March 30, 2006
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[1], 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.

Share |      Randall Parker, 2006 March 30 09:14 PM  Brain Development

Ahrimahn said at March 31, 2006 9:50 AM:

In Africa, parasitic diseases such as malaria are particularly lethal to children. This may mean that natural selection has favored a shorter childhood in black Africans so that they will be able to fight off malaria but with the downside that brain development also occurs more rapidly and, therefore, results in a lower IQ. As J. Philippe Rushton points out:

Black babies sit, crawl, walk, and put on their clothes earlier than Whites or East Asians. For walking: East Asians, 13 months; Whites, 12 months; Blacks, 11 months.

Blacks also have an earlier age of sexual maturity than do Whites, who in turn have an earlier age than do East Asians, whether measured by age of first menstruation, first sexual experience, or first pregnancy.

Ivan Kirigin said at April 1, 2006 7:46 AM:

I'm not sure I understand what the taboo is in the original post. Is it what Ahrimahn is saying, that correlation between brain size and IQ would imply something about different races and their tested differences in brain size?

Doug said at April 1, 2006 8:02 AM:

I'd like to see a longitudinal study like this in which the children have a diet supplemented with vitamin D3 and the omega-3 essential fatty acids. I wonder how much, if any, of this thinning of the cortex occurs because the children's sun exposure and intake of omega-3 EFAs is inadequate to support a large, well-formed brain. It seems this is especially a concern in the brains of the children with the highest IQs; as I understand it, brain is "metabolically expensive" tissue. It's a sickening thought, but it may be the otherwise better brains that are more harmed by inadequate vitamin D3 or undergo a greater percentage of pruning in the face of inadequate intake of EFAs. If there are such fitness penalties for high intelligence, then we have a good explanation of the fact that most animals have, well, the intelligence of an animal.

michael vassar said at April 1, 2006 10:27 AM:

Randal "seems unreasonable to expect no correlation between brain size and intelligence."
Somewhat unlikely a-priori and disproven, but not that unreasonable. Intelligence isn't that big a part of what the brain does. Bigger brains might be better at processing sensory information or at social skills that correlate poorly with g

Doug: I would REALLY like to see such a study. Concievably incredibly Useful.

All: I wonder what to think about extreme prodigies who seem to peak at superior mental performance by age 8 or 9 such as Gregory Smith, Sho Yano, and Michael Kearney

APC said at April 1, 2006 10:30 AM:

I too believe that nutritional factors can do wonders for both the dev. and function of the brain(personal experience), just like they seemingly can for lifespan(cr and cr mimetics, iron control, vit c, veggie rich diet, etc.). I generally tend to consume large doses of some of these nutrients(omega 3s, circulation improving nutrients, and mitochondria energy output increasing nutrients. Along with massive veggie consumption.) and I can attest that the difference I've experienced is monumental(I'm talking exponential increases in creativity, rates of new insight, ease of grasping new subjects, memory and the like.), I feel like I'm several fold smarter and capable of handling any subject with utter ease when supplementing.

Randall Parker said at April 1, 2006 11:13 AM:


Yes, a positive correlation between brain size and intelligence seems likely for all sorts of reasons. Intelligence is a bigger part of what humans brans do than for lower animals. Obviously higher intelligence was selected for in humans and that this came along with a larger brain. Also, we as a species are noted more for our accomplishments by reasoning than by our coordination or special sensory skills. Lots of species have better sensory processing for a variety of purposes (e.g. many predators have great senses of smell or special visual sensory processing abilities).

We do not get that much adaptive advantage from sensory capabilities - certainly not enough to account for such a huge increase in energy needs that comes from a bigger brain.


Whites have larger brains than blacks on average and East Asians have larger brains than whites on average. So the repeatedly measured positive correlation between brain size and intelligence is a taboo. See, for example:

Rushton (1995) reviewed 100 years of scientific literature and found that across a triangulation of procedures, brains of East-Asians and their descendants average about 17 cm3 (1 in3) larger than those of Europeans and their descendents whose brains average about 80 cm3 (5 in3) larger than those of Africans and their descendents. Although critics can pick outliers to show counter-examples and suggest opposite trends (as could critics of a statement that men are, on average, taller than women) the aggregated data are clear (see Rushton, 1995, for full discussion of alleged counter examples).

