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In my last article on brain size and IQ, I showed how people with half of their brains removed and people with microcephaly can have IQs in the normal/above average range. There is a pretty large amount of data out there on microcephalics and normal intelligence—even a family showing normal intelligence in two generations despite having dominantly inherited microcephaly.
Microcephaly is a condition in which an individual has a head circumference of 2 SD below the mean. Though most would think that would doom all microcephalics to low IQs, 15 percent of microcephalics have IQs in the normal range. This is normally associated with mental retardation, but this is a medical myth (Skoyles and Sagan, 2002: 239), though there are numerous cases of microcephalics having normal IQs (Dorman, 1991). Numerous studies show that it’s possible for normal people to have small brains. Giedd et al (1996) showed a wide variation in head circumference. Of the 104 individuals who had their heads scanned, volume for the cerebellum ranged from 735 cc in a 10 year old boy to 1470 cc in a 14 year old boy (Skoyles, 1999: 4, para 12). Though Giedd et al (1996) did not report total brain volumes in their subjects, brain volume can be inferred. Skoyles (1999; 4, para 12) writes:
The cerebral cortex makes up only 86.4% of brain volume when measured by MRI (Filipek, Richelme, Kennedy & Caviness, 1994), so the total brain volume of the 10-year-old would be larger at 850.7 cc. Brains at 10 years are about 4.4% smaller than adult size (Dekaban & Sadowsky, 1978), suggesting that that brain would grow to an adult size of 888 cc. Even using the lower figure of 80% cerebrum to brain ratio derived from anatomical studies suggests a figure of only 960 cc.
The variation of 888 cc to 960 cc depending on which value for the cerebrum to brain ratio you use still shows that people can have brains 450-300 cc lower than average and still be ‘normal’.
Researchers began noticing many cases of both individuals and families exhibiting features of microcephaly—but they had normal intelligence (Simila, 1970;Seemanova et al, 1985; Rossi et al, 1987; Teebi et al, 1987; Sherrif and Hegab, 1988; Desch et al, 1990; Opitz and Holt, 1990; Evans, 1991; Heney et al, 1991; Green et al, 1995; Rizzo and Pavone, 1995; Teebi and Kurash, 1996; Innis et al, 1997; Kawame, Pagon, and Hudgens, 1997; Abdel-Salam et al, 1999; Digweed, Reis, and Sperling, 1999; Woods, Bond, and Enard, 2005; Ghaoufari-Fard et al, 2015). This is a pretty huge blow to the brain size/IQ correlation, for if people with such small heads can have normal IQs, why do we have such large brains that leave us with such large problems (Skoyles and Sagan, 2002: 240-244)?
If we can have smaller heads—which would make childbirth easier and allow us to continue to have smaller pelves which would be conducive to endurance running since we are the running ape, why would brains have gotten so much larger from that of erectus (where modern people can have normal IQs with erectus-sized brains) if it is perfectly possible to have a brain on around the size of early erectus? In any event, these anomalies need an explanation, and Skoyles (1999) hypothesizes that people with smaller heads but normal IQs may have a lower capacity for expertise. This is something that I will look into in the future, as it may explain these anomalies, along with the true reason why our brains began increasing around 3 mya.
Sells (1977)—using the criteria of 2 SD below mean head size—showed that 1.9 percent of the children he tested (n=1009) had IQs indistinguishable from their normocephalic peers. Watemberg et al (2002) studied 1,393 patients. They found that almost half of their patients with microcephaly (15.4% of their patients studies had microcephaly) had IQs within the normal limits, while among those with sub-normal intelligence, 30 percent had borderline IQs or were mildly mentally retarded (it’s worth noting that l-glutamate can raise IQ scores by 5-20 points in the mild to moderate mental deficiency; Vogel, Braverman and Draguns, 1966 review numerous lines of evidence that glutamate raises IQ in mentally deficient individuals). Sassaman and Zartler (1982) showed that 31.9 percent of microcephalics had normal intelligence, 6.9 percent of them had average intelligence.
