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Biological Correlates of IQ

  1. Dec 25, 2004 #1
    Behavioral Genetics in the Postgenomic Era, published by the American Psychological Association, 2003. Editors: Plomin, Defries, Graig & McGuffin

    "Willerman, Schultz, Rutledge, and Bigler correlated brain size as measured through MRI within a sample of 40 unrelated participants. They found a correlation of 0.51, which was higher in men (0.65) than in women (0.35). In a follow-up study, Willerman, Schultz, Rutledge and Bigler suggested that, in men, a relatively larger left hemisphere better predicted verbal IQ than it predicted performance IQ, whereas in women the opposite was true. Since then, several studies have provided confirmative evidence that brain volume and IQ correlate around 0.40.

    "In a large MRI study including 111 twin pairs and 34 additional siblings, the heritability of volumes of several brain structures was investigated. Heritability estimates for intracranial volume, total brain volume, gray-matter volume, white-matter volume, and cerebellar volume were all between 80% and 90%. Genetic intercorrelations between these measures were all very high, indicating that a largely overlapping set of genes is responsible for individual differences in each of these measures…. This indicates that about half of the genetic influences on either cerebral brain volume or IQ is due to genetic factors influencing both. Put differently, 80% of the phenotypic [observed human differences] correlation is explained by genetic mediation."

    "This suggests that genes important for brain size, reaction times, inspection time, and theta coherence may also be important for intelligence, which fits very well in the myelination hypothesis as formulated by Miller. According to this hypothesis, generally, the relation between speed and intelligence can be explained if part of the interindividual variance in intelligence can be ascribed to interindividual variance in the degree of myelination of cortico-cortical connections. If true, this could explain why more intelligent brains show faster nerve conduction, faster reaction times, and faster inspection times. And, all other things equal, thicker myelin sheaths will result in larger brain volume, thus explaining the positive relation between brain size and IQ."

    -----------------------------------------------

    The Scientific Study of General Intelligence: Tribute to Arthur R. Jensen. Edited by Helmuth Nyborg, 2003

    "In Chapter 2, Britt Anderson takes a closer look at the brain size-g relationship. He first discusses the low and variable relationships found in earlier studies using such rough measures as external head circumference, and then presents the outcome of studies using more exacting in vivo neuro-imaging techniques. The overall conclusion is that anatomical and metabolic imaging techniques using magnetic resonance technology suggest a correlation in the order of 0.35 between brain size and IQ, a finding that is consistent across multiple experimental groups. Another important conclusion is that the majority of individual variation in intelligence is not explained by variation in brain volume. A third conclusion is, that we still do not know whether specific brain regions or compartments are the principal basis for the size-IQ correlation, and this sets the stage for further experiments exploiting the many new capabilities of magnetic resonance imaging and other brain image techniques."

    He also notes that others have concluded that, "between 80~90 percent of brain volume is heritable, and that the genetic correlation between brain volume and IQ is 0.48."

    "The causes of genetically based racial differences in intelligence should be sought in their evolutionary history. Differences in IQ must have developed together with differences in skin color, morphology and resistance to diseases as adaptations to the environments in which the races evolved. We can reconstruct the broad outline of how this occurred. Modern humans evolved in Central East Africa about a quarter of a million years ago. Their brain size was the same as that of living blacks and it can be assumed that their intelligence was the same, represented by an IQ of 69."

    "The morphological basis of the increase in intelligence in the Caucasoids and Mongoloids was an enlargement of brain size the evidence for which is set out by Rushton."

    "Myopia (near-sightedness) is positively correlated with IQ. The relationship appears to be pleiotropic, that is, a gene affecting one of the traits also has some effect on the other. Further, there are significant racial and ethnic differences in the frequency of myopia, with the highest rates found in East Asians, the lowest rates among Africans and Europeans intermediate. Among Europeans, Jews have the highest rate of myopia, about twice that of gentiles and about on a par with that of Asians. Miller suggested that myopia is caused by extra myelinization in the eye and is similarly linked to brain size."

    "Ulric Neisser, Chair of the recent American Psychological Association's Task Force Report on The Bell Curve, acknowledged that, with respect to 'racial differences in the mean measured sizes of skulls and brains (with East Asians having the largest, followed by Whites and then Blacks) ... there is indeed a small overall trend.'"
     
