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Do Physiologists Study General Intelligence?

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The general factor of intelligence (g) is said to be physiological. Jensen (1998: xii) states that “Students in all branches of the behavioral and social sciences, as well as students of human biology and evolution, need to grasp the essential psychometric meaning of g, its basis in genetics and brain physiology, and its broad social significance.” There are, furthermore, “a number of suggestive neurological correlates of g, but as yet these have not been integrated into a coherent neurophysiological theory of g” (Jensen, 1998: 257). I personally don’t care for correlations too much anymore, I’m interested in actual causes. Jensen (1998: 578) also states “Although correlated with g [size of the brain, metabolic rate, nerve conduction velocity, and latency and amplitude of evoked electrical potentials], these physiological variables have not yet provided an integrated explanatory theory.”

This seems suspiciously like Dreary’s (2001: 14) statement that there “is no such thing as a theory of human intelligence differences – not in the way that grown-up sciences like physics or chemistry have theories.” If is physiological, then where is the explanatory theory? On that same matter, where is the explanatory theory for individual intelligence differences? That’s one thing that needs to be explained, in my opinion. I could muster something up off the top of my head, such as individual differences in glucose metabolism in the brain, comparing both high and low IQ people (Cochran et al, 2006; Jensen, 1998: 137), however, that is still not good enough.

In physiology there is sliding filament theory which explains the mechanism of muscle contraction (Cooke, 2004). Why is there no such theory of why individuals differ in intelligence and why have these “suggestive neurological correlates of g” not been formulated into a coherent neurophysiological theory? There are numerous theories in physiology, but a theory of g or why individuals differ in intelligence is not one of them.

It’s like Darwin only saying “Species change“, and that’s it; no theory of how or why. He’s just stating something obvious. Similarly, saying “Person A is smarter or has a higher IQ than person B” is just an observation; there is no theory of how or why for why individuals differ in intelligence. There are theories for group differences (garbage cold winter theory), but no individual differences in intelligence? Hmmm… Sure it’d be a ‘fact that species change over time’, but without a theory of how or why, how useful is that observation? Similarly, it is true that some people are more intelligent than others (score higher on IQ tests), yet there is no explanatory theory as to why? I believe this ties back to the physiological basis for g: are physiologists studying it, and if not, why?

Reaction time (RT) is one of the most talked about physiological correlates in regards to IQ. However, as a fitness professional, I know that exercise can increase reaction time, especially in those with intellectual disabilities (Yildirim et al, 2001). I am now rethinking the correlate between reaction time and IQ, since it can be trained in children, especially those with intellectual disabilities. Clearly, RT can be trained by exercise, participating in sports, and even by playing video games (Green, 2008). So since RT can be trained, I don’t think it’s a good physiological measure for g.

Individuals do differ in individual physiology, however, I have never heard of a physiologist attempting to rank individuals on different traits, nevermind attempting to say that a higher level of one variable is better than a lower variable, say blood pressure or metabolic rate. In fact, individuals with high blood pressure and metabolic rates would need immediate medical attention.

There are also wide variations in how immune systems act when faced with pathogens, bacteria and viruses. Though, “no one dreams of ranking individual differences on a general scale of immunocompetence” (Richardson, 2017: 166). So if is physiological then why don’t other physiological traits get placed on a rank order, with physiologists praising certain physiological functions as “better”?

Richardson (2017: 166-167) writes:

In sum, no physiologist would suggest the following:

(a) that within the normal range of physiological differences, a higher level is better than any others (as is supposed in the construction of IQ tests);

(b) that there is a general index or “quotient” (a la IQ) that could meaningfully describe levels of physiological sufficiency or ability and individual differences in it;

(c) that “normal” variation is associated with genetic variation (except in rare deleterious conditions; and

(d) the genetic causation of such variation can be meaningfully separated from the environmental causes of the variation.

A preoccupation with ranking variations, assuming normal distributions, and estimating their heritabilities simply does not figure in the field of physiology in the way that it does in the field of human intelligence. This is in stark contrast with the intensity of the nature-nurture debate in the human cognitive domain. But perhaps ideology has not infiltrated the subject of physiology as much as it has that of human intelligence.