Head circumference does not directly correlate with IQ in microcephalic patients (Baxter et al, 2009). Dorman (1991: 268) writes: “Decreased head size may or may not be associated with lowered intelligence, indicating that small head size by itself does not affect intelligence. The presence of subgroups of microcephalic persons who typically have normal intelligence is sufficient to rule out a causal relationship between head size and intellect. … It can be added that reduction in brain size without such structural pathology, as mayvoccur in some genetic conditions or evenvas a result of normal variation, does not
affect intelligence. ”
Tenconi et al (1981) write: “We were able to examine five other members of this family (1-3; 11-1; 11-4; 11-5; 11-8) and found no abnormalities: they were of normal intelligence, head circumference, and ophthalmic evaluation. Members of the grandmother’s family who refused to be examined appeared to be of normal intelligence and head appearance and did not have any serious eye problems.”
Stoler-Poria et al (2010) write: “There was a K-ABC cognitive score < 85 (signifying developmental delay) in two (10%) children from the study group and in one (5%) child from the control group: one of the children in the study group (the one with HC below − 3 SD) scored significantly below the normal range (IQ = 70), while the other scored in the borderline range (IQ = 83); the child from the control group also scored in the borderline range (IQ = 84).” Whereas Thelander and Pryor (1968) showed that individuals with head circumferences 2-2.6 SDs below the mean had average IQs, though the smaller their HC, the lower their IQ. Ashwal et al (2009: 891) write: “The students with microcephaly had a similar mean IQ to the normocephalic group (99.5 vs 105) but had lower mean academic achievement scores (49 vs 70).” So it seems that microcephalics can have normal IQs, but have lower academic achievement scores.
Primary microcephalics have higher IQs than secondary microcephalics (Cowie, 1987). Primary microcephaly is microcephaly that one is born with whereas secondary microcephaly is acquired.
There is one case study of a girl with microcephaly where Tan et al (2014) write: “Most recent measures of general intelligence (performed at 6½ years of age) reveal a below average full scale IQ of 75 with greatest impairment in processing speed. On the Wechsler Preschool and Primary Scale of Intelligence III Revised (for children 2 years 6 m – 7 years 3 m), she obtained a Verbal IQ of 83, Nonverbal IQ of 75, and Processing speed 71. On the Wechsler Individual Achievement Testing (WIAT) she showed significant struggles in secondary language on tasks of early reading (SS 60), word reading (SS 70), reading comprehension (SS 69) and struggles in math on the task of numerical operations (SS 61) (WPPSI – R and WIAT mean = 100 and SD = 15). Parents report subjectively that differences in development relative to her sisters are becoming more apparent with time.”
It is not a foregone conclusion that if an individual has microcephaly that they will have a low IQ and be mentally retarded, as reviewed above, there are numerous cases of individuals with microcephaly and normal IQs, with this even being seen in families—that is, multiple families with normal IQs yet have microcephaly. Numerous people with Nijmegen breakage syndrome (a type of microcephaly) can have normal IQs. Rossi et al (1987) reported that for 6 Italian families (n=21 microcephalics) with autsomally inherited microcephaly, for those administered psychometric tests (n=12), all had normal IQs but one, with an IQ range of 99 to 112 for a mean of 99.3.
In conclusion, microcephalics can have normal IQs and live normal lives, despite having heads, on average, that are 2 SDs below the mean. These anomalies (and there are many, many more) need explaining. This is great evidence that a larger brain does not always mean a higher IQ, as well as yet more evidence that it was possible for Homo erectus to have an IQ in our range today, which means that we may not need brains our current size for our intellect and achievements. To conclude, I will provide a quote from Dorman (1991):
The normal intelligence found by SELLS in school children with small head size also militates against any straightforward relationship between diminished head size and lowered intelligence.
With the correlation between brain size and IQ being .4 (Gignac and Bates, 2017), this does not rule out the ‘outliers’ reviewed in this article. These cases deserve an explanation, for if large brains lead to high IQs, why do these people with heads significantly smaller have IQs in the normal range? (See Skoyles, 1999: 8, para 31 for an explanation for the brain size/IQ correlation.)
