Is there any scientific basis for the belief that head circumference determines IQ?

"Big Head Son, Small Head Father" is a childhood memory for many people. However, can a father with a small head give birth to a son with a big head? Are people with big heads smarter? What determines the size of our brains?

Before we start, let's take a look at this picture:

Comparison of three different brain sizes in MRI

Image source: doi:10.31887/DCNS.2018.20.4/gmirzaa

Is this a comparison of the brains of monkeys, normal babies, and super babies? Or is it a comparison of the brain changes during the evolution from apes to humans?

In fact, this is all part of the modern human brain.

However, on the far left is a person with microcephaly (MIC): head circumference is more than two standard deviations (SD) below the mean for age and gender. Microcephaly is often associated with epilepsy, autism and other birth defects.

Usually, we measure the head circumference of newborns within 24 hours after birth. For boys, for example, the average head circumference is about 36 cm, and the lower two standard deviations are about 32 cm.

In the middle are normal people.

The rightmost image shows a patient with megalencephaly (MEG) : compared to the average, their head circumference is significantly larger than 2.5SD (still taking boys as an example, it is about 38.5 cm). Despite their abnormally large brains, their IQ has not increased, but has decreased, and their reaction time has become slow[1].

It seems that brain size and IQ are not simply positively correlated.

This is an extreme contrast. So under normal circumstances, what is the relationship between brain size and IQ?

Part 1

How much do brain sizes vary between people in different conditions?

First, it's important to clarify how brain size is measured. Sometimes we measure it by weight, and sometimes by volume (via MRI scans or skull volume). For this article, we'll focus on measuring brain size using volume.

Also, we're not talking about the size of the head here, but the size of the brain inside the head.

We are talking about the brain above.

(Photo source: Veer Gallery)

How big is the difference in brain size between people from different regions, genders, and ages?

Statistically speaking, there is a slight difference in the average brain capacity of East Asians, Europeans, and Africans, and there is also a slight difference in the brain capacity between men and women.

Thirty years ago, American scientists conducted the world’s largest brain capacity survey on more than 20,000 modern human skulls worldwide. The results showed that the average brain capacity of East Asians was 1,415 cc, while the average brain capacity of Europeans and Africans was 1,362 cc and 1,268 cc, respectively[2].

Brain volume map as of August 21, 2014

Image source: https://commons.wikimedia.org/wiki/File:Brain_Size_Map.png

The average brain volume for men is 1273.6cc, ranging from 1052.9cc to 1498.5cc; the average brain volume for women is 1131.1cc, ranging from 974.9cc to 1398.1cc. The total brain volume of men is 10.8% larger than that of women, a difference of 2.1SD, or 131cc.[3]

In addition to geographic location and gender, there are indeed differences in brain size among people of different ages.

The brain of a newborn baby develops rapidly, but the brain capacity starts to decline after the age of 35. After the age of 35, you can legitimately complain that your brain is getting worse day by day.

The human brain develops rapidly before birth, during the first year of life and throughout childhood.

Image source: https://humanorigins.si.edu/human-characteristics/brains

At birth, the average size of a newborn's brain is 341cc, and at 90 days old it is 558cc.

In three months, a newborn's brain grows from about 33% to 55% of the average adult brain size.

During childhood, brain mass continues to increase, but at a slower rate, reaching 95% of adult size by age 5 and 98% by age 10.

Brain growth then continues to slow, but another wave of growth may occur during young adulthood, between the ages of 18 and 35.

After the age of 35, brain volume decreases by 0.2% per year, gradually accelerating to 0.5% per year by the age of 60. After the age of 60, the volume loss is stable at more than 0.5% per year [4, 5].

Specifically, children with autism tend to have larger brains (and earlier and disproportionate brain growth) than their non-autistic peers [6].

In addition, there are more extreme cases caused by genes. ASPM-as an important gene for determining head size, was first reported in 2002. It is a key gene controlling brain development and is essential for the function of mitotic spindles in embryonic neuroblasts.

