Why did brain size decrease after the Ice Age?

Nubians (Wikipedia). After the last ice age, brain size decreased in Europeans and East Asians. In western Europeans, this trend continued until some time before 1800. No decrease is observable in a large series of crania from Nubia.



In my latest paper I argue that northern hunting peoples were the first to break free from the cognitive straitjacket of hunting and gathering. Because women at northern latitudes had few opportunities for food gathering, they took on new, more cognitively demanding tasks, like garment making, needlework, weaving, leatherworking, pottery, and kiln operation. This increase in task complexity, led by women, provided these peoples and their descendants with the mental toolkit for later developments: farming, more complex technology and social organization, and an increasingly future-oriented culture (Frost 2019).

That paper left out a key piece of evidence. As these northern hunting peoples expanded southward into the temperate zone, they must have had excess mental capacity, especially the women, who were now redirected toward the cognitive demands of food gathering and, later, farming. Cognitive demand also decreased for men, who no longer had to store huge quantities of spatiotemporal information for tracking game and finding their way home. On the other hand, men put some of this excess mental capacity to new uses, by exploiting many of the technologies that women had pioneered.

So is there evidence of decreased cognitive demand after the last ice age? According to a study by Maciej Henneberg (1988), brain size steadily shrank from the Mesolithic to modern times, on the order of 9.9% for men and 17.4% for women. This is consistent with the reduction in cognitive demand being greater for women than for men.

Henneberg ignored the sex difference, preferring to attribute the decrease in brain size to a corresponding decrease in body size for both men and women. This explanation has been challenged by John Hawks, who reanalyzed Henneberg's data and showed that the decrease in body size explains only one-fifth to one-seventh of the one in brain size. He also showed that the declining ratio of brain size to body size did not affect all human populations. In fact, it can be securely demonstrated only for Europeans and Chinese. Indigenous southern Africans and Australians may have had similar declines, but the sample sizes are too small to conclude with certainty. No overall change is seen in the one case where we have a large cranial sample from a non-Eurasian population (Nubians):

A large series of crania from ancient Nubia covers the period from roughly 3400 years ago to 600 years ago [20, 21]. Samples show a slight trend toward decrease in the major length, breadth and height measurements from Iron Age (Meroitic, external cranial module 145.2) to Medieval (Christian, external cranial module 143.9) times, but the intermediate series of crania (X-Group, external cranial module 147.1) is somewhat larger in these dimensions than either of the other groups. In this context it would be misleading to speak of a reduction in cranial vault size in this region. (Hawks 2011)

A recent reversal

This trend reversed itself at some point in time, apparently before the 1800s. Jantz and Jantz (2016) and Jellinghaus et al. (2018) found an increase in brain size from at least 1800 in Germans and 1820 in white Americans. When I asked John Hawks, he attributed this reversal to improvements in nutrition and a reduction of childhood disease. That, too, was what I thought, initially.

But, then, the reversal would surely have been stronger in women than in men. If brain size had decreased twice as much in women, shouldn't the rebound have been twice as strong in women? Yet this is not what we see in the brain size of Americans born from 1820 to 1990: "Both sexes changed, but female change was less pronounced than male change" (Jantz and Jantz 2016).  In Germans born between 1800 and 1950, no clear sex difference was observable in the magnitude of this change over time (Jellinghaus et al. 2018).

Both Jantz and Jantz (2016) and Jellinghaus et al. (2018) are skeptical that these changes could be explained by improvement in nutrition or reduction of childhood disease. Infant mortality is a good proxy for both, and it did not begin to decline until circa 1900. At the very least, the increase in brain size should have accelerated during the twentieth century, yet it didn't (Jellinghaus et al. 2018).


Conclusion

Our knowledge on this subject comes largely from Maciej Henneberg, who concluded that brain size had decreased in all human populations and that this decrease continued into modern times. Both conclusions have been disproven. The decrease did not affect all human populations, and it had already reversed by 1800 in northern Europeans, as shown by two recent studies on white American and German samples. 

Perhaps the reason lies in changing patterns of natural selection. After the last ice age, northern hunting peoples had excess mental capacity, particularly the women. This excess capacity enabled them to create and exploit new and more complex social environments—farming, towns and cities, civilizations … It was still more than what was needed, however, and a long-term decline set in. Then, in early modern times, this decline reversed in western Europeans, and brain size once more began to increase. Why? Perhaps this is related to evidence, summarized in my last paper, that mean intelligence steadily rose in western European societies during late medieval and early modern times.

Hawks' study is the only comprehensive critique of Henneberg's work. Unfortunately, it has never appeared in a peer-reviewed journal. When asked why, he replied: "I did not feel it was necessary to pursue formal journal publication for this, because I did not think it fit well into the journals at the time." When asked why he had removed a post on that study from his weblog (it was put up in 2012 and taken down in 2017), he answered: "I used to have a section on my blog for research manuscripts that were in prep, but I decided to discontinue this as I became involved in more collaborative work."

Is there another reason? I can understand not publishing a post because other work is more pressing, but why delete an existing post? What made it less blogworthy by 2017?

The study in itself seems uncontroversial. Indeed, it leads to the amusing conclusion that European brains got smaller while Nubian brains remained unchanged. But talk about "smaller brains" can trigger some people, and John Hawks is already viewed with suspicion because of his work with Henry Harpending and Greg Cochran. Henry once told me—not long before his untimely death in 2016—about the mounting pressures he was facing to discontinue his research. Have similar pressures been brought to bear on John Hawks? One may wonder. The last three years have seen a remarkable escalation of deplatforming and outright violence in the name of "antiracism." When Steve Sailer (2019) charted the number of New York Times articles that mention the word "racism," he found that this number took off during the mid-decade, rising from 291 in 2011 to 2,353 in 2018. The mentions also changed qualitatively, becoming much more vociferous.

Today, John is a tenured professor, yet he is now much more reticent to say what he thinks than when he was a graduate student. His example should be sobering. The pressure to be "correct" doesn't end when you get tenure.


References

Frost, P. (2019). The Original Industrial Revolution. Did Cold Winters Select for Cognitive Ability? Psych 2019, 1(1), 166-181
https://doi.org/10.3390/psych1010012

Hawks, J. (2011). Selection for smaller brains in Holocene human evolution. arXiv:1102.5604 [q-bio.PE]
https://arxiv.org/abs/1102.5604 

Henneberg, M. (1988). Decrease of human skull size in the Holocene. Human Biology 60: 395-405.
https://www.jstor.org/stable/41464021

Jantz, R.L., and L.M. Jantz. (2016). The Remarkable Change in Euro-American Cranial Shape and Size, Human Biology 88(1), 56-64 
https://doi.org/10.13110/humanbiology.88.1.0056

Jellinghaus, K., H. Katharina, C. Hachmann, A. Prescher, M. Bohnert, and R. Jantz. (2018). Cranial secular change from the nineteenth to the twentieth century in modern German individuals compared to modern Euro-American individuals. International Journal of Legal Medicine 132: 1477-1484.
https://www.researchgate.net/profile/Michael_Bohnert/publication/323952852_Cranial_secular_change_from_the_nineteenth_to_the_twentieth_century_in_modern_German_individuals_compared_to_modern_Euro-American_individuals/links/5ab8dcdcaca2722b97cfe45f/Cranial-secular-change-from-the-nineteenth-to-the-twentieth-century-in-modern-German-individuals-compared-to-modern-Euro-American-individuals.pdf 

Sailer, S. (2019). Graphing the Great Awokening. May 28, The Unz Review
http://www.unz.com/isteve/graphing-the-great-awokening/