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Thursday, July 5, 2012

The genetics of stupidity


What if we’ve been thinking about the genetics of intelligence from completely the wrong angle?  Intelligence (as indexed by IQ or the general intelligence factor “g”) is clearly highly heritable in humans – people who are more genetically similar are also more similar in this factor. (Genetic variance has been estimated as explaining ~75% of variance in g, depending on age and other factors).  There must therefore be genetic variants in the population that affect intelligence – so far, so good.  But the search for such variants has, at its heart, an implicit assumption: that these variants affect intelligence in a fairly specific way – that they will occur in genes “for intelligence”. 

An implication of that phrase is that mutations in those genes were positively selected for at some stage in humanity’s descent from our common ancestor with apes, on the basis of conferring increased intelligence.  This seems a fairly reasonable leap to make – such genes must exist and, if variation in these genes in humanity’s evolution could affect intelligence, then maybe variation in those same genes can explain variation within the human species. 

The problem with that logic is that we are talking about two very different types of variation.  On the one hand, mutations that arose during human evolution that conferred increased intelligence (through whatever mechanism) will have been positively selected for and fixed in the population.  How this happened is unknown of course, but one can imagine an iterative process, where initial small changes in, say, the timing of processes of brain development led to small increases in intelligence.  Increased cognitive capabilities could have led in turn to the emergence of crude communication and culture, opening up what has been called the “cognitive niche” – creating an environment where further increases in intelligence became selectively more and more advantageous – a runaway process, where genetic changes bootstrap on cultural development in a way that reinforces their own adaptiveness. 

That’s all nice, though admittedly speculative, but those mutations are the ones that we would expect to not vary in human populations – they would now be fixed.  In particular, there is little reason to expect that there would exist new mutations in such genes, present in some but not all humans, which act to further increase intelligence.  This is simply a matter of probabilities: the likelihood of a new mutation in some such gene changing its activity in a way that is advantageous is extremely low, compared to the likelihood of it either having no effect or being deleterious.  There are simply many more ways of screwing something up than of improving it.

That is true for individual proteins and it is true at a higher level, for organismal traits that affect fitness (the genetic components of which have presumably been optimised by millions of years of evolution).  Mutations are much more likely to cause a decrement in such traits than to improve them.  So maybe we’re thinking about the genetics of g from the wrong perspective – maybe we should be looking for mutations that decrease intelligence from some Platonic ideal of a “wild-type” human.  Thinking in this way – about “mutations that affect” a trait, rather than “genes for” the trait – changes our expectations about the type of variation that could be contributing to the heritability of the trait. 

Mutations that lower intelligence could be quite non-specific, diverse and far more idiosyncratic.  The idea of a finite, stable and discrete set of variants that specifically contribute to intelligence levels and that simply get shuffled around in human populations may be a fallacy.  That view is supported by the fact that genome-wide association studies for common variants affecting intelligence have so far come up empty

Various researchers have suggested that g may be simply an index of a general fitness factor – an indirect measure of the mutational load of an organism.  The idea is that, while we all carry hundreds of deleterious mutations, some of us carry more than others, or ones with more severe effects.  These effects in combination can degrade the biological systems of development and physiology in a general way, rendering them less robust and less able to generate our Platonic, ideal phenotype.  In this model, it is not the idea that specific mutations have specific effects on specific traits that matters so much – it is that the overall load cumulatively reduces fitness through effects at the systems level.  This means that the mutations affecting intelligence in one person may be totally different from those affecting it in another – there will be no genes “for intelligence”.

Direct evidence for this kind of effect of mutational load was found recently in a study by Ronald Yeo and colleagues, showing that the overall burden of rare copy number variants (deletions or duplications of segments of chromosomes) negatively predicts intelligence (r = -0.3).

If g really is an index of a general fitness factor, then it should be correlated with other indices of fitness.  This indeed appears to be the case.  G is weakly positively correlated with height, for example, and also strongly correlated with various measures of health and longevity.  In a recent, outstanding review by Ian Deary, the following statistics are cited:

“One standard deviation advantage in intelligence was associated with 24% lower risk of death over a  follow-up range of 17 to 69 years (Calvin et al. 2011 – [meta-analysis]).  … The range of causes of death with which intelligence is significantly associated… include deaths from cardiovascular disease, suicide, homicide, and accidents, but not cancer.”