ASPM nonsense mutations cause primary microcephaly, characterized by a 70% reduction in brain size, producing a throwback phenomenon similar to that of Australopithecus. The incidence of microcephaly is very rare, with an average of 2-12 cases per 10,000 births.

Phylogenetic tree of the ASPM gene, with the numbers being the ratio of the nonsynonymous to synonymous substitution rates, Ka/Ks, for each branch of the phylogenetic tree. Because nonsynonymous changes alter the biochemical properties of the protein product, they are often subject to selection. A high (or low) Ka/Ks ratio means that the protein encoded by the gene evolves rapidly (or slowly). When the ratio is greater than 1, it indicates that the nonsynonymous substitution rate is faster than expected under selective neutrality, possibly due to the presence of positive selection. The primate lineage leading to humans is shown in red, and we can see that the ratio is as high as 1.44 in the last step of selection.

Cited from: doi:10.1038/nrg1634

Overall, there are indeed people with relatively large brains and relatively small brains in the population, but the gap is not that big.

Part 2

Why are our brains the size they are today?

Complex thinking abilities may result from incremental changes in brain function. Once the size and structural complexity of the brain exceeded a certain threshold, cognitive abilities may have increased disproportionately with brain improvements, leading to human enlightenment. Moreover, brain size is more evolutionarily stable than any other organ in the body[7].

So how did our brains evolve to their current size?

In short, the brain grew rapidly during early human evolution, with a burst of growth 3 million years ago and then a plateau. During this 3 million-year sprint, the human brain almost quadrupled in size from what our ancestors had grown during the previous 60 million years of primate evolution.

Model of brain size in our ancestors and modern humans

Image source: https://www.eurekalert.org

One of the main ways to track the evolution of the human brain is through fossils. Because fossil brain tissue is rare, a more reliable method is to look at the anatomical features of the skull, which can provide insights into the characteristics of the brain. One way to do this is to look at endocranial casts, but endocranial casts cannot reveal underlying brain structures.

Number and average brain size of various fossil specimens

Image source: https://www.britannica.com/science/human-evolution/Increasing-brain-size

Fossils show that Australopiths, which lived between 3.85 million and 2.95 million years ago, had brains of about 300-500cc, about the same as those of living chimpanzees. As we entered the age of Homo sapiens, brain size continued to increase steadily.

Homo habilis, which lived between 2.4 million and 1.4 million years ago, had a skull capacity of about 600cc, while the more modern Homo heideibergenesis, which lived between about 700,000 and 200,000 years ago, had a brain capacity of about 1,290cc.

Neanderthals (Homo neanderthalensis) lived between 400,000 and 40,000 years ago. Their skull capacity averaged 1,500–1,600 cc, comparable to that of modern humans, and even larger than that of modern humans. However, due to their low brain mass relative to body mass, Neanderthals were not as intelligent as modern humans [8,9].

Around 200,000 years ago, when the brain reached the physical limit of the pelvic size, its growth stagnated. We can get a sense of this from the following figure.

Based on the skulls of ancient primates and early hominids

Human evolution as shown in the cast, with the horizontal axis showing the age and the vertical axis showing the brain capacity

Figure from: https://doi.org/10.15761/imm.1000287

Why hasn't the human brain continued to increase in size indefinitely?

First, the brain is one of the most metabolically expensive organs in the body; even though it makes up only 2% of body weight, it accounts for approximately 20% of total energy expenditure at rest.

Secondly, the increased cranial capacity made childbirth difficult, which directly led to an increase in maternal mortality (0.5%), which was the highest among mammals before the advent of modern medicine. To compensate for the difficulty of childbirth, the pelvic opening was enlarged, and the efficiency of bipedal locomotion was reduced.

Third, larger brains take longer to mature, which significantly prolongs gestation and child-rearing periods, thereby placing greater demands on the mother and reducing the total number of children she can bear.[10]

So, we always say that the modern human brain is the product of a balance game between costs and benefits.