We are approaching the end of the taboo period. DNA sequencing becomes cheaper every year. MRI scans and other measures also become cheaper and the accumulated knowledge about intelligence differences and their causes is eventually going to become so large that the rationalizations used to explain away the evidence will become too weak to hold back the truth. I'd be surprised if the taboos lasted another 5 years.

Ahrimahn said at April 3, 2006 7:55 AM:

MRI scans and other measures also become cheaper and the accumulated knowledge about intelligence differences and their causes is eventually going to become so large that the rationalizations used to explain away the evidence will become too weak to hold back the truth. I'd be surprised if the taboos lasted another 5 years.

I think you are underestimating the human ability to hide from the truth.

Randall Parker said at April 3, 2006 3:26 PM:


Most people are in the closet about their views on racial differences for intelligence and other cognitive qualities. I know grad students and professors in biological sciences and social sciences at some of the most elite universities in America who are closet believers but who stay silent because they are afraid of the taboo enforcers. Some are angry about being in the closet. Once the genetic bases of IQ differences are demonstrated a whole lot of people will come out of the closet.

CASpears said at April 3, 2006 6:37 PM:

Can someone explain this....

" 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."

Truth is larger people have larger brains, that is why we have to correct for body size. We all know the stereotype of the dumb high school jock, I'm sure many of those people have larger brains than many of you, but that does not equate to intelligent. In the article I read on this site the correlation is far from clear.

Somewhat related to this...a small group of hominid pygmies were found in Indonesia. They are believed to be some offshoot of Homo Erectus. These people were less than 4 feet tall and had very very small heads, but produced complex tools. This was considered impossible...for the very reasons that many of you above sited, it was counterintuitive to modern science.

I have met people with "small heads" who are very intelligent, and go to leading schools. How does this happen? Anyone care to explain? Einstein's brain was measured and studied and it was not particularlly large, but certain areas were very dense. This says a lot. I seriously doubt that a large head or brain equals high intelligence in most people it is about the shape and density of certain areas of the brain.

I've known a lot of black people all my life, growing up in the US, and even if the average black IQ is truely 85 and even if that in relation to the average white American's IQ is unchangable by any outside environmental factors, I wonder if the resulting 6 million blacks who have a higher IQ than the average whites, have larger or small heads. I'm guessing smaller. Most whites have larger heads, but I've met some dumb big headed white people, sorry to burst your bubble.

CASpears said at April 3, 2006 6:46 PM:

Q&A : Indonesian hominid find
LB1, Image: National Geographic
Legends on Flores talk of small "murmuring" people called the Ebu Gogo
The remains of LB1, or Homo floresiensis, tell us our global dominance is far more recent than we thought.

We shared the planet with other intelligent beings until a mere 12,000 years ago.

BBC News Online looks at some of the questions that the "Hobbit" raises.

Q: Why is this discovery so exciting?

In short, it could change our understanding of human evolution. For example, we tend to think that complex tool making comes with big brains. But LB1 had a tiny brain and yet made quite sophisticated tools. This will probably cause researchers to re-think the behavioural complexity and capabilities of our small brained ancestors.

And, according to conventional wisdom, humans have walked the Earth alone since Neanderthals died out about 27,000 years ago.

But if LB1 was alive until 12,000 years ago, what other hominids also shared our planet? This discovery has reignited the debate.

Bob Badour said at April 4, 2006 6:31 AM:


"Can someone explain this...."

The correlation between brain size and intelligence is 0.4, which means 0.6 of intelligence correlates with other factors. We know that intelligence also correlates with varying densities of white and gray matter in various parts of the brain. There may be other factors too.

CASpears said at April 4, 2006 4:30 PM:

Another question...I read in another article on this site, that women and men's brains have been shown to be structured differently (big surpise...ha) having more grey matter and women more white matter, but the average IQ difference between men and women is negligable.

So, I wonder how this effects IQ inheritability. A smart woman might not be able to pass down "intelligent" genes to a boy, being that the structure that makes her brain intelligent only applies to women.

