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Here is my reply to Jared Taylor’s new article over at AmRen Breakthroughs in Intelligence:
“The human mind is not a blank slate; intelligence is biological”
The mind is not a ‘blank slate’, though there is no ‘biological’ basis for intelligence (at least in the way that hereditarians believe). They’re just correlations. (Whatever ‘intelligence’ is.)
“there is no known environmental intervention—including breast feeding”
There is a causal effect of breast feeding on IQ:
While reported associations of breastfeeding with child BP and BMI are likely to reflect residual confounding, breastfeeding may have causal effects on IQ. Comparing associations between populations with differing confounding structures can be used to improve causal inference in observational studies.
Brion, M. A., Lawlor, D. A., Matijasevich, A., Horta, B., Anselmi, L., Araújo, C. L., . . . Smith, G. D. (2011). What are the causal effects of breastfeeding on IQ, obesity and blood pressure? Evidence from comparing high-income with middle-income cohorts. International Journal of Epidemiology, 40(3), 670-680. doi:10.1093/ije/dyr020
Breastfeeding is related to improved performance in intelligence tests. A positive effect of breastfeeding on cognition was also observed in a randomised trial. This suggests that the association is causal.
Horta, B. L., Mola, C. L., & Victora, C. G. (2015). Breastfeeding and intelligence: a systematic review and meta-analysis. Acta Paediatrica, 104, 14-19. doi:10.1111/apa.13139
“before long we should be able to change genes and the brain itself in order to raise intelligence.“
Which genes? 84 percent of genes are expressed in the brain. Good luck ‘finding’ them…
These results corroborate with the results from previous studies, which have shown 84% of genes to be expressed in the adult human brain …
Negi, S. K., & Guda, C. (2017). Global gene expression profiling of healthy human brain and its application in studying neurological disorders. Scientific Reports, 7(1). doi:10.1038/s41598-017-00952-9
“Normal people can have extraordinary abilities. Prof. Haier writes about a non-savant who used memory techniques to memorize 67,890 digits of π! He also notes that chess grandmasters have an average IQ of 100; they seem to have a highly specialized ability that is different from normal intelligence. Prof. Haier asks whether we will eventually understand the brain well enough to endow anyone with special abilities of that kind.”
Evidence that intelligence is not related to expertise.
“It is only after a weight of evidence has been established that we should have any degree of confidence in a finding, and Prof. Haier issues another warning: “If the weight of evidence changes for any of the topics covered, I will change my mind, and so should you.” It is refreshing when scientists do science rather than sociology.”
Even with the “weight of evidence”, most people will not change their views on this matter.
“Once it became possible to take static and then real-time pictures of what is going on in the brain, a number of findings emerged. One is that intelligence appears to be related to both brain efficiency and structure”
Patterns of activation in response to various fluid reasoning tasks are diverse, and brain regions activated in response to ostensibly similar types of reasoning (inductive, deductive) appear to be closely associated with task content and context. The evidence is not consistent with the view that there is a unitary reasoning neural substrate. (p. 145)
Nisbett R. E., Aronson J., Blair C., Dickens W., Flynn J., Halpern D. F., Turkheimer E. Intelligence: New findings and theoretical developments. American Psychologist. 2012;67:130–159. doi: 10.1037/a0026699.
“Early findings suggested that smart people’s brains require less glucose—the main fuel for brain activity—than those of dullards.”
Cause and correlation aren’t untangled; they could be answering questions in a familiar format, for instance, and this could be why their brains show less glucose consumption.
“It now appears that grey matter is where “thinking” takes place, and white matter provides connections between different areas of grey matter. Some brains seem to be organized with shorter white-matter connections, which appear to allow more efficient communication, and there seem to be sex differences in the ways the part of the brain are connected. One of the effects of aging is deterioration of the white-matter connections, which reduces intelligence.”
Read this commentary (pg. 162): Norgate, S., & Richardson, K. (2007). On images from correlations. Behavioral and Brain Sciences, 30(02), 162. doi:10.1017/s0140525x07001379
“Brain damage never makes people smarter”
This is wrong:
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 diploma . . . [and now holding] a responsible administrative position with a local authority.”
Skoyles, J. R., & Sagan, D. (2002). Up from dragons: the evolution of human intelligence. New York: McGraw-Hill (pg. 282)
“Prof. Haier wants a concerted effort: “What if a country ignored space exploration and announced its major scientific goal was to achieve the capability to increase every citizen’s g-factor [general intelligence] by a standard deviation?””
Don’t make me laugh. You need to prove that ‘g’ exists first. Glad to see some commentary on epigenetics that isn’t bashing it (it is a real phenomenon, though the scope of it in regards to health, disease and evolution remains to be discovered).