Part 3

Does a bigger head mean smarter people?

In 1836, German anatomist and physiologist Friedrich Tiedemann wrote that there was no doubt that there was a very close connection between the absolute size of the brain and intelligence and mental function. This sparked a fierce debate and controversy, with hundreds of studies to date.

However, when I reviewed the early literature, I found that these studies were too controversial and had obvious flaws: for example, how people defined and measured intelligence, whether the subjects' body shape, age and gender were considered when conducting relevant analyses, and which part of the brain should be observed when making judgments.

In the past, three main methods have been used to estimate brain size: weighing the wet brain at autopsy, measuring the volume of the empty skull with stuffing, and measuring the size of the external head and estimating the volume. Early studies used postmortem brains, but the longer the time after death, the more the brain will weigh due to edema. And the intelligence level of the deceased cannot be well measured, and can only be judged by occupation and social status during life.

After the civil rights movement came to prominence in the United States in the 1960s, research on brain size and intelligence, and group differences therein, came to a halt and the literature came under intense criticism.

In the 1990s, as new technologies for scanning the brain became available, including computer-assisted tomography (CAT) and magnetic resonance imaging (MRI), more sophisticated techniques were added to the arsenal, and interest was rekindled. At the same time, well-established intelligence assessment criteria became important, and one of the most widely cited was the g-factor.

g factor, general mental ability, the predecessor of what we often call IQ, was proposed by Spearman in 1904. It was later used for the selection of the US army in World War I and gradually gained world recognition. The small squares represent 16 different cognitive ability tests. These 16 tests are combined into 5 factors: reasoning, spatial ability, memory, processing speed, and vocabulary. Each test has a set of factors: these numbers can be considered as correlations between individual tests and higher-order latent traits or ability areas. All five areas are highly correlated with the general intelligence factor (g). Cited from doi: 10.1038/nrn2793

It is now widely agreed that total brain volume (measured using structural MRI) is only moderately correlated with intelligence, with a correlation coefficient of approximately 0.30-0.40 [15,16].

Neuroscience research has identified several structural and functional correlates of individual differences in intelligence, including functional parietofrontal networks, neuronal efficiency, and white matter integrity. The overall developmental stability of the brain should also be taken into account, and acquired practice and experience can lead to increases in the volume of relevant brain regions [18]. A 2006 study found that IQ was not related to cortical thickness per se, but rather to the plasticity of cortical thickness during childhood [19]. These factors appear to affect intelligence alternately, resulting in heterogeneity in the extent to which each factor contributes to each person's IQ level.

Different neural behaviors during reasoning are controlled by different brain regions. 29 neural networks were plotted under resting state and Raven Progressive Matrices (RPM, commonly used to test abstract reasoning ability): 6 attention neural networks (A1 A6), 6 cognitive neural networks (C1 C6), 6 visual neural networks (V1 V6), 6 sensorimotor neural networks (S1 S6), 3 default neural networks (D1 D3), auditory neural networks (AU), and basal ganglia neural networks (BG). In the Raven test, the spatial distribution and performance of neural networks during rest were plotted as statistics, and warm and cold colors marked the areas where the intrinsic network coherence increased and decreased significantly during the task, respectively.

Cited from: http://dx.doi.org/10.1016/j.neuroimage.2014.09.055

So while brain size is partly responsible for influencing IQ, more research is necessary to fully elucidate the interplay between genes, environment, brain anatomy, and cognitive development.

The brain is so complex that the research of neuroscience and cognitive science is still in its infancy. Perhaps as the research methods become more in-depth, there will be a clearer answer to this question.

References:

[1]Pirozzi F, Nelson B, Mirzaa G. From microcephaly to megalencephaly: determinants of brain size. Dialogues Clin Neurosci. 2018;20(4):267-282. doi:10.31887/DCNS.2018.20.4/gmirzaa

[2]Kenneth L. Beals. Brain Size, Cranial Morphology, Climate, and Time Machines CURRENT ANTHROPOLOGY V01. 25, NO 01984

[3]Allen, John S.; Damasio, Hanna; Grabowski, Thomas J. (August 2002). "Normal neuroanatomical variation in the human brain: An MRI-volumetric study". American Journal of Physical Anthropology. 118 (4): 341–358. doi: 10.1002/ajpa.10092. PMID 12124914.

