How do we do it? Volume!

In current MRI studies, the correlation of brain volume and IQ is about 0.4 – and a good thing, too.  If there was no significant correlation, we would have to conclude that natural selection didn’t work.   Here’s why.

Brain tissue is expensive: altogether it burns up something like 20% of the food we eat.  A big brain is expensive in more ways than that: for example, it makes human birth much more difficult and risky than in other mammals.  If we could think just as well with a smaller brain,  individuals with smaller brains would obviously have higher fitness.  They’d do better when food was short, and for them having babies would as easy as spitting out a watermelon seed.   Since humans brains are expensive and troublesome, there must be a positive relationship between brain size and brain performance – otherwise natural selection would already left us with shrunken heads.

That doesn’t mean that size is the only factor affecting IQ: obviously there can be differences in neural organization, in the performance of individual neurons, etc. So it is not surprising that the brain size/IQ correlation is significantly above zero while at the same time being well below 1.0.   That is just what you would expect.

Of course, it is possible in principle that the brain contributes to fitness in some way other than thinking.  Aristotle taught that its primary purpose is to cool the blood,  but as Will Cuppy pointed out, this is true only of certain persons.

We know that IQ increases with brain size, on average – but the costs increase as well.  It seems likely that costs grow more and more rapidly as brain size increases.  If nothing else, the difficulty of giving birth to a big-headed baby must grow rapidly as head size increases. It could also be the case that the payoff of extra brain volume gradually decreases.  For one thing, as the brain expands, more and more of its volume  has to be used for connections between neurons (white matter), leaving less room for neurons.

This line of analysis suggests that there is an optimum brain size – smaller brains are cheaper but performance is lower still (resulting in lower net fitness), while larger brains have higher performance, but not enough to pay for their higher costs.  Of course optimum means optimum for past environments. Changes in the environment  – which includes the aspects of the environment created by humans – could move that optimum up and down. Since brain size has decreased noticeably during the Holocene (by about 10%), we know that evolutionary response to changes in selection pressures affecting the brain is certainly possible in 10,000 years or less. Since major human populations spent tens of thousands of years with almost zero gene flow, average brain size could differ between populations, and of course it does.

One of the interesting implications is that brains that work less well – that produce less fitness per cubic centimeter – will  also be smaller.  If the costs are the same, lower brain performance means that the optimum shifts downward.  That lower fitness payoff per cubic centimeter could stem from physiological differences that caused lower performance, or, in principle, from  environmental factors that were not well addressed by human intelligence: in some sense incomprehensible, at least for humans  in prehistory.

Consider a graph that shows fitness as a function of brain size. It has an optimum, but  since we see quite a bit of variation in brain size within populations, we suspect that the  curvature is not that high near the optimum. Modest differences in IQ do not (or, to be more exact, did not) cause huge changes in fitness.  Mutational pressure will also cause deviations from the optimum: adaptation happens, but there is always some noise in the system.

By the way, there is a lot of talk about how nobody can really define or measure intelligence, IQ is not related to brain size, brain size doesn’t vary between human populations, and brain volume is hard to measure.  That’s all nonsense.

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28 Responses to How do we do it? Volume!

  1. Gorbachev says:

    Last sentence:

    Of course it’s all nonsense. But because it’s important for people to believe it’s nonsense, you need references.

    Or you’re a nazi bastard.

    (tongue <= cheek)

  2. ben g says:

    Women seem to do fine with smaller brains.. How does that fit into your theory? Why didn’t natural selection give us all female brains?

    • I don’t think women do fine with smaller brains. I think that is political correctness and affirmative action.

      In the ancestral environment, a smart man was likely to achieve something that made a very large difference to the number of offspring he produced, while it probably made no difference whether a woman was smart or stupid. So why invest in useless brain, when she could invest in useful milk production? We should therefore expect to see substantial differences in intelligence, much as we see substantial differences in upper body strength. And I am pretty sure that that is what we do see.