Bob Badour said at April 4, 2006 5:25 PM:


While I may have been misled and I am too lazy to look up citations, I believe that, while the average measured IQ of men and women is the same, the bell curves are different. The female bell curve is steeper with a smaller standard deviation.

The most eloquent way I heard the phenomenon put is: "Reproductively, the average female is much more valuable than the average male."

The shallower male curve with a larger standard deviation means there are fewer men clustered near the median and more at both extremes. While obviously the Y chromosome has an effect, the effect is not always toward greater intelligence.

Whether a more intelligent woman is likely to contribute to a more intelligent child probably depends on whether the genes that make her more intelligent are paired like recessives or whether the intelligent set "won the draw" during the disabling of the set on one X chromosome. Overall though, I suspect intelligent men will have more intelligent children by mating more intelligent women than by mating less intelligent women.

Once you get to the low side of the bell curve, the question becomes how much the mother's contribution will hold down the child's IQ.

Ahrimahn said at April 5, 2006 12:04 PM:

Most whites have larger heads, but I've met some dumb big headed white people, sorry to burst your bubble

Are you talking about the ones that vote Democrat? ;)

sandra said at May 13, 2007 3:55 AM:

How can those who believe in intelligence believe in evolution? Your concepts seem very racist and sexist to me. I believe in intelligent design. I believe in an intelligent and loving God who makes our little intelligence "negligible."

Romans 11:33
O the depth of the riches both of the wisdom and knowledge of God! how unsearchable are his judgments, and his ways past finding out!
1 Corinthians 3:18-20

18Let no man deceive himself. If any man among you seemeth to be wise in this world, let him become a fool, that he may be wise.

19For the wisdom of this world is foolishness with God. For it is written, He taketh the wise in their own craftiness.

20And again, The Lord knoweth the thoughts of the wise, that they are vain.
James 1:5
If any of you lack wisdom, let him ask of God, that giveth to all men liberally, and upbraideth not; and it shall be given him.

signed, a black female with a Ph.D.--and a high IQ, according to mainstream doctors.

Counter View said at July 4, 2007 3:21 PM:

Sandra, you can believe in intelligent design all you wish. Fine then, the Canaanites, inheriting the curse of Ham forever (the Talmud makes it perfectly clear what that curse was), the curse persisting even after the 10th generation, were known in the O.T. as stupid, violent and said to be that way always and said to never be able to change. It would seem to me that the Hebrews who wrote that knew exactly what we know today about the same people, 13% of the population, committing 90% of the crime. That is due not just to low IQ, but to much higher (measurable) testosterone levels.

And by the way, their prognathism is what precludes higher intelligence and what makes the brain sutures close much earlier. They keep up when they are children, even seem ahead, but when they get to be about 12 of 13 years of age, they retrogress. The jaw is now known to be intricately connected to the brain. Prognathism developed in them over the last 200,000 years. But in terms of intelligent design, I would say that this prognathism is part of the "mark" that God put on the entire race, as a mark that they are cursed. Hebrews and Israelites were FORBIDDEN to mate with them.

It's in the O.T. Extreme segregationism is in the O.T.

Manj said at July 30, 2007 2:45 AM:

People with larger heads have larger brains. University study makes you more intelligent and therefore makes your head grow. How would you test this hypothesis?

Bernie D. said at April 13, 2008 6:40 PM:

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; ).

Referenced Literature:

Allen B.P. (2006). If No “Races,” No Relevance to Brain Size, and No Consensus on Intelligence, Then No Scientific Meaning to Relationships Among These Notions: Reply to Rushton11. General Psychologist, Summer, 2003 Volume 38:2 Pages 31-32.

Bernstein L, Ross RK, Judd H, et al (1986). Serum testosterone levels in young black and white men. J Natl Cancer Inst 76:45—48, 1986

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Liviana said at November 3, 2008 8:57 PM:

I had a longer childhood compare to my relatives and had higher test scores, I always figured this idea....

Clyde said at December 25, 2010 7:40 PM:


but perhaps as a smaller child you studied more in school because that was your niche? bigger kids always had the advantage in sports, and that's their niche. sorry non-native english speaker

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