Higher Intelligence Caused By Slower Brain Development
My immediate reaction is what genetic variations cause this trajectory that leads to higher intelligence?
Youth with superior IQ are distinguished by how fast the thinking part of their brains thickens and thins as they grow up, researchers at the National Institutes of Health's (NIH) National Institute of Mental Health (NIMH) have discovered. Magnetic resonance imaging (MRI) scans showed that their brain's outer mantle, or cortex, thickens more rapidly during childhood, reaching its peak later than in their peers — perhaps reflecting a longer developmental window for high-level thinking circuitry. It also thins faster during the late teens, likely due to the withering of unused neural connections as the brain streamlines its operations. Drs. Philip Shaw, Judith Rapoport, Jay Giedd and colleagues at NIMH and McGill University report on their findings in the March 30, 2006 issue of Nature.
"Studies of brains have taught us that people with higher IQs do not have larger brains. Thanks to brain imaging technology, we can now see that the difference may be in the way the brain develops," said NIH Director Elias A. Zerhouni, M.D.
Here is where political correctness enters in. Zerhouni holds a highly visible position as head of a large government research agency. So in today's intellectual environment we can't expect much from him on the topic of intelligence. There is a positive correlation between IQ and brain size. There's an even higher positive correlation between IQ and brain gray matter size. But when it comes to differences in intelligence the taboos kick in with a vengeance. See links below for the truth of the matter.
While most previous MRI studies of brain development compared data from different children at different ages, the NIMH study sought to control for individual variation in brain structure by following the same 307 children and teens, ages 5-19, as they grew up. Most were scanned two or more times, at two-year intervals. The resulting scans were divided into three equal groups and analyzed based on IQ test scores: superior (121-145), high (109-120), and average (83-108).
The researchers found that the relationship between cortex thickness and IQ varied with age, particularly in the prefrontal cortex, seat of abstract reasoning, planning, and other "executive" functions. The smartest 7-year-olds tended to start out with a relatively thinner cortex that thickened rapidly, peaking by age 11 or 12 before thinning. In their peers with average IQ, an initially thicker cortex peaked by age 8, with gradual thinning thereafter. Those in the high range showed an intermediate trajectory (see below). While the cortex was thinning in all groups by the teen years, the superior group showed the highest rates of change.
"Brainy children are not cleverer solely by virtue of having more or less gray matter at any one age," explained Rapoport. "Rather, IQ is related to the dynamics of cortex maturation."
The observed differences are consistent with findings from functional magnetic resonance imaging, showing that levels of activation in prefrontal areas correlates with IQ, note the researchers. They suggest that the prolonged thickening of prefrontal cortex in children with superior IQs might reflect an "extended critical period for development of high-level cognitive circuits." Although it's not known for certain what underlies the thinning phase, evidence suggests it likely reflects "use-it-or-lose-it" pruning of brain cells, neurons, and their connections as the brain matures and becomes more efficient during the teen years.
The development of higher intellectual abilities required longer childhoods for humans than for other primates. Therefore it is not surprising that those who are smartest have longer periods of brain development.
"People with very agile minds tend to have a very agile cortex," said Shaw. The NIMH researchers are following-up with a search for gene variants that might be linked to the newly discovered trajectories. However, Shaw notes mounting evidence suggesting that the effects of genes often depends on interactions with environmental events, so the determinants of intelligence will likely prove to be a very complex mix of nature and nurture.
I'd really like to see a massive search for the genetic variations that boost intelligence. Identification of those genetic variations will lead to identification of targets for drug development and other means for boosting IQ in children whose brains are still developing.
As for the claim above that IQ does not correlate with brain size: Studies of brain size and intelligence have found correlations around r = 0.4. One study found that after controlling for body size the correlation with brain size was 0.65. Wikipedia has a short survey of brain size and IQ research results.
Modern studies using MRI imaging shows a weak to moderate correlation between brain size and IQ (Harvey, Persaud, Ron, Baker, & Murray, 1994) and have shown that brain size correlates with IQ by a factor of approximately .40 among adults (McDaniel, 2005). In 1991, Willerman et al. used data from 40 White American university students and reported a correlation coefficient of .35. Other studies done on samples of Caucasians show similar results, with Andreasen et al (1993) determining a correlation of .38, while Raz et al (1993) obtained a figure of .43 and Wickett et al. (1994) obtained a figure of .40. The correlation between brain size and IQ seems to hold for comparisons between and within families (Gignac et al. 2003; Jensen 1994; Jensen & Johnson 1994). However, one study found no within family correlation (Schoenemann et al. 2000).
The brain is a metabolically expensive organ, and consumes about 25% of the body's metabolic energy. Because of this fact, although larger brains are associated with higher intelligence, smaller brains might be advantageous from an evolutionary point of view if they are equal in intelligence to larger brains. Skull size correlates with brain size, but is not necessarily indicative.