      The fact that you (incorrectly) remember who supposedly discovered radium (actually it was Pierre Curie, not Marie Curie, in so far as it was any one person) but do not remember who discovered the other one hundred elements should tell you all about female scientists

      There have been some great female political leaders, but no great corporate leaders – on the contrary just about every big corporation with a female CEO gets into trouble. There have been no great female composers, no great female scientists (Marie Curie was the least important person on the three man team that discovered radium. The reason everyone remembers radium and no one remembers the far more important discovery of radon, is that there was a woman somewhere within shouting distance of the discovery of radium, but no where near the discovery of radon. The fact that Marie Curie is so famous shows how hard up we are for alleged female scientists.) scientists.

      And though statistics supposedly show that females can drive cars just as well as males, this alleged fact fails to agree with casual observation.

  3. Gorbachev says:

    Individual variability. Variable efficiency. Modular brain structure. Larger size as a function of larger body size between otherwise similar individuals.

    The smaller brain gets less functional not in a direct linear relationship – there’s a fuzzy wiggle room around the edges.

    But curiously, women and men do have very different brains. I wonder if size correlates.

  4. Polynices says:

    I wonder if you could attempt some correlation between increased Caesarean delivery in recent years and higher IQs. In theory, some of it is kids with bigger heads and thus bigger brains.

  5. Anonymous says:

    Paleontologist Stephen Jay Gould strongly felt science should be used to further social and political goals. He accused anthropologist Samuel George Morton of having had subconciously manipulated data to show smaller brain volumes for African skulls. But this charge has been recently debunked. See article in New York Times by Nicholas Wade.

  6. That Guy says:

    @ben g,

    Women have evolved to outsource rationality, decision making and civilization building to men, and have a brain more specialized towards child-rearing – hence smaller overall, even allowing for sexual dimorphism.

  7. That Guy says:


    Curiously my oldest daughter had a head diameter 2″ wider than her mother’s pelvis – and we had no option but to have a Caesarean birth. My daughter turned out to be 95%, 95%, 99% for Height, Weight and Head Circumference. Today she still has an oversize head – looks kind of like a mix of Jessica Alba and Alizee, with a high, broad forehead – and is intellectually gifted.
    Before modern times, she certainly would have perished.

  8. Tod says:

    The problem of giving birth to babies with large heads was solved, certainly by the time of CroMagnons, (their brains were a whopping 15 to 20 % bigger than those that came after them in Europe). So did having big heads suddenly become a problem again, natural selection forget the solution ?

    Maybe there were compensatory changes in the brain to maintain iQ but surely the descendants of the Cromags could have got doubly smarter if they’d evolved superior neural organisation while keeping their near-Neaderthal cranial capacity, child’s play for evolution to solve that I would think. It seems that, in effect, it’s being claimed that head size was being selected against after the time of the CroMagnons without considering the evidence (CroMag crania) that the birth canal and calorie consumption problem had already been solved.

    Consider the possibilty that head size was being selected aganst for a new reason: appearance. The sharpest reduction in cranial capacity in Europe came during the Magdelenian which is also when the first known case of an impacted wisdom tooth occured. There was also a reduction in height. Women have smaller jaws and heads. If most of the less feminine females failed to reproduce then over the generations those features would alter in both sexes.

  9. LK says:

    Or larger brain size might be needed for other purposes than IQ.


    “Why do modern humans have larger brains than earlier people such as Homo erectus? As large brains cause problems in childbirth, infancy and locomotion, the advantage they offer must be substantial. This advantage might be associated with increased IQ, but there is a problem: evidence from MRI volumetric surveys, microcephaly and hemispherectomy shows that there exist individuals with psychometrically normal IQ but Homo-erectus-sized brains. Why did evolution increase brain size (with its associated costs) when humans (as these individuals demonstrate) can have normal IQ without bigger brains? I propose that the advantage may be related to increased capacity for an aspect of intelligent behaviour not measured by IQ tests but critical to the survival of our simple hunter-gatherers ancestors: the capacity to develop expertise. ”

    Article + lots of responses:

  10. blert says:

    My head is +4 SD, yet I was delivered normally — the easiest birth of my siblings.