Emil Kirkegaard left a short commentary on John Skoyles’ 1999 paper Human Evolution Expanded Brains to Increase Expertise Capacity, not IQ, in which Emil writes in his article Evolution and imperfect mediators:
If we condense the argument, it becomes a little clearer:
John Skoyles (1999) [Condensed argument from Emil; paragraph 2] Brain expansion causes problems. Thus, whatever selected for increased brain size must have offered compensating benefits. People can have below average size brains yet exhibit normal intelligence. Thus, the compensating benefit offered by large brains is unlikely to be intelligence. Why should evolution have increased brain size with its associated problems for something smaller sized brains could have without expansion?
I merely edited out the unnecessary parts. Now try substituting some other trait, say fighting ability and some mediator of it.
Muscle size increases causes problems. Thus, whatever selected for increased muscle size must have offered compensating benefits. People can have below average size muscles yet exhibit normal fighting ability. Thus, the compensating benefit offered by large muscles is unlikely to be fighting ability. Why should evolution have increased muscle size with its associated problems for something smaller sized muscles could have without increase?
See the issue? This argument works for any imperfect physical underpinning of a trait, which is to say, basically all of them. Longer legs didn’t evolve for running well for some people with short legs run well. Bigger/stronger hears didn’t evolve for better cardio, because some people smaller/weaker hearts have good cardio. Longer arms didn’t evolve for fighting because some short armed people fight well. Darker skin didn’t evolve as a protection against sun exposure for some relative light skinned people don’t get skin cancer or sunburns. Larger eyes didn’t evolve for seeing better for some people with smaller eyes see well. Bigger ears… Bigger noses… Stronger hands… …
I don’t agree. Our brains sap about 20 percent of our daily energy needs while being 2 percent of our overall body mass whereas, in other primates, their brains cost about 9 percent of their daily energy needs (Fonseca-Azevedo and Herculano-Houzel, 2012).
In regards to Emil’s counterarguments, I’ll address them one by one:
Long legs: People with longer legs were better runners and could escape from predators and chase prey. People with shorter legs were killed.
Bigger/stronger hearts: Those with a larger heart (sans cardiomegaly) could run for longer distance (remember, we are distance runners; Carrier, 1984; Skoyles and Sagan, 2002; Bramble and Lieberman, 2004; Mattson, 2012) and so long legs and bigger/stronger hearts tie in with each other.
Long arms: This, again, goes back to our morphology in Africa. Long limbs are more conducive to heat dissipation (Lieberman, 2015). So those who had the right body plan for distance running could survive better during our evolutionary history.
Dark skin: A light-skinned person who spends enough time without protection in a tropical climate will develop skin cancer. (It is hypothesized that skin cancer is what caused the evolution of dark skin; Greaves, 2014, though this was contested by Jablonksi and Chaplin, 2014.)
Large eyes: Bigger eyes don’t mean better eyesight in comparison to smaller ones.
All in all, the brain size argument is 100 percent different from these arguments: large brains come with large problems. Further, there is evidence (which will be reviewed below) that people can live long, normal lives with half of their brain missing
The brain-size/IQ puzzle
The oft-repeated wisdom is that our brains evolved to such a large size so we could become more intelligent. And looking at when our brains began to increase (starting with erectus, which had to do with the advent of cooking/fire use), we can see that that’s when our modern body plan appeared. We can ascertain this by looking at Nariokotome boy, an erectus that lived about 1.6 mya.
Further, in regards to brain size, there was a man named Daniel Lyon. What was so extraordinary about this man is that, at the time of his death, had a brain that weighed 1.5 pounds (see Wilder, 1911)! Skoyles and Sagan (2002: 239) write:
Upon examination, anatomists could find no difference between it [Lyon’s brain] and other human brains apart from its size with one exception: The part of his brain attached to the brainstem, the cerebellum, was near normal size. Thus, the total size of Lyon’s cerebral hemisphere was smaller than would be suggested by a total brain weight of 1.5 lb. We do not know how bright he was—being a watchman is not particularly intellectually demanding—but he clearly was not retarded. A pound and a half brain may not be enough to manage a career as an attorney, a professor of theology, or a composer, but it was sufficient to let Lyon survive for 20 years in New York City.
