The science of intelligence
The brain and intelligence
Short answer
Brain size matters, but only a little. A 2015 meta-analysis of 88 studies and more than 8,000 people found a correlation of .24 between brain volume and IQ. The link held up after correcting for publication bias, but it was smaller than earlier studies suggested, and the authors warned that brain size is not a stand-in for intelligence.
Want your own number? The test is 25 visual puzzles, about 12 minutes, no sign-up.
Brain size: a real but modest link
The question of whether bigger brains mean higher intelligence is old, and it now has a large evidence base. A 2015 meta-analysis pooled 88 studies covering 148 samples and more than 8,000 people. It found a positive association between brain volume and IQ of r = .24, and the association generalized across age, IQ domain and sex (Pietschnig et al., 2015).
The same team found that the link had been overestimated in the published literature because of publication bias: studies with striking results were more likely to be published. After correcting for that, the association was smaller but still robust (Pietschnig et al., 2015). Their conclusion was blunt: brain size should not be treated as an isomorphic proxy for intelligence differences (Pietschnig et al., 2015). In plain terms, you cannot read someone's intelligence off the size of their head or their brain scan.
What a correlation of .24 means
Squaring a correlation gives the share of variation two measures have in common. For .24 that is about 6%. The other 94% of the differences in IQ between people go with something other than brain volume. A correlation of this size is meaningful for science, because it shows biology and test scores are connected, but it is weak for any individual prediction. The correlation glossary entry explains how to read these numbers.
Try one · Quantitative Reasoning
Which number comes next?
4, 30, 7, 32, 10, 34, ?
Tap the option you think is right, or show the answer.
Show the answer
The next number is 13 (option 1).
How to solve it
- The gaps jump around, so look at every other number.
- The 1st, 3rd, 5th numbers (4, 7, 10) go up by 3 each time.
- The 2nd, 4th, 6th numbers (30, 32, 34) go up by 2 each time.
- The missing number continues the first series: 10 + 3 = 13.
That was 1 puzzle. The full test has 25.
A network, not a single spot
If size explains little, where does intelligence live? The most cited answer is the Parieto-Frontal Integration Theory (P-FIT). Rex Jung and Richard Haier reviewed 37 modern neuroimaging studies and found that variation in a distributed brain network predicts individual differences on intelligence and reasoning tasks (Jung & Haier, 2007). The network includes (Jung & Haier, 2007):
- the dorsolateral prefrontal cortex, toward the front of the brain
- the inferior and superior parietal lobule, toward the top and back
- the anterior cingulate
- regions of the temporal and occipital lobes
- white matter tracts such as the arcuate fasciculus, which connect these areas
The idea fits what tests show from the outside: reasoning draws on several abilities at once, such as holding information in mind and controlling attention. Those abilities are covered on the working memory and executive function pages.
Two popular brain myths
The first is that we use only 10% of our brains. The Association for Psychological Science notes that PET scans show the entire brain is constantly active, even during sleep (Chew, 2018 (APS)). More on that in the 10% myth.
The second is that people are "left-brained" or "right-brained." A resting-state imaging study of 1,011 people aged 7 to 29 found data not consistent with that idea, and found lateralization to be a local property of brain networks rather than a global one (Nielsen et al., 2013). See left brain vs right brain.
Brains, genes and experience
Brain studies are correlational. They show which features go with higher scores, not which way the influence runs. Genes contribute to both brain development and test performance, and the heritability of intelligence rises with age (Plomin & Deary, 2015), a topic covered in genes and intelligence. The honest summary is that the brain basis of intelligence is real, spread across many regions, and only loosely captured by any single measurement such as total volume.
Sources
- Pietschnig, J., Penke, L., Wicherts, J. M., Zeiler, M., & Voracek, M. (2015). Meta-analysis of associations between human brain volume and intelligence differences: How strong are they and what do they mean? Neuroscience & Biobehavioral Reviews, 57, 411-432. doi.org/10.1016/j.neubiorev.2015.09.017
- Jung, R. E., & Haier, R. J. (2007). The Parieto-Frontal Integration Theory (P-FIT) of intelligence: Converging neuroimaging evidence. Behavioral and Brain Sciences, 30(2), 135-154. doi.org/10.1017/S0140525X07001185
- Chew, S. L. (2018, August 29). Myth: We only use 10% of our brains. Association for Psychological Science. www.psychologicalscience.org/uncategorized/myth-we-only-use-10-of-our-brains.html
- Nielsen, J. A., Zielinski, B. A., Ferguson, M. A., Lainhart, J. E., & Anderson, J. S. (2013). An evaluation of the left-brain vs. right-brain hypothesis with resting state functional connectivity magnetic resonance imaging. PLOS ONE, 8(8), e71275. doi.org/10.1371/journal.pone.0071275
- Plomin, R., & Deary, I. J. (2015). Molecular Psychiatry, 20, 98-108. doi.org/10.1038/mp.2014.105