    In a world where nothing was written — visual spatial memories demanded extra DRAM.

    Many generations later, famine favors the smaller mind. Famously, the Germans encircled at Stalingrad all received the same rations. ( boys in the line, that is ) Within days the strongest, largest troops died off. What was barely enough for a 140# man was prompt death to a 210# man.

    You can lose an awful lot of big brain genes with a major famine.

    And then, Nappy shot up Europe so much that male height went down.

    Deep abstract thinking requires more brain volume. It’s what separates the men from the girls.

    Do I have to quote the Godfather?

  11. blert says:

    BTW, a large male skull is a sex draw. Women correlate it to smarts across the board.

    When I was measured for a hat — the sales babe just about jumped the counter — in four years of selling she’d never seen my stats — and was lusty just on that alone.

    So, it’s like a massive chest or tremendous legs — peacocking becomes automatic. This is certainly a factor in human dimorphism. (In the Ice Age the only body part a male could peacock would be his skull, all else being very well furred over. )

  12. Steve Sailer says:

    I suspect conserving heat was more of a problem for Ice Age Man, while shedding heat generated by the brain is more of a problem for us, who do most of our hard thinking at room temperature.

  13. bgc says:

    The trade of involved in developing a higher IQ may be illustrated if reports of a short life history (shorter childhood and earlier sexual maturity) in pygmies are correct:

    Presumably, pygmies have smaller heads as well as shorter height.

    Interestingly, this section appears at the end the second paper; “Finally, the data presented here show that pygmy body size evolved through earlier cessation of growth, being therefore the result of changes in late rather than early stages of growth. This explains why brain growth, which is completed years before the onset of adolescence (28), is not affected in human pygmies (29).” –

    …which strikes me as so obviously wrong as to be either incompetent or dishonest – probably the latter since the authors (and I have e-mailed several a few times) will not respond to (polite) queries about this.

    Jason Malloy (who has a big database of international IQ estimates) puts average adult pygmy IQ at roughly 59 – which is roughly that of a European aged nine and a half. This would seem to fit with the necessarily shortened period of brain growth and maturation entailed by early sexual maturity.

  14. Fenris says:

    My understanding is that relatively to Lean body mass males and female brains are of equivalent size. Encephalization rate is identical once you control for size and body fatness. Men and women have very different brains and think and emote very different but the general capacity seems similar, like a mac and pc with the same proccesing speed, ram etc but still running differently.

  15. Flip says:

    I work for a large corporation with a lot of professional women with college degrees and MBAs from top schools. Virtually none of them can do the sort of abstract strategic thinking that men can.

  16. gcochran says:

    Female brains are indeed smaller, even controlling for body size. Quite a few people say otherwise, but they’re wrong. Women do about the same as men on IQ tests, but do slightly better in some verbal tests and noticeably worse in spatial visualization (by about 0.7 standard deviation). Maybe spatial visualization is resource intensive. There could be other considerations: maybe male brains had to withstand more blows, on average.

    As for the notion that normal IQ does not require typical brain size, you could in the same way say that Spud Webb ( who was 5 3 ) proves that professional basketball players do not need to be tall. When a number of factors are involved in IQ, when there are several kinds of variability, the fact that a few people manage a normal IQ with a smaller brain does not imply that volume doesn’t increase IQ. Similarly, Robert the Bruce won while greatly outnumbered at Bannockburn, but that hardly shows that numbers do not matter in war.

  17. Fenris says:

    This ( ) is were I got the idea that if you controlled for body fat the gap in brain size disapears. Are most studies of brain size controlled for body fatness?

  18. gcochran says:

    The real question is why you would control for body weight at all.

  19. Unamused says:

    “That’s all nonsense.”

    Short and to the point. No further rebuttal necessary.

  20. Jason Malloy says:

    “If nothing else, the difficulty of giving birth to a big-headed baby must grow rapidly as head size increases.”