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  3. Jan 1, 2005 #2

    loseyourname

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    Higher myelination is one factor that can increase the speed of computation, although it would have a far greater effect on the speed with which one expresses a computation outwardly. This is because myelination is more relevant to nerve conductance outside of the brain than inside of the brain, simply because the intercellular environment within the brain is already adapted to be conducive to nervous activity and does not require as much insulation for the wiring. Actual computational ability should be effected very little and should mostly be effected by the dendritic volume in proportion to the axonal volume. Other factors important to both computational speed and ability would include the integrity of cytoskeletal structures in the neurites, performance of axonal receptors. This is, of course, assuming that all holistic factors are equal (i.e. there are no lesions or functional deficiencies in any particular portion of the brain).

    In assessing the importance of myelination to intelligence, I suggest we look to demyelinating diseases, the most common of which is MS. MS patients display marked motor dysfunction and serious cognitive deficits. Their ability to recognize patterns, retrieve memory, and such, can be severely affected. A decrease in intellect, however, is almost never seen. This article (the first I found, but it isn't much, admittedly) explains that the speed of information-processing is greatly impaired, and so a lowering of performance on IQ tests does result. However, if we check simply for accuracy and not speed, we find that there is no impairment. Thus, we can see how less myelin, and so less brain mass, can result in a lower IQ and give the illusion that the subject is less intelligent, but, in fact, computational accuracy is not affected. (The fact that someone is "slow" doesn't mean he's stupid.) In fact, this might even suggest that, to be a more accurate reflection of real intelligence, IQ tests should be adjusted for brain size.
     
  4. Jan 1, 2005 #3
    myelination

    The hypothesis that was referenced bases a significant part of its explanation on the role played by myelin in reducing the impairing role of neural noise in cognitive activities. The findings that RT as measured by ECTs correlates negatively with intelligence can be interpreted with respect to improved efficiency of the WM, since WM is known to be highly volatile. But it has been soundly demonstrated that the variance in RT measures correlates independently and negatively with intelligence. The latter correlation can be explained by the neural noise model. I have not seen any other explanation for it.
    What definition are you using for "actual computational ability?" Do you wish to imply that _g_ is computational ability? Maybe it is. I am not arguing, but there are some very interesting aspects of _g_ which do not appear to relate to computations, unless one has a carefully crafted definition. What is that definition?

    This reference says otherwise:
    Medaer, R., de Smedt, L, Swerts, M., & Geutjens, J. (1984) Use of rating scales to reflect cognitive and mental functioning in multiple sclerosis. Acta Neurologica Scandinavica, 70 (Supp. 101), 65-67.

    Although the sample size was small, they observed that 64% of the MS patients suffered IQ losses of over 1 standard deviation. Some were in excess of 2 standard deviations.

    You provided a URL link that is locked. That is not helpful.

    However, if we check simply for accuracy and not speed, we find that there is no impairment. Thus, we can see how less myelin, and so less brain mass, can result in a lower IQ and give the illusion that the subject is less intelligent, but, in fact, computational accuracy is not affected.

    As Miller has pointed out, there are various aspects of myelination that point to it playing a role in explaining the variance in intelligence. Neural noise is one. The T2 relaxation time is another:
    The only reason for using IQ tests at all is that they have useful external validity. Do you have any evidence that adjusting IQ test scores for brain size would increase the external validity (academic performance, job performance, learning rate, etc.)? MI would expect that such adjustments would degrade the external validity. We already know that about 16% of the variance in intelligence is explained by brain volume.
     
  5. Jan 1, 2005 #4

    loseyourname

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    I'm just referring to the accuracy of computations. I'm not entirely certain why neural noise would be all that relevant to this. I know that demyelination results in a loss of attention-span, but that still doesn't necessarily affect computational accuracy. From what you've told me of _g_, it's a measurement of overall cognitive ability, not intelligence. There are many cognitive abilities that are seriously affected by demyelination, such as memory-retrieval, attention-span (as mentioned above), and spatiovisual pattern recognition.

    My URL said the same thing. I don't know why it is locked. But notice that it only says tested IQ is lower. All of the literature I have read indicates that it is only computational speed and spatiovisual pattern recognition that are affected. Both of these are measured by IQ tests, which explains the lower scores, but are not reflective of computational accuracy or ability. Let me explain a little more deeply what I mean by "ability." A person with less myelin, but more computational ability (say, higher dendritic volume in proportion to axonal volume) would be able to perform more difficult and complex computations than a person with more myelin but less dendritic volume, but would not perform normal computations with anywhere near the same speed. They would underperform on IQ tests, but still be able to perform computations that the person of higher IQ would not be able to make.

