brain size, reporting a correlation of r = 0.19 (N = 13,608; Nave, Jung, Linnér, Kable, & Koellinger, 2019)."
The second paper referred to is "Are Bigger Brains Smarter? Evidence From a Large-Scale Preregistered Study" which you can read here. The authors of that paper falsely described the negligible correlation they found between brain volume and fluid intelligence, describing the negligible correlation they found of only .19 as "robust." As the table above tells us, all correlations of less than .30 are properly described as "negligible."
Different types of cognitive tests are usually measurements of things more than just intelligence, because the scores in the test scores can be influenced by things such as manual dexterity, visual perception and a tendency of an unmotivated mind to wander. The type of very low correlations reported above can be easily accounted for under the idea that your brain is not the source of your mind, by supposing that slight differences in things such as manual dexterity and visual perception are showing up in the test scores.
"The largest and most recent meta-analysis (a study of studies) on the topic was led by Martin Voracek of the University of Vienna in 2015. Voracek and colleagues found that, although there is evidence for a relationship between overall brain volume and various intelligence measures, the strength of the relationship is very weak.
Encompassing 148 samples totaling more than 8,000 individuals in total, the Voracek study found that about 6 percent of the variance in intelligence performance can be attributed to differences in brain volume. This means that a very 'intelligent' person (by one metric or another) can attribute only a tiny fraction of their smarts to having a big brain, while a highly 'unintelligent' person can assign their brain size the same very small degree of blame.
However, even these weak relationships may be illusory. Showing that there is a positive relationship between brain size and smarts is an inherently headline-grabbing finding. Researchers who believe in such a relationship are more likely to undertake these kinds of studies, more likely to analyze their data in a way that generates the desired outcome, and more likely to be able to publish their positive results. Biases like these, which have riled the psychological sciences in recent years, make me doubt that any meaningful relationship exists between brain size and intelligence."
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