The metabolic expense of the brain is the reason why brain size positively correlates with intelligence. Calorie malnutrition has been one of the biggest causes of death of humans since humans came into existence. The cost of a larger brain is such that it will get selected against unless it provides a selective advantage. Therefore it seems unreasonable to expect no correlation between brain size and intelligence.
P. Tom Schoenemann, an anthropologist at UC Berkeley, had this to say about brain size and IQ:
More interestingly, 4 recent studies of this question for the first time
derived estimates of brain size from high quality magnetic resonance
imaging (MRI), instead of using external cranial dimensions. All 4
studies show much higher correlations: Willerman et al. (1991) report an
estimated correlation of r = .35 (N = 40); Andreasen et al. (1993) found
a correlation of r= .38 (N = 67); Raz et al (in press) found a
correlation of r = .43 (N = 29); and Wickett et al. (in press) report a
correlation of r = .395 (N = 40, all females). These are all
statistically significant. It is quite simply a myth that brain size and
IQ are empirically unrelated in modern populations.
But it is a popular myth among public intellectuals.
Also see my post Brain Gray Matter Size Correlated To Intelligence.
Update: The New York Times coverage by Nicholas Wade notes that Dr. Paul Thompson of UCLA also found in 2001 that frontal lobes gray matter volume correlates with IQ.
In 2001, Dr. Thompson reported that based on imaging twins' brains the volume of gray matter in the frontal lobes and other areas correlated with I.Q. and was heavily influenced by genetics.
Wade also reports that the team around Shaw is doing many genetic studies on intelligence and have taken genetic samples from the Bethesda children used in this study.
Race and Brain size: Blacks have larger brains...
The majority of empirical studies on the matter of racial differences in Brain size suggest that blacks have larger brains than do others groups. Brain sizes vary considerably within any species, but this variation is not usually related to intelligence. Instead, it correlates loosely with body size: large people tend to have larger brains (Gould, 1981). As a result, women on average will have smaller brains than men (Peters, 1991). However, this does not indicate that the level of male intelligence is higher than female intelligence; Neanderthals had on average larger brains than do anatomically modern humans (Tattersall, 1995; Gould, 1981) but most would agree that they were considerably less intelligent than Homo sapiens (Tattersal, 1995, 2004; Gould, 1981; Mithen 1998).
Tobias (1970) compared 7 racial and national groups in a study on brain size, in which he reported that the brain size of American blacks was larger than any white group, (which included American, English and French whites) except those from the Swedish sub sample (who had the largest brains of any of the groups measured), and American blacks were also estimated to have some 200 million more neurons than American whites (See Tobias 1970; Weizmann et al. 1990). While Gould (1981) discovered upon recalculating Morton’s skull data that the skulls of blacks in his sample were on average larger than those of whites. Morton included in his sample of black skulls more females than he included in the white sample. After correcting this error it was shown that the black sample had larger skulls (and presumably, larger brains) than did whites.
Genetic studies of human brainsize have discovered two genes that when mutated can result in a severely reduced brain volume, or ‘Autosomal recessive primary microcephaly’. The gene microcephalin (MCPH1) regulates brain size during development and has experienced positive selection in the lineage leading to Homo sapiens (Zhang, 2003; Evans et al, 2005). Within modern humans a group of closely related haplotypes, known as ‘haplogroup D’ arose from a single copy at this locus (Evans, 2006). Globally, D alleles are young and first appeared about 37,000 years ago; with high frequency haplotypes being rare in Asia, and particularly Africa. The highest frequencies are seen in Europe/Eurasia. The second microcephalin gene, ‘ASPM’ (abnormal spindle like Microcephaly associated), went an episode of positive selection that ended some time ago (between 6–7 million and 100,000 B.P.), with newer D variants showing positive selection arising about 5,800 years ago (Evans et al, 2005; Zhang, 2003).
Microcephaly genetic researchers believe that D alleles may have first arisen in an archaic homo species about 1.1 million years ago before introgression into modern Homo sapien sapiens about 37, 000 years ago; possibly as the result of interspecies breeding (Evans et al, 2006). In fact, microcephalin shows by far the most compelling evidence of admixture among the human loci examined thus far (Evans et al, 2006). Modern humans arose only 100,000 years ago in Africa (Horan et al, 2005), which would make D alleles more than 1million years “older” than modern humans, and certainly very primitive by any stretch.
Normal D variants of both ‘MCPH1’ and ‘ASPM’ genes have been shown to have mild affects on human brainsize with empirical evidence demonstrating the alleles to reduce brain volume, slightly (Woods et al, 2006). For example, each additional ASPM allele was associated with a non significant 10.9 cc decrease in brain volume. For MCPH1, each additional allele was associated with a non significant 19.5 cc decrease in brain volume (Woods et al, 2006).