    This is all theoretical, of course. There exists no way to even test for dendritic volume or axonal volume (at least while the subject is alive), but this can be figured simply by making an analogy to computational machines. The more circuits, the more computational ability. The more conductance, the more speed.

    This is still talking about speed. As Miller says, the ions in the cytoplasm cannot return to their rest states as quickly. This is just another way of saying that conductance is less (ions carry the current down the axon).

    You know that 16% of the variance in IQ is explained by brain volume. I'm suggesting that IQ is not a measure purely of computational capacity because it also factors in the speed of computation. Heck, any test that must be completed in one sitting does, which is why tests in general aren't necessarily an accurate reflection of ability separate from speed. If all you're looking for in a test is external validity, I'm sure you're right to say that the current IQ tests are pretty good for that. I'd say that's because performance markets (such as the job market or school grades) rely as much on the ability to multi-task (which requires attention-span) and speed as they do on accuracy. Even in the academic market, tenure is rewarded more on the frequency of publication than the quality of what is published. Don't you watch the commercials, Mandrake? This is an on-demand world.

    In short, though, I do think current IQ tests are pretty good at testing for what they test for, including job performance, school performance, and other external indicators. IQ tests are as good a correlate to success in these matters as anything else.
     
  6. Jan 2, 2005 #5
    Speed necessary for working memory capacity vs speed of task completion

    g is not defined as a measurement. g is defined as a latent variable, or a factor:

    • Factor. The word "factor" has a number of dictionary definitions, but the term as used here has a very restricted, specialized meaning. A factor is a hypothetical variable that "underlies" an observed or measured variable. Thus a factor is also referred to as a latent variable. It is best thought of initially in terms of the mathematical operations by which we identify and measure it.

      Although a factor is identifiable and quantifiable, it is not directly observable. It is not a tangible "thing" or an observable event. So we have to be especially careful in talking about factors, lest someone think we believe that we are talking about "things" rather than hypothetical and mathematical constructs. But one can say the very same thing about the many constructs used in the physical sciences (gravitation, magnetism, heat, valence, and potential energy, to name a few). They are all constructs.
    (Arthur Jensen.The g Factor. p55.)



    Intelligence has not been objectively defined here. The word is off-topic for this discussion.



    Some of those might be cognitive abilities. Attention span does not seem to qualify as an ability. This is how ability is defined in differential psychology:

    • Item Performance (IP). This term, henceforth abbreviated IP, refers to any distinct voluntary behavioral act. It can be any such overt action provided it is also observable or recordable in some way. Saying (or writing) "four" (or any other response) to the question "what is two plus two?", if the response can be agreed upon by observers, is an IP. Doing a triple axel, writing one's name, hitting middle C on the piano, performing the Tchaikovsky violin concerto, solving (or failing) an attempted math problem, jumping over a two-foot hurdle, cutting an apple in half, hitting a baseball into left field, and parking parallel-these are all IPs. An IP may be a discretely classifiable act (e.g., either hitting or missing a target) or some action that can be graded on a continuum (e.g., speed of response to a signal, time taken to complete a task, distance run in a given time). The universe of possible IPs is obviously unlimited. The definition of IP also includes, of course, a voluntary response to an item in any kind of test or to any laboratory procedure that measures, for example, reaction time, sensory threshold, speed of rote learning, or memory span.

      Excluded from the category of IPs are unconscious, involuntary, or accidental acts, such as tripping on a stair, eye blinks, facial tics, unconditioned and conditioned reflexes, reactions of the autonomic nervous system, somnambulistic actions, drug reactions per se, fainting, and the like. Organismic events that are not strictly behavioral acts are also excluded, such as changes in brain waves, glandular secretions, pulse rate, blood pressure, skin conductance, and pupillary dilation, although these phenomena may be correlated with certain IPs. Most importantly, it should be noted that an IP is not an inference, or an abstraction, or an interpretation. It is a single objective raw datum--some overt act, directly observable by other persons or immediately recordable by an apparatus.

