Mailbag: The teacher who wouldn't believe in shrinking brains

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My son, a student at [redacted university], was recently ridiculed by his professor in class when my son suggested that the human brain has been shrinking for the last 20,000 years (the teacher insists that it has not changed). When my son cited the article in September Discover, he was (somewhat understandably) further ridiculed for such a source. My question is: can you provide me with some links to credible sources for this information? He has started to work on this, but wading through so many sources that mention brain size, etc. has proven difficult. We would appreciate any help you could provide.

Thank you for writing – that is indeed sad but not surprising; I hear all too often from people who have teachers that can’t be bothered to read.

The reduction in brain size during the last 10,000 years is a really well-known fact in anthropology, it is not at all controversial. It is, for example, discussed in the introductory-level textbook that I assign my students. I have attached several papers that include primary data from archaeological or historical samples. In Europe the trend is most clearly documented, because of the large number (many thousands) of skeletons that have been studied, but the trend is also apparent in South Africa, China, and Australia. Some of the papers include many other characters that also changed during the same time span.

We cannot rule out that other locations might have had stasis instead of reduction in brain size, but there is not yet a well-documented example of it.

The most common explanations for a reduction in brain size are (a) diet and the consequent reduction in body size; and (b) warmer Holocene climate. Larger brains are always bad, in that they require more development and time, so what we are looking at in the Holocene is a change in the stabilizing selection – either an intensification of selection against larger brains, or a relaxation in selection against small brains.

Body size also did get smaller during the past 50,000 years, which gives rise to the question of whether the brain has reduced more than should be expected from the reduction in body size. My research indicates that it has done so (this was one topic covered in the Discover article), but I would not say this is yet a consensus view.