This correlation can be interpreted in two ways: one, less intelligent people have less healthy and/or riskier lifestyles (i.e., direct causation), or, two, both intelligence and rates of mortality at least partially reflect an underlying factor – general fitness. 

Another good marker of general fitness is developmental stability.  This refers to the robustness of the system and the ability of the genotype to reliably generate a phenotype within the species-specific normal range, despite genetic and environmental perturbations and intrinsic noise or randomness.  It is a property that varies between people. 

One can get a good measure of developmental stability by looking at how symmetric someone is.  The two sides of the body develop independently from the same set of genomic instructions – if a particular genotype is very robust then it should generate a very similar outcome on each side of the body.  If, however, the system is less robust, then the person may be more asymmetric in any number of features (arm lengths, finger widths, earlobe lengths, eye widths, etc.).  This kind of asymmetry is called fluctuating asymmetry as the direction is random – one arm may be longer than the other, but it is equally likely to be the left or right (unlike the asymmetry of internal organs, for example, which is directional and a species-specific trait).    

Fluctuating asymmetry should thus be a good indicator of general fitness and is fairly easy to measure (though it is important to look at multiple features to get an aggregate score in each individual).  It is also a heritable trait – monozygotic twins are more similar to each other in degree of asymmetry than are dizygotic twins.  There is no reason, however, to think this reflects variation in a set of genes whose function it is to make the organism more symmetric, or to make developmental systems more robust.  Rather, mutations in any genes affecting development are likely to not just contribute to some specific phenotype, but also to generally decrease robustness of the system and increase variability. 

You can probably guess what’s coming next – fluctuating asymmetry correlates negatively with various IQ measures.  At least, most of the studies that have looked at it have found such a correlation – ranging from –0.2 to –0.4, which is fairly substantial.  Not all studies have found this but a meta-analysis confirms a correlation with a value between –0.12 and –0.2.  This correlation is weaker, but still significant, and means that there is at least some relationship between intelligence and symmetry.  (It may also be an underestimate, as one study found that psychometric tests with heavier loadings on g showed greater correlations with fluctuating asymmetry).  The most plausible interpretation is that this correlation reflects the effects on both parameters of a “latent variable” – general fitness.

This may, incidentally, also explain the recently demonstrated correlation between intelligence and physical attractiveness, which itself has been correlated with facial symmetry.  (Gratuitously exemplified by the lovely, symmetrical Kate Beckinsale and her mirror-image right- and left-side dopplegangers).

Correlations of intelligence with measures of brain size, white matter integrity, network efficiency or other parameters may be similarly explained.  They could be either independent correlates of general fitness or the structural measures could be the substrates of phenotypic differences in intelligence – the means by which general fitness affects g.  According to that model, expecting to find defined sets of genes “for white matter integrity” would be as misguided as looking for genes “for intelligence”. 

Instead, we may all carry many mutations that affect intelligence, negatively and mostly non-specifically, with the total burden determining how far away we each are from our archetypal Homo platonis.  

80 comments:

  1. This is an interesting perspective and I think it fits with the emerging evidence.

    Interestingly it offers a solution to the deadlocked debates over "race", IQ and genetics.

    A lot of people worry that if IQ "is genetic" then IQ differences between ethnic groups might be genetic as well which would mean that the racists were right all along.

    But actually that doesn't follow. IQ could be largely genetic as a result of the fact that diverse deleterious mutations can lower IQ. But if these mutations are constantly selected out of the population, and are therefore generally "new" mutations (de novo or inherited over a few generations max) , they would not be expected to vary across populations.

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    1. I agree - see also reply to comment below.

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    2. I don't see why moralizing bullshit has any place in science, anyway. The Universe is a nihilist, all this egalitarian crap is just invented by credulous, mystical apes.