      Ability. Going from an IP to an ability is going from a direct observation to an abstraction or inference, although of the lowest order. The universe of abilities is open-ended but bounded by certain qualifications.

      An ability is an IP that meets the following three criteria:
      • (1) it has some specified degree of temporal stability (consistency or repeatability); (2) it can be reliably classified, measured, ranked, rated, graded, or scored in terms of meeting some objective standard of proficiency; and (3) it has some specified degree of generality.
    (Arthur Jensen.The g Factor. pp50-51.)



    There is no speed-of-task-completion factor in g. As far as a given IQ test factors in performance speed, it does not test g as well as it could otherwise. This is why Jensen administers Raven's Matrices untimed. Need for neural conduction speed in untimed mental performance tasks is explained by working-memory (WM) theory (RT stands for reaction time; ECT stands for elementary cognitive task; LTM stands for long term memory):

    • The Importance of Processing Speed in Working Memory. RT on the kinds of ECTs that involve the functions that cognitive theorists ascribe to WM are always g loaded, usually more so than tasks that make less demand on WM. Hence an important part of the connection between RT on ECTs and performance on untimed psychometric tests in which item responses are scored as correct or incorrect seems to center on the functions ascribed to WM. The connection can be accounted for theoretically in terms of individual differences in the processing capacity of WM, which is a function of the speed of its information-processing operations, in addition to another variable to be discussed shortly.

      Two properties of WM make speed of operation crucial: (1) its limited capacity (i.e., the amount of information WM can hold at one time), and (2) the short duration of information in WM (i.e., the rapid rate of loss of information in WM). The operations performed on incoming information by WM must occur before the information is lost. Otherwise it has to be entered again. Hence we need to jot down long phone numbers and work most arithmetic problems with paper and pencil. If the amount of newly input information exceeds the capacity of WM, it has to be transferred to LTM in successive stages, so all of it can be retrieved in WM for answering the question or solving the problem that was posed. The transfer from WM to LTM itself occupies some of the WM capacity, so there is a necessary trade-off between processing and storage of incoming information.

      If the amount of information that must be processed overloads the WM's capacity for performing these functions, there is a breakdown of processing and some essential information is lost. Incomplete processing of the incoming information results in erroneous output, or response. Speedier processing, therefore, is advantageous because more information can be processed or stored before it decays beyond retrieval.
    (Arthur Jensen.The g Factor. p253.)
     
  7. Jan 3, 2005 #6
    Neural noise presumably causes the brain to work harder to perform any given function, as is demonstrated by observing glucose uptake. As problem complexity increases, the brain has to call upon more resources, thereby increasing its neural noise load. When retransmissions are required, Miller suggests that the effects of noise increase due to cascading. This leads to a point where the brain cannot proceed because it is overwhelmed by its own attempts to function.

    I enjoyed a visit of about two hours with Ed Miller about a month ago. I asked him if there had been any new developments, concerning myelination. He said that a medical researcher had contacted him and claimed to have findings that support Ed's model, but so far, Ed has not examined the material.

    Are you just guessing, or do you know this from studies? If the latter, what computations were used? If the former, how do you explain the large IQ declines that have been reported (see my prior reference) for MS patients (in which demyelination is only partial)?

    Most psychometricians consider _g_ to be the best representation of intelligence that has ever been devised. It is a common factor for all cognitive abilities.

    Of course. IQ is defined by tests.

    How much literature have you examined? Have you read about this for months? Years? Or, did you do a quick Google search? I just now checked Google to see what would be listed. This site:
    http://www.albany.net/~tjc/cog-dys2.html
    Lists a lot of papers that relate to cognitive ability and MS. One paper from the site:
    http://www.albany.net/~tjc/cog-dys2.html#6
    Seems to confirm my observation. If this interests you, the links are there for you to read.

    It strikes me as a wild guess and one that is most likely incorrect. It is my understanding that we already know that demyelination causes significant cognitive impairment and is measured as low IQ.

    I disagree. We know that timed tests are degraded when the time allowance is reduced to the point where the testee is rushed. That is necessarily going to happen, since the time limit places a premium on specific knowledge. For that reason, some tests are not timed (the Raven's is usually not) and some tests that typically specify a time limit are given without time limits by researchers who are concerned about the time effect. As long as the testee is not rushed, reductions in the time limit do not alter the g-loading.

