Shrinking brains and intelligence

Average human brain size has declined for 20,000 years, a stark contrast to the steady increase over the preceding millions. While I would argue that translation of this reduced brain size into lower intelligence would have a negative result, I am not convinced that intelligence has dropped over that time.

Eighteen months ago, Discover magazine published an article by Kathleen McAuliffe on the shrinking brain phenomenon and what this means for intelligence. On the idiocracy side of the debate was David Geary and Drew Bailey:

[T]he Missouri team used population density as a proxy for social complexity, reasoning that when more people are concentrated in a geographic region, trade springs up between groups, there is greater division of labor, the gathering of food becomes more efficient, and interactions among individuals become richer and more varied.

Bailey and Geary found population density did indeed track closely with brain size, but in a surprising way. When population numbers were low, as was the case for most of our evolution, the cranium kept getting bigger. But as population went from sparse to dense in a given area, cranial size declined, highlighted by a sudden 3 to 4 percent drop in EQ starting around 15,000 to 10,000 years ago. “We saw that trend in Europe, China, Africa, Malaysia—everywhere we looked,” Geary says.

The observation led the researchers to a radical conclusion: As complex societies emerged, the brain became smaller because people did not have to be as smart to stay alive. As Geary explains, individuals who would not have been able to survive by their wits alone could scrape by with the help of others—supported, as it were, by the first social safety nets.

In a Malthusian world where incomes hover around subsistence levels, population density is proportional to the level of technology available to the population. Population density is not something that simply arises, but it is enabled by the technology which allows for the required level of subsistence to be produced for a larger population in a given area. As a result, I am not convinced by Bailey and Geary’s conclusion. If we need a higher level of technology to support higher population density, and average intelligence of the population is related to technology, you would expect higher intelligence populations to have higher population densities.

As for their concerns about the dysgenics of the first social safety nets (I wonder what their views are about the modern ones), I am not convinced that higher populations delivered such survival benefits to the dumb. With a larger, more dense society comes trade, specialisation and more complex social arrangements, all of which are likely to favour those with higher intelligence. Plus, if there was such a social net, why wasn’t it also supporting those huge brained people who were burning too much energy to survive? If we throw sexual selection into the mix, the benefits to intelligence appear larger.

John Hawks puts a different spin on Bailey and Geary’s data, which may reconcile the Malthusian assumptions with the brain size observations:

The organ is such a glutton for fuel, he says, that it gobbles up 20 percent of all the calories we consume. “So although a bigger brain can presumably carry out more functions, it takes longer to develop and it uses more energy.” Brain size probably depends on how those opposing forces play out.

The optimal solution to the problem, he suggests, “is a brain that yields the most intelligence for the least energy.” For evolution to deliver up such a product, Hawks admits, would probably require several rare beneficial mutations—a seeming long shot. But a boom in the human population between 20,000 and 10,000 years ago greatly improved the odds of such a fortuitous development. He cites a central tenet of population genetics: The more individuals, the bigger the gene pool, and the greater the chance for an unusual advantageous mutation to happen. …

Hawks notes that such changes would be consistent with the many brain-related DNA mutations seen over the past 20 millennia. He speculates that the organ’s wiring pattern became more streamlined, the neurochemistry shifted, or perhaps both happened in tandem to boost our cognitive ability.

While bigger brains lead to greater intelligence (there is a positive correlation in modern populations), intelligence is not all about size. More efficient arrangements may result in a better energy-intelligence trade-off, delivering more efficient, smaller but smarter brains. The continued strong selection of alleles associated with brain size is also suggestive of this.

As an end note, brain size has been back on an increasing trend for the last 200 years, and there is some suggestion that this is not all environmental:

Since evolution does not happen overnight, one would assume this sudden shift (much like the increase in height and weight) is unrelated to genetic adaptations. Hawks, for instance, says the explanation is “mostly nutrition.” Jantz agrees but still thinks the trend has “an evolutionary component because the forces of natural selection have changed so radically in the last 200 years.” His theory: In earlier periods, when famine was more common, people with unusually large brains would have been at greater peril of starving to death because of gray matter’s prodigious energy requirements. But with the unprecedented abundance of food in more recent times, those selective forces have relaxed, reducing the evolutionary cost of a large brain.

Of course, as Geary may point out, the survival cost of a small brain’s lack of intellectual horsepower is also well down.


  1. “As an end note, brain size has been back on an increasing trend for the last 200 years, and there is some suggestion that this is not all environmental”

    Didn’t W.D. Hamilton somewhere raise the possibility that c-sections played a role?

    1. Hamilton was worried about the dysgenic effects of c-sections, and wanted their use limited. I don’t recall any mention of brain size by him, but it’s a good point.

      1. Found what I was thinking of in his review of Lynn’s Dysgenics: 

        “The first is that perinatal and surgical gynaecological skill is permitting the birth of larger-headed babies than formerly. There certainly was a mortality which used to curtail perinatal survival of the largest babies (Karn & Penrose, 1951), and this probably also affected the largest head sizes most. Since head size is positively correlated with intelligence (Penrose, 1961 ; Lynn, 1990 ; Rushton, 1995), the upper tail of intelligence may be becoming less `thinned’ than it once was.”

        But specifically about c-sections he takes, as you said, a negative view, although I haven’t read the paper he cites (“Sex and Disease”) that puts forward his argument.

  2. I’m not sure I am convinced by your reasoning. In your last post you argued that “there are massive positive externalities to a population having a higher average IQ”. If this is the case, shouldn’t we expect IQ to be “under-provided”, with individuals free-riding on the intelligence of those around them? Having a big brain is very expensive both in terms of calories and birthing difficulty, so I would expect this to be a powerful effect. Hard to say how it would interact with technological development and Malthusian dynamics I suppose, but without actually building a little model it seems reasonable that the net effect of increasing population density (or technology?)  might cause brain size to decline. 

    1. Fair point, but we need to separate the two posts between pre and post-1800. In the Malthusian world, there were no (or weak) positive externalities to average IQ levels, as technological advances were swallowed by population growth. Living in the higher technology England of 1500 was no better for the average person than it would have been for most hunter gatherers of the last 40,000 years. Living in a population of high intelligence people was of little benefit to your survival before 1800.

      That is the weak point of Geary’s argument. It implies that there is some surplus that could be used to keep low-IQ people alive in more complex societies, but the evidence suggests that denser societies were just as Malthusian as the less dense.Also, if there were too many free riders in a dense, high-IQ population, it would ultimately cause income levels to decrease, which would then remove the ability to free ride. However, for the reasons mentioned above, I am not convinced that would be the case.

      I am developing a working paper which has some models relevant to this question, so hopefully they will be of some use when I finally get them ready for public consumption.

      1. Well, I do agree it will come down to the details. I’ll be interested to see how your models turn out.

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