    Like most human biodiversity this has largely gone under the scientific radar, but the large headed Inuit actually have some of the easiest childbirths of any population:

    “The limited scientific evidence that is available does support the fact that the Inuit appear to be virtually free of dystocia. Baskett in 1978, observed that Inuit patients tend to have very efficient uterine action, endure labor well, and rarely have dystocia. He recorded only ten cesareans in a study of 622 Inuit deliveries, for a rate of only 1.6%, dramatically lower than the contemporaneous rate of 6.9% for the province of Manitoba and 10.5% for the Winnipeg teaching hospitals. Of these ten cesarean deliveries, only four were performed for cephalopelvic disproportion.”

  21. jb says:

    I don’t understand the idea that the size of heads and brains has been limited by heat generation. If the brain uses up 20 percent (?) of the body’s energy budget, and thus generates 20 percent of the body’s heat, I would expect that increasing the volume of the brain by 10 percent would increase the body’s total heat production by 2 percent. That just doesn’t seem like a huge big deal. For one thing, the brain is well supplied with blood, so much of the excess heat will be passed to and dumped by the body. But beyond that, how big an issue is heat stress anyway? Of all the things that can prevent people from passing their genes to the next generation, how often is the deciding factor the fact that they just couldn’t take the heat? Even in the tropics, I don’t see how that can outweigh the advantage of additional intelligence. (Assuming of course that additional intelligence is in fact an advantage in the tropics!)

  22. Fenris says:

    Re: Greg i would presume its important to control for body size because encephalization rate is better guide to cognitive capacity then raw brain size.

  23. gcochran says:

    But is it? In humans? If we’re talking frogs or dinosaurs, it has been argued that most of the brain is either taking in sensory information or controlling muscles. Maybe they don’t spend much of their mental energy thinking deep thoughts. Along those lines, those who think about encephalization usually normalize by the animal’s surface area – or, more exactly, the two-thirds power of body mass. In fact, in mammals generally, brain volume tends to be proportional to to the two-thirds power of body mass.

  24. Fenris says:

    I don’t\have a deep enough knowledge of the subject to really offering anything in a deeper debate of correlations between cognitive capacity and absolute brain size or brain size ratio or how body fat would play into that. I though matilda suggestion was intuitive considering we usually don’t see a gap in straight IQ between males and females despite the gap in brain size. I don’t have the priors to examine the idea more deeply. I am curious if you have more in depth argument as to why absolute size is more important then encephelization ratio or why body fatness wouldn’t be a consideration.

  25. Anonymous says:

    Nice blog.
    From an engineering perspective, problems are not scalable with the same solutions, at each step, you quickly hit a threshold that requires different solutions. So, going from IQ 75 to 90 might just have required bigger brain with more food and going 90 to 105 might require more neuronal connections with the extra ability to feed only a subset of neurones to keep the energy consumption low. On the link below the brain is rated at about 20 watts with Einstein’s brain not being the biggest and therefore the hungriest.
    So it is likely that hight IQ brains such as Einstein’s don’t run on more energy. It would be advantageous for brains with IQ>100 to run with actually less energy/neuronal connexion than brains with IQ<100. Interestingly, there are now lots of evidence that glial cells, that were thought to have only a support/nutritive roles for the neurones, have a role in the neuronal connectivity as well. Some have stated that the gain in connexion complexity, versus glial cells not involved in cognitive process, would be one order of MAGNITUDE!
    " The new study (2) extends these results by showing that astrocytes increase by 7-fold the number of synapses on each neuron and enhance synaptic efficacy by altering both presynaptic and postsynaptic functions". Also, check this stunning statement:
    "…In this theory, neurons are tied to our muscular action and external senses. We know astrocytes monitor neurons for this information. Similarly, they can induce neurons to fire. Therefore, astrocytes modulate neuron behavior. This could mean that calcium waves in astrocytes are our thinking mind. Neuronal activity without astrocyte processing is a simple reflex; anything more complicated might require astrocyte processing", at:

  26. Pingback: IQ Ceilings? | JayMan's Blog

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