    Your comment seems to imply that this has not been studied and that we must guess. That is not true. It has been studied and we know that an "adequate" time limit does not alter the g-loading, nor does it introduce errors.

    As Miller has pointed out, when problem complexity is increased, a point will be reached where the testee cannot proceed, even given unlimited time. There are two reasons that presumably explain this: the WM capacity has been exceeded by the problem and the brain reaches a point where errors cascade, causing the brain to effectively hit a wall.

    What other merits should a test have? The external validity is presumably already going to reflect the test error and the reliability coefficient.

    One of the papers I heard last year was the analysis of very bright people, as filtered by the SAT taken at age 12. The study was longitudinal (presenter: David Lubinski, title: Tracking exceptional human capital). It showed that those very bright people who entered academics were awarded tenure considerably sooner than the other intelligent, but less brilliant teachers. Maybe this was only because they published more; the study didn't say. Perhaps more publishing was due to more results to publish? Whatever the cause, the age 12 test was a robust predictor of superior adult success.

    As a matter of fact, I do not. When I do watch TV (not often), I either use the remote to mute ads, or I record the material and skip the ads on playback.
     
  8. Jan 4, 2005 #7

    loseyourname

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    That's true. I probably should not have even posted in here given that fact. I obivously use it to mean "computational capacity" and Darwin isn't using it the same way. I just realized he's only claiming correlates of IQ, not intelligence. That's fine.

    That's all right. I'm not a psychologist, and neither are the people that study neurological disorders. Every discipline has its own definitions.

    That's fine, but this has nothing to do with Jensen, nor does it have anything to do with g. It is suggested by neurological studies that the lower IQ resulting from demyelinating diseases is a lowering of attention-span and computational speed, not of computational capacity. Problem-solving abilities are not much effected. Since I now realize that Darwin probably wasn't claiming otherwise to begin with, the point is likely irrelevant.
     
  9. Jan 4, 2005 #8

    loseyourname

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    I'll explain it the same way your links do below.

    I've studied the functions of glial cells for about a year, but only as a student and strictly through literature. MS comes up every now and then.

    Well, this seems to confirm what I've said as well. Severe underperformance is seen when structural atrophy and axonal degeneration starts to kick in, not from myelin loss alone.

    It's not a wild guess. It's the function of glial cells to insulate the neuronal circuitry and increase the speed of computation. That much is know. Read the links you provided more carefully. Most of the cognitive dysfunction is correlated with noticeable enlargement of ventricular volume and lesioning to particular parts of the brain, brain stem, or spinal cord. The overall myelin lost might not be great, but if enough is lost in one place to cause lesioning, then of course it will affect functioning!

    None. I have no problems with the tests. I just don't think IQ is the same thing as intelligence, but everyone is going to have an opinion on that and they won't really mean a whole lot because, as hits points out, "intelligence" is pretty poorly defined.

    Okay, this is way off-topic, but do you have any idea why people at age 12 take the SAT? Me and one other student from my middle school both were bussed down to UCI to take the SAT in sixth grade and I can't for the life of me remember why this was done or what the scores were even used for.
     
  10. Jan 4, 2005 #9
    More myelin usually means faster NCV, which in turn means higher intelligence. Among the mechanisms that may cause that gain are the reduction in neural noise and increased efficiency of brain transmissions (explaining why more intelligent brains consume less energy). Faster NCV is presumed to cause a more efficient use of WM.

    What definition of intelligence do you think is most precise and acceptable to your personal preference?

    Yes, I know exactly why people at age 12 take the SAT.

    Perhaps they were impressed with your English skills and wanted to evaluate them?

    Okay, the reason some students take the SAT at age 12 is that the talent search program (started at and administered by Johns Hopkins University) uses the SAT as a vehicle for identifying students who are smart enough to be asked to participate in their summer programs. At one time this operation was called CTY (Center for Advancement of Academically Talented Youth). Several sites around the country accepted students for summer advancement or enrichment work. Some were dedicated to science and some to humanities (people who use proper sentence structure). Students who scored at about the 99.5th percentile were invited. Of those, the upper 1/3 were given a special award for their high scores. Within the science program, the advancement classes were devoted to such subjects as chemistry, physics, and math. Typically, the students could complete one year of work in three weeks, but exceptional math students often did multiples of that. Enrichment courses were those that were not available in most public schools and included such courses as archaeology, astronomy, marine ecology, geology, and paleobiology. Humanities included etymologies, logic, Latin, Ancient Greek, writing, history, poetics, history, etc.