As most readers may know, I’m skeptical here and a huge contrarian. I do not believe that g is physiological and if it were then they better start defining it/talking about it differently because I’ve shown that if it were physiological then it would not mimick any known physiological process in the body. I eagerly await some good neuroscience studies on IQ that are robust, with large ns, their conclusions show the arrow of causality, and they’re not just making large sweeping claims that they found X “just because they want to” and are emotionally invested in their work. That’s my opinion about a lot of intelligence research; like everyone, they are invested in their own theories and will do whatever it takes to save face no matter the results. The recent Amy Cuddy fiasco is the perfect example of someone not giving up when it’s clear they’re incorrect.
I wish that Mr. Taylor would actually read some of the literature out there on TBI and IQ along with how people with chunks of their brains missing can have IQs in the normal range, showing evidence that most a lot of our brain mass is redundant. How can someone survive with a brain that weighs 1.5 pounds (680 gms) and not need care for the rest of his life? That, in my opinion, shows how incredible of an organ the human brain is and how plastic it is—especially in young age. People with IQs in the normal range need to be studied by neuroscientists because anomalies need explaining.
If large brains are needed for high IQs, then how do these people function in day-to-day life? Shouldn’t they be ‘as dumb as an erectus’, since they have erectus-sized brains living in the modern world? Well, the human body and brain are two amazing aspects of evolution, so even sudden brain damage and brain removal (up to half the brain) does not show deleterious effects in a lot of people. This is a clue, a clue that most of our brain mass after erectus is useless for our ‘intelligence’ and that our brains must have expanded for another reason—family structure, sociality, expertise, etc. I will cover this at length in the future.
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.)
What is the relationship between traumatic brain injury (TBI) and IQ? Does IQ decrease? Stay the same? Increase? A few studies have looked at the relationship between TBI and IQ, and the results may be quite surprising to some. Tonight I will look through a few studies and see what the relationship is between TBI and IQ—does IQ decrease substantially or is there only a small decrease? Does it decrease for all subtests or only some?
TBI and IQ
In a sample of 72 people with TBI who had significant brain injuries had an average IQ of 90 (study 1; Bigler, 1995). Bigler also says that whatever correlation exists between brain size and IQ “does not persist post injury” (pg 387). This finding has large implications: can there be a minimal hit to IQ depending on age/severity of injury/brain size/education level?
As will be seen when I review another study on IQ and brain injury, every individual in the cohort in Bigler (1995) was tested after 42 days of brain injury. This does matter, as I will get into below.
Table 1 in study 1 shows that whatever positive relationship between IQ and brain size that is there before injury does not persist after injury (Bigler, 1995: 387). Study 1 showed that, even with mild-to-severe brain damage, there was little change in measured IQ—largely because the correlation between brain size and IQ is .51 at the high end (which I will use—the true correlation is between .24 [Pietschnig et al, 2015] to .4 [Rushton and Ankney, 2009]), this means that if the correlation were to be that high, brain size would only explain 25 percent of the variation in IQ (Skoyles, 1999). That leaves a lot of room for other reasons for differences in brain size and IQ in individuals and groups.
In study 2 (Bigler, 1995: 389-391), he looked into whether or not there were differences in IQ between high and low brain volume people (95 men). Results summed in table 3 (pg 390). Those with low brain volume (1185), aged 28, had an IQ of 82.61 while those with high brain volume (1584), aged 34 had an IQ of 92 (both cohorts had similar education). Bigler showed in study 1 IQ was maintained post injury, so we can say that this was their IQ preinjury.
In table 2, Bigler (1995) compares IQs and brain volumes of mild-to-moderate and moderate-to-severe individuals with TBI. Brain volume in the moderate-to-severe group was 1289.2 whereas for the mild-to-moderate TBI-suffering individuals had a mean brain volume of 1332.9. Amazingly, both groups had IQ scores in the normal range (90.0 for moderate-to-severe TBI and 90.7 for individuals suffering from mild-to-moderate TBI. In study 3, Bigler (1995) shows that trauma-induced atrophic changes in the brain aren’t related to IQ postinjury, nor to the amount of focal lesion volume.
Nevertheless, Bigler (1995) shows that those with bigger brains had less of a cognitive hit after TBI than those with smaller brains. PumpkinPerson pointed me to a study that shows that TBI stretches far back into our evolutionary history, with TBI seen in australopithecine fossils along with erectus fossils found throughout the world. This implies that TBI was a driver for brain size (Shivley et al, 2012); if the brain is bigger, then if/when TBI is acquired, the cognitive hit will be lessened (Stern, 2002). This is a great theory for explaining why we have large brains despite the negatives that come with them—if we were to acquire TBI in our evolutionary past, then the hit to our cognition would not be too great, and so we could still pass our genes to the next generation.
