How does brain structure create different levels of intelligence? I was somewhat surprised to find that the leading researcher of the relationship between neurobiology and IQ, Richard Haier, is not mentioned anywhere on LessWrong, at least according to the search function. The following post reviews current leading academic literature on the neurobiology of intelligence which is highly suggestive of the role of white matter tracts in the deep brain for elevating psychometric g. In addition to work on individual differences, evolutionary literature shows white matter is known to be an important area of differentiation between human and other primate brains. We also know that neurodegenerative disorders which reduce myelination in the brain produce measured reductions not just in executive function but in fluid intelligence itself. Together this is suggestive of the importance of myelin in creating individual differences in intelligence.

Haier: The NEH and P-FIT

The two major contributions of Haier are his “Neural Efficiency Hypothesis,” sometimes called the brain efficiency hypothesis, and his Parietal-Frontal Integration Theory of intelligence (P-FIT).

Haier et al. (1988) was the first study to combine modern brain imaging techniques with psychological intelligence testing. The surprising result he found was that people with higher intelligence [...]

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Outline:

(01:04) Haier: The NEH and P-FIT

(05:39) Myelination and IQ Research

(10:16) Myelination and Human Evolution

(11:40) Reaction Time and IQ

(14:14) Ephaptic Coupling

(18:41) Discussion

(21:26) References

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First published:
May 19th, 2026

Source:
https://www.lesswrong.com/posts/uYXjSHmHyjbNzuZqk/brain-structure-and-iq-how-myelin-elevates-intelligence

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Narrated by TYPE III AUDIO.

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Images from the article:

Three brain images comparing white matter connections in Rhesus monkeys and humans.

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