This is all true. I know of no physiologist who would suggest such a thing. So does it make sense to compare with physiological variables—even when classic physiological variables do not have some kind of rank order? Heritabilities for BMR are between .4 and .8, which is in the same range as the heritability of IQ. Can you imagine any physiologist on earth suggesting a rank order for physiological traits such as BMR or stroke volume? I can’t, and if you knew anything about physiological variables then you wouldn’t either.

In sum, I believe that conflating with physiology is erroneous; mostly because physiologists don’t rank physiological traits in the same ways that human intelligence researchers do. Our physiology is intelligent in and of itself, and this process begins in the cell—the intelligent cell. Our physiological systems are intelligent—in our bodies are dynamic systems that keenly respond to whatever is going on in the environment (think of how the body always attempts to maintain homeostasis). Physiology deals with the study of living organisms—more to the point, how the systems that run the organisms work.

Looking at physiological variables and attempting to detangle environmental and genetic effects is a daunting task—especially the way our physiological systems run (responding to cues from the environment, attempting to maintain homeostasis). So if general intelligence—g—had a true biological underpinning in the body, and if physiologists did study it, then they would not have a rank ordering for like psychologists do; it’d just be another human trait to study.

So the answer to the question “Do physiologists study g?” is no, and if they did they would not have the variable on a rank order because physiologists don’t study traits in that manner—if a true biological underpinning for exists. Physiology is an intelligent and dynamic system in and of itself, and the process begins in the intelligent cell, except it is on a larger scale, with numerous physiological variables working in concert, constantly attempting to stay in homeostasis.



  1. You really make it appear so easy together with your presentation but I find this matter to be actually one thing that I feel I would by no means understand.
    It sort of feels too complex and very extensive for me.
    I am having a look forward for your subsequent submit, I’ll attempt to get the grasp of it!


    • RaceRealist says:

      Physiology is in no way, shape, or form simple. You need to know how physiological systems (i.e., intelligent systems) interact, especially the human body’s physiology.

      I have another post in mind. I saw Jensen say:

      Jensen: I hypothesize that the brain contains no module for general problem solving. Correlations between individuals’ performances in various cognitive tasks result from quantitative individual differences in physiological conditions that do not constitute the brain’s modular and other neural design features but do influence their speed and efficiency of information processing.

      The g factor: psychometrics and biology.

      Oh really? Individual differences in physiological conditions? As noted above, there is a wide variation in human physiology and all off that falls within the “normal” range of variation. General intelligence, on the other hand, if physiologists were to study it, wouldn’t be put into a rank order because physiologists don’t rank physiological traits in an order. Can you imagine a physiologist saying that person A has a higher, say, stroke volume and that’s better than a normal or lower value? I can’t, because that’s not how physiology works.

      This is why general intelligence is not physiological.


  2. Malcer says:

    I’m failing to see the point of this post. Are we denying there’s basis for intelligence gaps among humans in biology? Have you looked into Neuroscience using work by researchers like Stuart J. Ritchie?


    • RaceRealist says:

      I’m failing to see the point of this post.

      Psychologists say that g is physiological. Physiologists don’t study g as far as I know, correct me if I’m wrong. If physiologists did study g then they wouldn’t put it in a rank order since physiological traits are not ranked in such a way. I’ve never heard a physiologist praise a a higher value in a certain physiological traits as better than a normal or lower value.

      My example in the article—heritability of BMR being between .4 and .8—is apt. That’s in the range of the heritability of IQ. Would it be weird to you if a physiologist said that someone having a higher BMR is better than one who has a BMR in the normal range or below it?

      Are we denying there’s basis for intelligence gaps among humans in biology?

      I’m saying that general intelligence isn’t studied by physiologists and if it were it wouldn’t be put on a rank order, that’s something psychologists do, physiological traits aren’t ordered like that. That’s the point of the article. I also have an in-use anatomy and physiology textbook, the the g factor is not brought up once. I’m talking about this in terms of physiology, not psychology.

      Have you looked into Neuroscience using work by researchers like Stuart J. Ritchie?

      I keep up with Herculano-Houzel’s work. I just ordered the book Intelligence by Ritchie. But he’s a psychologist, not a physiologist. The point of the article is that if g is physiological, then physiologists wouldn’t put it in a rank order. Do you believe the argument is wrong?