The fact that changes in IQ are minimal when brain damage is acquired shows that brain size isn’t as important as some brain-size-fetishists would like you to believe. Though, preinjury (PI) IQ was not tested, I have one study where it was.
Wood and Rutterford (2006) showed results similar to Bigler (1995)—minimal change to IQ occurs after TBI. The whole cohort pre-injury (PI) had a 99.79 IQ. T1 (early measure) IQ for the cohort was 90.96 while T2 (late measure) IQ for the cohort was 92.37. For people with greater than 11th-grade education (n=30), IQ decreased from 106.57 PI to 95.19 in T1 to 100.17 in T2. For people with less than an 11th-grade education (n=44), IQ PI was 95.16 and decreased to 86.99 in T1 and increased to 87.96 in T2. Male (n=51) and female (n=23) were similar, with male PI IQ being 99.04 to women’s 101.44 with a 90.13 IQ in T1 for men with a 90.72 IQ in T1 for women. In T2 for men it was 92.94 and for women, it was 92.83. So this cohort shows the same trends as Bigler (1995).
The most marked difference in subtests post-injury was in vocabulary (see table 3) with similarities staying the same, and digit symbol, and block design increasing between T1 and T2. Neither group differed between T1 and T2. The only significant association in performance change over time was years of education. Less educated people were at greater risk for cognitive decline (see table 2).
The difference for PI IQ after T2 for less educated people was 7.2 whereas for more educated people it was 6.4. Though more educated people gained back more IQ points between T1 and T2 (4.98 points) compared to less educated people (.97 IQ points). And: “The participants in our study represent a subgroup of patients with severe head injury reported in a larger study assessing long‐term psychosocial outcome.”
Bigler (1995) didn’t have PI IQ, but Wood and Rutterford (2006) did, and from T1 to T2 (Bigler 1995 tested what would be equivalent to T1 in the Wood and Rutterford 2006 study), IQ hardly increased for those with lower education (.97 points) but substantially increased for those with higher education (4.98 points) with there being a similar difference between PI IQ and T2 IQ for both groups.
Brain-derived neurotrophic protective factor (BDNF) also promotes survival and synaptic plasticity in the human brain (Barbey et al, 2014). They genotyped 156 Vietnam War soldiers with frontal lobe lesion and “focal penetrating head injuries” for the BDNF polymorphism. Though they did find differences in the groups with and without the BDNF polymorphism, writing that there were “substantial average differences between these groups in general intelligence (≈ half a standard deviation or 8 IQ points), verbal comprehension (6 IQ points), perceptual organization (6 IQ points), working memory (8 IQ points), and processing speed (8 IQ points) after TBI” (Barbey et al, 2014). This supports the hypothesis that BDNF is protective against TBI; and since BDNF was important in our evolutionary history which is secreted by the brain while endurance running (Raichlen and Polk, 2012), this could have also been another protective factor against hits to cognition that were acquired, say, during hunts or fights.
Nevertheless, one study found in a sample of 181 children Crowe et al (2012) found that children with mild-to-moderate TBI had IQ scores in the average range, whereas children with severe TBI had IQ scores in the low average range (80 to 90; table 3).
Infants with mild TBI had IQ scores of 99.9 (n=20) whereas infants with moderate TBI has IQs of 98.0 (n=23) and infants with severe TBI had IQs of 90.7 (n=7); preschoolers with mild TBI had IQ scores of 103.8 (n=11), whereas preschoolers with moderate TBI had IQ scores of 100.1 (n=19) and preschoolers with severe TBI had IQ scores of 85.8 (n=13); middle schoolers with mild TBI had IQ scores of 93.9 (n=10), whereas middle schoolers with moderate TBI had IQ scores of 93.5 (n=21), and middle schoolers with severe TBI had IQ scores of 86.1 (n=14); finally, children with mild TBI in late childhood had a mean FSIQ of 107.3 (n=17), while children with moderate TBI had IQs of 99.5 in late childhood (n=15), and children with severe TBI in late childhood had FSIQs of 94.7 (Crowe et al, 2012; table 3). This shows that age of acquisition and severity influence IQ scores (along with their subtests), and that brain maturity matters for maintaining average intelligence post-TBI. Königs et al (2016) also show the same trend; the outlook is better for children with mild TBI, while children faired far worse with severe TBI compared to mild when compared to adults (also seen in Crowe et al, 2012).