Skoyles and Sagan (2002) review numerous lines of evidence of individuals with small brains/people with severe TBI living full lives, even having IQs in the average/above average range. They write (pg 238):
You would think that cutting out one-half of people’s brains would kill them, or at least leave them vegetables needing care for the rest of their lives. But it does not. Consider this striking story. A boy starts having seizures at 10 years of age when his right cerebral hemisphere atrophies. By the time he is 12, the left side of his body is paralyzed. When he is 19, surgeons decide to operate and remove the right side of his brain, as it is causing gits in his intact left one. You might think this would lower his IQ or leave him severely retarded, but no. His IQ shoots up 14 points, to 142! The mystery is not so great when you realize that the operation has gotten rid of the source of his fits, which had previously hampered his intelligence. When doctors saw him 15 years later, they described him as “having obtained a university doploma . . . [and now holding] a responsible administrative position with a local authority.” (18)
They also write about the story of an Argentinian boy who had a right hemispherectomy when he was 3-years-old who was notable for “the richness of his vocabulary and syntax” and also “attends English classes at school, in which he attains a high level of success (20; quote from Skoyles and Sagan, 2002: 238).
It is also a “medical myth that microcephaly (having a head smaller than two standard deviations (SD) below average circumference) is invariably linked to retardation.” (Skoyles and Sagan, 2002: 239).
There are some important things to be noted in regards to the study of Nariokotome boy’s skeleton and skull size. Skoyles and Sagan (2002: 240) write (emphasis mine):
So how well equipped was Homo erectus? To throw some figures at you (calculations shown in the notes), easily well enough. Of Nariokotome boy’s 673 cc of cortex, 164 cc would have been prefrontal cortex, roughly the same as half-brained people. Nariokotome boy did not need the mental competence required by cotemporary hunter-gatherers. … Compared to that of our distant ancestors, Upper Paleolithic technology is high tech. And the organizational skills used in hunts greatly improved 400,000 years ago to 20,000 years ago. These skills, in terms of our species, are recent, occurring by some estimates in less than the last 1 percent of our 2.5 million year existence as people. Before then, hunting skills would have required less brain power, as they were less mentally demanding. If you do not make detailed forward plans, then you do not need as much mental planning abilities as those who do. This suggests that the brains of Homo erectus did not arise for reasons of survival. For what they did, they could have gotten away with much smaller, Daniel Lyon-sized brains.
Lastly, I will touch on the fact that since we are running apes, that we need a narrow pelvis. As I stated above, our modern body plan came to be around 1.6 mya with the advent of erectus, which could be inferred from footprints (Steudel-Numbers, 2006; Bennett et al, 2009). Now the picture is beginning to become clearer: if people with brains the size of erectus could have intelligence in the modern range, and if our modern body plans evolved 1.6 mya (which is when our brains began to really increase in size due to metabolic constraints being unlocked due to erectus’ cooking ability), then you can see that it’d be perfectly possible for modern Homo sapiens to have brains the size of erectus while still having an IQ in the normal range.
Lastly, Skoyles and Sagan (2002: 245) write (emphasis mine):
Kanzi seems to do remarkably well with a chimp-sized brain. And while we tend to link retardation with small brains, we have seen that people can live completely normal lives while missing pieces of their brains. Brain size may enhance intelligence, but it seems we can get away without 3 pounders. Kanzi shows there is much potential in even 13 oz.
So Skoyles and Sagan do concede that brain size may enhance intelligence, however, as they have argued (and as Skoyles does in his 1999 paper), it is perfectly possible to live a normal life with half a brain, as well as have an average/above average IQ (as reviewed in Skoyles, 1999). So if people with erectus-sized brains can have IQs in the normal range and live normal lives, then brains must have increased for another reason, which Skoyles has argued is expertise capacity.
Large brains are, clearly, not needed for high IQs.
(Also search for this paper: Reiss, A. L., Abrams, M. T., Singer, H. S., Ross, J. L. & Denckla, M. B. (1996). Brain development, gender and IQ in children: A volumetric imaging study. Brain, 119, 1763-1774. where they show that there is a plateau, and a decrease in IQ in the largest brains; see table 2. I also reviewed some studies on TBI and IQ and how even those with severe TBI can have IQs in the normal range (Bigler, 1995; Wood and Rutterford, 2006; Crowe et al, 2012). Yet more evidence that people with half of their brains missing can lead normal lives and have IQs in the modern range.)