Saturday, July 6, 2019

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).


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.


Frost, P. (2019). The Original Industrial Revolution. Did Cold Winters Select for Cognitive Ability? Psych 2019, 1(1), 166-181

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

Henneberg, M. (1988). Decrease of human skull size in the Holocene. Human Biology 60: 395-405.

Jantz, R.L., and L.M. Jantz. (2016). The Remarkable Change in Euro-American Cranial Shape and Size, Human Biology 88(1), 56-64

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. 

Sailer, S. (2019). Graphing the Great Awokening. May 28, The Unz Review  


Anonymous said...

Why do you assume the cause of the decrease after the ice age and increase after 1800 needs to be the same? Seems perfectly possible that the decrease was genetic and the increase after 1800 was because of better nutrition.

Santo said...

Women were tamed by patriarchy?

Increase of brain size during 1800's maybe a result of intense selective pressure (and also very bad socio economic standards among lower classes) with the initial period of demographic transition.

Santo said...

I mean, caused by industrial rev

John said...

I would guess diet has somewhat of an influence, but that difficult to decipher for "industrial" populations, because of food processing and variety. I believe Mongols and Inuits have high cranial capacity compared to peoples with genetic overlap. One obvious factor is that it is cold for both, but, also, they both have unusual diets in that they are very "high quality" (nutrient and calorie dense), mostly carnivorous. I think I remember Maasai having larger cranial capacity versus nearby African groups as well, but I can't remember when those measurements were done.

Santo said...

Environmental hypotheses are absolutely generalists or causalists.... period. Just one example can prove it wrong.

The correlation, or, possible correlation between diet lifestyle and intelligence mirror behavioral lifestyle... In nature, hunters need use more their brains to find food.. So, this correlation is unlikely to be a causation.

Sean said...

Greenland Inuit do not have the largest braincases though, what about the Hobbits' tool use, and does smaller mean stupider, or just less brutish dominance interactions as the large braincases of the relatively immobile Neanderthals suggest? I think there are a lot of things going on; the hypothesis of humans having followed the path of bonobos rather than chimps as Wrangham suggests seems to be an important part of it. Also here where Hawks is extensively quoted. Wasn't the reduction in brain size most drastic between Cro magnons and the Magdalenian, but the Magdalenian was when the steppe tundra reindeer hunters had the greatest need for "to store huge quantities of spatiotemporal information for tracking game and finding their way home", and that was when there was the greatest increase in use of technical devices and of course sewing ect by women.

Peter Frost said...


The increase in brain size doesn't really correspond to improvements in nutrition. Infant mortality is a good proxy for the latter, and it didn't begin to decline until the late 19th century. Moreover, the big improvements in nutrition happened during the early to mid 20th century, when the increase in brain size was tapering off.


Yes, in a sense. With the development of farming and social complexity, men took over tasks that were formerly done mainly or exclusively by women. Women became confined to a narrower range of duties.

John and Sean,

Keep in mind that brain size isn't the only factor in mean intelligence. Brain size is quite large in the indigenous inhabitants of Siberia and Arctic North America, yet their mean intelligence is below that of East Asians, whose brain size is somewhat smaller (see my last paper for an overview of those IQ studies).

"Wasn't the reduction in brain size most drastic between Cro magnons and the Magdalenian"


The sample sizes are too small to say with certainty.

clar said...

Good post!

tomR said...

It's well researched that hunter gatherers have good visual memory and spatial memory, and are great at tasks requiring it eg. pathfinding. Elephants have bigger briains than humans, and they are even better at long-term storage of mapping of terrain. In modern times some Asperger/Autism people have the ability to think visually.

And visual information requires more neurons per a piece of information than verbal representation that is more useful in the civilization. Eg. a known poet Anatole France had brain size 2/3 of normal, he was recognized as intelligent, but his output, and most likely thoughts were expressed in words. You need less neurons, but with good myelination for such uses. I wouldn't trust Anatole France to be a navigator or a pilot.

In modern times, after the industrial revolution, the need for processing visual information become greater, due to the need to first engeeneer then maintain all those mechanical machines and infrastructure. Then these needs even increased with the requirements of designing electronic circuits. At the same time the need for abstract thinker is also higher... There's a need for both types in teams designing high tech, or even normal tech.

Also modern brain shape is very different than the old human one. The old ones were elongated - universally. New ones are more globular - this means ability to have a large brain size "cheper" - with better volume to surface ratio. Longer heads have longer back, meaning better balance between front and back - are better for running.
My unproven suspicion about globular heads is that it is better for fighting - eg less likely to move when hit or pushed (short arches), more resistant to being hit heavily due to "ball" shape rather than "flat from the side", but also perhaps shorter distances between extremes (ath the same volume) giving lower decision times - faster refelexes? If you look at UFC champions then you see mostly men with shorter heads.