People who got into motor vehicle accidents suffered a loss of 14 IQ points (n=33) after being tested 20 months postinjury (Parker and Rosenblum, 1996). The WAIS-IV Technical and Interpretive Manual also shows a similar loss of 16 points (pg 111-112), however, the 22 subjects were tested within 6 to 18 months within acquiring their TBI, with no indication of whether or not a follow-up was done. IQ will recover postinjury, but education, brain size, age, and severity all are factors that contribute to how many IQ points will be gained. However, adults who suffer mild, moderate, and severe TBIs have IQs in the normal range. TBI severity also had a stronger effect on children aged 2 to 7 years of age at injury, with white matter volume and results on the Glasgow Coma Scale (which is used to assess consciousness after a TBI) were related to the severity of the injury (Levin, 2012).
TBI can occur with a minimal hit to IQ (Bigler, 1995; Wood and Rutterford, 2006; Crowe et al, 2012). IQs can still be in the average range at a wide range of ages/severities, however the older one is when they suffer a TBI, the more likely it is that they will incur little to no loss in IQ (depending on the severity, and even then they are still in the average range). It is interesting to note that TBI may have been a selective factor in our brain evolution over the past 3 million years from australopithecines to erectus to Neanderthals to us. However, the fact that people with severe TBI can have IQ scores in the normal range shows that the brain size/IQ correlation isn’t all it’s cracked up to be.
Barbey AK, Colom R, Paul E, Forbes C, Krueger F, Goldman D, et al. (2014) Preservation of General Intelligence following Traumatic Brain Injury: Contributions of the Met66 Brain-Derived Neurotrophic Factor. PLoS ONE 9(2): e88733. https://doi.org/10.1371/journal.pone.0088733
Bigler, E. D. (1995). Brain morphology and intelligence. Developmental Neuropsychology,11(4), 377-403. doi:10.1080/87565649509540628
Crowe, L. M., Catroppa, C., Babl, F. E., Rosenfeld, J. V., & Anderson, V. (2012). Timing of Traumatic Brain Injury in Childhood and Intellectual Outcome. Journal of Pediatric Psychology,37(7), 745-754. doi:10.1093/jpepsy/jss070
Green, R. E., Melo, B., Christensen, B., Ngo, L., Monette, G., & Bradbury, C. (2008). Measuring premorbid IQ in traumatic brain injury: An examination of the validity of the Wechsler Test of Adult Reading (WTAR). Journal of Clinical and Experimental Neuropsychology,30(2), 163-172. doi:10.1080/13803390701300524
Königs, M., Engenhorst, P. J., & Oosterlaan, J. (2016). Intelligence after traumatic brain injury: meta-analysis of outcomes and prognosis. European Journal of Neurology,23(1), 21-29. doi:10.1111/ene.12719
Levin, H. S. (2012). Long-term Intellectual Outcome of Traumatic Brain Injury in Children: Limits to Neuroplasticity of the Young Brain? Pediatrics, 129(2), e494–e495. http://doi.org/10.1542/peds.2011-3403
Parker, R. S., & Rosenblum, A. (1996). IQ loss and emotional dysfunctions after mild head injury incurred in a motor vehicle accident. Journal of Clinical Psychology,52(1), 32-43. doi:10.1002/(sici)1097-4679(199601)52:1<32::aid-jclp5>3.3.co;2-1
Pietschnig, J., Penke, L., Wicherts, J. M., Zeiler, M., & Voracek, M. (n.d.). Meta-Analysis of Associations Between Human Brain Volume And Intelligence Differences: How Strong Are They and What Do They Mean? SSRN Electronic Journal. doi:10.2139/ssrn.2512128
Raichlen, D. A., & Polk, J. D. (2012). Linking brains and brawn: exercise and the evolution of human neurobiology. Proceedings of the Royal Society B: Biological Sciences,280(1750), 20122250-20122250. doi:10.1098/rspb.2012.2250
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Shively, S., Scher, A. I., Perl, D. P., & Diaz-Arrastia, R. (2012). Dementia Resulting From Traumatic Brain Injury: What Is the Pathology? Archives of Neurology, 69(10), 1245–1251. http://doi.org/10.1001/archneurol.2011.3747
Skoyles R. J. (1999) HUMAN EVOLUTION EXPANDED BRAINS TO INCREASE EXPERTISE CAPACITY, NOT IQ. Psycoloquy: 10(002) brain expertise
Stern, Y. (2002). What is cognitive reserve? Theory and research application of the reserve concept. Journal of the International Neuropsychological Society,8(03), 448-460. doi:10.1017/s1355617702813248
Wood, R. L., & Rutterford, N. A. (2006). Long‐term effect of head trauma on intellectual abilities: a 16‐year outcome study. Journal of Neurology, Neurosurgery, and Psychiatry, 77(10), 1180–1184. http://doi.org/10.1136/jnnp.2006.091553