    • Malcer says:

      Richardson denies education problems have to do with biology. This is classic Left speak for the underperformance of victim groups like Negroes even after decades of pandering to tbem.

      His talk of ideology infesting research on intelligence is projection.


    • RaceRealist says:

      Richardson denies education problems have to do with biology. This is classic Left speak for the underperformance of victim groups like Negroes even after decades of pandering to tbem.

      I don’t care about his views on education. At the moment, I’m talking about physiology and general intelligence. When I read about his views on education we can discuss that (I can discuss views that I don’t hold to see how arguments hold up, can you?). I buy books to read them for the information in it. I don’t care about someone’s politics or ideology. I care about if something makes logical sense and if something is backed by solid data. What he says about physiology and intelligence and how physiologists would treat general intelligence if they were to study it is true. Let’s discuss that before discussing other things. We can discuss his views on education after this. What can you tell me about his views on education?

      His talk of ideology infesting research on intelligence is projection.

      That’s nice. Everyone has an idealogical bias. I don’t care about that. I care about data and logical arguments. I buy books for new information and to learn new things. As I said, I don’t agree with everything Richardson writes. I take something away from everything that I read. And in regards to general intelligence and physiology, Richardson is on the money. We can discuss that before we discuss his views on education.


  3. Malcer says:

    You’re using a book made by a transparent Leftist.


    • RaceRealist says:

      … Literally meaningless. Do physiologists study g? If they did would there be a rank order? Yes or no. I’m talking about general intelligence and physiology, which is pretty clear. My in-use anatomy and physiology textbook by Kenneth Saladin doesn’t talk about g, is he a leftist? Saying that someone is a leftist or rightest is literally irrelevant. What matters is what they write.

      The book is pretty good. You shouldn’t judge books by their covers. You should buy it, read it and get back to me. I don’t agree with everything that Richardson writes, but I have a good understanding of physiology and what he writes there in regards to intelligent cells and intelligent physiology is on the money.


    • RaceRealist says:

      My point here is that psychologists should stick to psychology and not try to edge into physiology, when what they’re saying doesn’t make sense in a physiological context. Is a higher BMR better than a lower or normal one? BMR has the same heritability range—.4 to .8—as IQ, is a lower or higher BMR better or worse than the other? Think about that before you respond and know that I’m talking in the context of physiology.


  4. meLo says:

    “So since RT can be trained, I don’t think it’s a good physiological measure for g.”

    Can you explain to me why plasticity is a sign of incompatibility with said hypothesis? Or am I misunderstanding something?


    • RaceRealist says:

      Numerous of factors affect RT.

      I wonder if some of these variables were controlled for in RT/IQ studies.

      Since RT can be trained (Dye, Green, and Bavelier, 2010), it’s not a good physiological variable in the first place. There’s also the fact that detangling genetic/environmental effects when it comes to physiological variables is pretty tough. Further, if physiologists did study it, they wouldn’t suggest that genetic causation can be separated from environmental causation.

      See above to those factors that affect RT (and there are still more) and see why a physiologist (if they did study general intelligence) wouldn’t 1) rank the variable and 2) why RT is not a good physiological measure for general intelligence due to said confounding variables.

      Read this review on RT and intelligence:

      Relationship between Intelligence and Reaction Time; A Review Study

      More study needed; RT and IQ “is too complicated and revealing a significant correlation depends on various variables (e.g. methodology, data analysis, instrument etc.)” (Khodadadi et al, 2014).

      So since RT can be trained with numerous tasks, especially at a young age, and numerous variables affect RT, then I don’t think it’s a good physiological variable for IQ/intelligence. I’m going to tackle this much more in depth soon.

      Can you see physiologists ranking general intelligence if they studied it?


    • John Connor says:

      “Can you see physiologist ranking general intelligence if they studied it?”

      If higher “g” is better than lower “g” (which it is), than yes, clearly.


    • RaceRealist says:

      Whatever that is.


  5. […] production is a real, measurable physiologic process, as is the hormone itself; which is not unlike the so-called physiologic process that ‘g’ is supposed to be, which does not …, which is covered with unscientific metaphors like ‘power’ and ‘energy’ and […]


  6. […] This, is in my view, one reason why RT should be tossed out as a ‘predictor of g‘ (whatever that is), as it is not a reliable measure and does not ‘test’ what it is purported to […]


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