[4]Hedman AM, van Haren NE, Schnack HG, Kahn RS, Hulshoff Pol HE. Human brain changes across the life span: a review of 56 longitudinal magnetic resonance imaging studies. Hum Brain Mapp. 2012;33(8):1987-2002. doi:10.1002/hbm.21334

[5]Rushton JP, Ankney CD. Brain size and cognitive ability: Correlations with age, sex, social class, and race. Psychon Bull Rev. 1996;3(1):21-36. doi:10.3758/BF03210739

[6] Sacco R, Gabriele S, Persico AM. Head circumference and brain size in autism spectrum disorder: A systematic review and meta-analysis. Psychiatry Res. 2015 Nov 30;234(2):239-51. doi: 10.1016/j.pscychresns.2015.08.016. Epub 2015 Sep 28. PMID: 26456415.

[7]Holden C. An evolutionary squeeze on brain size. Science. 2006 Jun 30;312(5782):1867. PMID: 16809505.

[8]http://thealternativehypothesis.org/index.php/2016/04/15/brain-size-and-iq/

[9] Ginneken, VV et al. “Hunter-prey correlation between migration routes of African buffaloes and early hominids: Evidence for the “Out of Africa” hypothesis.” Integrative molecular medicine 4 (2017): n. pag.

[10]Gilbert SL, Dobyns WB, Lahn BT. Genetic links between brain development and brain evolution. Nat Rev Genet. 2005;6(7):581-590. doi:10.1038/nrg1634

[11]Bond J, Roberts E, Mochida GH, et al. ASPM is a major determinant of cerebral cortical size. Nat Genet. 2002;32(2):316-320. doi:10.1038/ng995

[12]Zhang J. Evolution of the human ASPM gene, a major determinant of brain size. Genetics. 2003;165(4):2063-2070.

[13]Van Valen L. Brain size and intelligence in man. Am J Phys Anthropol. 1974;40(3):417-423. doi:10.1002/ajpa.1330400314

[14]Deary IJ, Penke L, Johnson W. The neuroscience of human intelligence differences. Nat Rev Neurosci. 2010;11(3):201-211. doi:10.1038/nrn2793

[15]Schoenemann PT, Budinger TF, Sarich VM, Wang WS. Brain size does not predict general cognitive ability within families. Proc Natl Acad Sci US A. 2000;97(9):4932-4937. doi:10.1073/pnas.97.9.4932

[16] Lee JJ, McGue M, Iacono WG, Michael AM, Chabris CF. The causal influence of brain size on human intelligence: Evidence from within-family phenotypic associations and GWAS modeling. Intelligence. 2019;75:48-58. doi: 10.1016/j.intell.2019.01.011

[17] Pietschnig J, Penke L, Wicherts JM, Zeiler M, Voracek M. Meta-analysis of associations between human brain volume and intelligence differences: How strong are they and what do they mean?. Neurosci Biobehav Rev. 2015;57:411-432. doi: 10.1016/j.neubiorev.2015.09.017

[18] Pietschnig, J. et al. “Meta-analysis of associations between human brain volume and intelligence differences: How strong are they and what do they mean?” Neuroscience & Biobehavioral Reviews 57 (2015): 411-432.

[19]Shaw, P., Greenstein, D., Lerch, J., Clasen, L., Lenroot, R., Gogtay, NEEA, et al., 2006. Intellectual ability and cortical development in children and adolescents. Nature 440, 676–679.

Produced by: Science Popularization China

Produced by: Clover Qingzi (Institute of Biomedical Sciences, Fudan University)

Producer: Computer Network Information Center, Chinese Academy of Sciences

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