    I was impressed that the Asian students took only the advancement courses and the Causcasian students took mostly the enrichment courses. The split was almost precisely along those lines. The participants were almost all excited about their summers and were eager to return the next year.
     
  11. Jan 4, 2005 #10
    Last edited: Jan 4, 2005
  12. Jan 5, 2005 #11
    http://socserv.mcmaster.ca/anthro/course.und/3L/105-1_gravleeetal.pdf
     
    Last edited: Jan 5, 2005
  13. Jan 5, 2005 #12
    http://www.psychology.adelaide.edu.au/members/staff/nickburns/O_B1.pdf.
     
  14. Jan 5, 2005 #13
    Chris Brand, Clarence Gravlee, Franz Boas and the case of the malleable heads

    Chris Brand discusses the Boas case, including the above-linked 2003 reanalysis by Gravlee et al., here:
    http://theoccidentalquarterly.com/vol3no2/cb-boasa.html

    • In March, the AAA published an article by Gravlee et al. [2003] defending Boas’s claims; but this article did not discuss the analysis by Sparks and Jantz and its own defense used data published by Boas in 1928, by which time—as was acknowledged—some 4,000 of the original subjects had mysteriously gone missing. Gravlee et al. say, in what is surely no overstatement: "It would be a worthwhile project for future researchers to explain this discrepancy and locate the missing data...."

      Of course, modern science does not depend on scholastic arguments and counterarguments. In science, the blunt way is replication, replication, replication.

    Apparently, Boas's head-malleability findings still have yet to be replicated.
     
  15. Jan 5, 2005 #14
    Regarding the "The Occidental Quarterly",
    http://www.gnxp.com/MT2/archives/003003.html

    Instead looking at peer-reviewed articles:
    http://lance.qualquant.net/gravlee03b.pdf
     
  16. Jan 5, 2005 #15
    http://www3.interscience.wiley.com/cgi-bin/abstract/71007970/ABSTRACT
     
  17. Jan 5, 2005 #16
    http://www.ncbi.nlm.nih.gov/entrez/...ve&db=PubMed&list_uids=11451056&dopt=Abstract
     
  18. Jan 5, 2005 #17
    "...Modern humans evolved in Central East Africa about a quarter of a million years ago. Their brain size was the same as that of living blacks and it can be assumed that their intelligence was the same, represented by an IQ of 69."
    I don't care to assume that, this is too opionated and covertly suggestive for me but then opionions sell more books, and most things in this world that appear to be correlated don't really have a directed cause and effect to one another, that is there may be a wide variety of symptoms that we detect one and see it correlated to a one of a wide variety of effects and falsely assume that the connection is of great importance. What difference does it make if people are different? If we all have the same opportunity and laws that should be "equality", but the truth is people are people they are going to form groups and fight and bicker and envy others and try to get what best for their group and basically we need to evolve on the individual level first before and the macro level will fix itself as a consequence. Hijack the thread anyone?
     
  19. Jan 5, 2005 #18
  20. Jan 6, 2005 #19
    Chris Band did not publish in a peer-reviewed journal and did not mention the other studies mentioned above, which clearly showed gains in cranial size during the last century.

    As has been shown in this thread, inspection time cannot be used as an argument that the Flynn effect does not increase fluid g. And the increase in cranial size between generations during the last century is another evidence that the Flynn effect is real and is increasing intelligence.
     
  21. Jan 6, 2005 #20
    Brain matter for vegetative and sensorimotor functions vs brain matter for cognition

    Since people require brain mass to control sensorimotor functions, one might expect that their intergenerational crania would enlarge as their intergenerational body height increased.

    • (The correlation between body size and brain size in adult humans is between +.20 and +.25.) Large parts of the brain do not subserve cognitive processes, but govern sensory and motor functions, emotions, and autonomic regulation of physiological activity. Controlling body size removes to some extent the sensorimotor aspects of brain size from the correlation of overall brain size with IQ.
    (Arthur Jensen. The g Factor. p147.)


    How large were the reported gains in cranial size? Jensen has reported "an increase of approximately one standard deviation in both adult height and head size over the same period as the secular rise in IQ has taken place." (Ibid., pp325-326.)
     
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