While selective pressure in favor of smaller brain volume might seem counterintuitive, it should be noted that the fossil records suggest that brain size in humans - particularly Europeans - has decreased over the past 35,000 years, and on through the Neolithic period (Frayer, 1984; Ruff et al, 1997; Woods, et al, 2006). Interestingly, the selected variant of MCPH1 is thought to have arisen about 37,000 years ago (Evans et al, 2006) making it a candidate gene responsible for this general decline (Woods et al, 2006), while the ASPM variant is thought to have arisen only 5,800 years ago. These archaeological changes in brain size are paralleled by changes in body size (Ruff et al, 1997; Woods et al., 2006), and it is possible that decreases in brain size may have exerted selective pressure for corresponding decreases in body size in Europeans (Ruff et al, 1997; Frayer, 1984; see also, Woods et al., 2006).
The rate of selection for these particular variant MCPH1 and ASPM alleles might also indicate that the genes are relatively unexpressed in the human brain, outside of causing ‘Autosomal recessive primary microcephaly.’ In one study it was shown that genes with maximal expression in the human brain tend to show little or no evidence for positive selection (Nielsen et al, 2006). For example, the microcephaly genes in question have also been implicated in the development of breast cancer (Xu et al, 2004), and other non brain related conditions (Trimborn et al, 2004). Implying that the mild brain volume reductions observed with each additional variant of ASPM and MCPH1 may in fact be adaptively unimportant. It should be further noted that one microcephalin gene (CDK5RAP2) has shown evidence of positive selection in West African Yoruba (Voight, 2006; bond et al, 2005), however, this gene at the MCPH3 locus has been least involved in causing a microcephalin phenotype (Hassan et al, 2007), and is not believed to have arisen in an archaic homo species.
S.O.Y. KEITA (2006) in his principal components analysis on male crania from the northeast quadrant of Africa and selected European and other African series found no consistent size differences in the skulls he measured. Stating: “The plots are immediately striking in that sharp patterns of segregation of individuals by group origin do not emerge in the two dimensional plots. It is striking how much ‘‘size’’ varies by individual within the European and African regions, assuming that PC 1 captures primarily this quality; Bergman’s rule is not demonstrated in these data in any easily recognizable way, since individuals from all regions exhibit variation.” Herskovits’s (1930) data also suggest that there is no consistent Black/ White difference with respect to stature or crania.
Cernovsky (1990), however, reported that American blacks were superior in brain weight when compared with American whites. It is also known that the largest portions of the human brain are devoted to sensory and motor functions, which would mean that people with especially acute senses or strong motor skills can be expected to have larger brains than do others (Allen, 2002). It has been shown in several studies that blacks in general possess superior motor skills when compared to whites (Super, 1976; Wilson 1978; DiNucci, 1975); some believe that this may be the result of environmental and cultural factors (Super, 1976). The overall implications are the same, however, and suggest that blacks have larger brains.
Testosterone, Brain size and Penis size…?
Some of the more desperate claims for racial differences in brain size are accompanied by unusual arguments suggesting racial differences in penis size (that they are inversely correlated). Thorough investigation of the formal neuroscience, anthropology, paleontology, anatomy, physiology, and ‘sex psychology’ literature reveal that legitimate references to this - ridiculous (?) - notion are not only remote, but in fact, “nonexistent.” The development and size of one’s penis tend to be controlled by testosterone levels during puberty; and it is testosterone (and body size) that determine penis size. Testosterone: “Primary male hormone, causes the reproductive organs to grow and develop; responsible for secondary sexual characteristics, and promotes erections and sexual behavior.” Definition from: University of Michigan comprehensive Cancer Center; Fertility & Cryopreservation Glossary.
With this in mind; employing elementary logic one may safely arrive at the conclusion that because men tend to have dramatically higher levels of testosterone than do women (about 10 times the level), and on average have larger brains (due mostly to body size); that testosterone not only increases body and penis size, but also brain size! In fact, the relationship between brain size and testosterone is one of common knowledge, and is well documented in the literature (e.g. Solms and Turnbull, 2002).
Moreover, low testosterone has been associated with smaller penises and testes, failure to go through full normal puberty, poor muscle development, reduced muscle strength, low interest in sex (decreased libido), osteoporosis (thinning of bones common in whites and Asians), poor concentration, difficulty getting and keeping erections, low semen volume, longer time to recover from exercise, and easy fatigue, in men (McLachlan and Allan, 2005). On the flipside, high testosterone has been associated with improved health, superior motor abilities, increased reproductive value (in men), increased mental focus, larger brain volume, superior bone density and “boldness” (Dabbs and Dabbs, 2000; Solms and Turnbull, 2002; ).
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