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  2. This is an interesting piece but I don't believe that intelligence could be primarily the balance between positive & negative random mutations. Although psychology generally agrees that there is a general intelligence factor, g, this is a statistical phenomenon, rather than a structure or function that is uniformly affected by external or internal environments. My understanding is that g is an emergent quality for which we have found measurement methods although it is derived from a balance among multiple component cognitive abilities. I have always imagined that these different abilities can all vary independently (excluding a few that seem linked) according to the usual variation of genes in assortative mating, meiosis & mitosis. In addition, some of these components of intelligence probably vary according to how many genetic contributions come from the male vs female sides, how far apart on chromosomes they are situated and how near the telomere ends they fall. Mutations are more "sudden" in an evolutionary sense I think, than variations arising from some of the other factors, which can accumulate or diminish across generations. Therefore, exploring mutations that appear linked with lower IQ may not be any more useful than chasing mutations which appear to increase g. Has anyone discovered the gene or mutation that makes Chinese people score better than Caucasians on traditional tests of g? Isn't that already a genetic difference between races? Maybe there are thousands of genes combined that give the Chinese an advantage, especially with numbers, rather than a definite mutation. There's definitely a mountain of work to do to decide these questions, whether or not we have maintained a group of "better evolved" humans over time or whether our invention of technology was merely the product of time & environment involving a basically intelligent brain that has changed little in a million years.

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    1. Kay, thanks for your comment. I didn't follow all of it, to be honest, but I will reply to a couple specific points: I use the term variant or mutation interchangeably, for the most part. The mutation can refer to the difference or to the event that results in the difference. Whether any specific mutation rises in frequency in a population depends on its effect on fitness (and also random drift). Mutations reducing intelligence will likely all be rare, and therefore the spectrum of mutations having that effect will differ in different people.

      Regarding supposed racial differences in IQ, there is an important misunderstanding. Just because variation in a trait is heritable within a specific population does not mean that differences in the mean values of that trait between two populations are caused by genetic differences. They could be entirely environmental or cultural (the most likely explanation in the example you cite).

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  3. This is an interesting theory. It fits with theories of natural selection in terms of the death (or failure to reproduce) of animals with deleterious mutations, and the survival (and reproduction) of those animals with a phenotype which is most fit (irrelevant of the specific genes). In other words, instead of survival of the fittest selecting for positive genetic mutations, its strongly selecting against negative ones. There is a positive selection pressure for "good" mutations, but this is a much weaker mechanism than the very strong selection against "bad" mutations. Both mechanisms can work alongside one another.
    However, the correlations you've cited above are not actually that strong, in comparisons with other g correlations, such as working memory. Some theories suggest that working memory is almost identical to g (very strong correlation), whereas the leading critiques find a correlation of about 0.25. I personally am somewhere between these two extremes, and consider working memory to be a kind of "cognitive bottleneck", i.e. necessary for certain complexity of cognition (holding lots of information in mind simultaneously really helps) but not the be all and end all cognitive function of IQ.
    Of course, IQ isn't the best measure of intelligence. IQ is a valid and reliable measure of IQ (one of the most valid and reliable measures in psychometrics in fact) but it does not take into account other cognitive abilities such as motor skills (not measured by psychometrics, but correlate highly with motor cortex volume and activity) and theory of mind skills (measured by tests like Reading the Mind in the Eyes psychometrically, and correlating with BA10 and tempero-parietal junction and other RH areas in the brain).
    These abilities do not necessarily correlate with g, and therefore don't necessarily correlate with IQ particularly highly, but they are real aspects of cognitive function (and can be selectively damaged in brain injury, or malfunction in high functioning autism for example).
    For example, a person with a relatively large DLPFC may have a relatively small motor cortex (as the areas are adjacent). This may cause that person to have a higher than average working memory, and a higher than average IQ, but also cause a deficit like dyspraxia. It corresponds to a cultural stereotype too; that of the clumsy geek (very clever, but picked last for sports).

    As another example, a person with a relatively large motor cortex might have a relatively smaller DLPFC. This person might have excellent motor skills but below average working memory and IQ. Hence this person might correspond to another cultural stereotype, the dumb jock (not that smart but great at physical things).

    Another example could be a person with a smaller frontal pole (Brodmann’s area 10) and larger DLPFC. This person might also have higher than average WM and IQ, but worse than average Theory of Mind skills (ToM). This might explain part of the picture of conditions such as high functioning autism/apergers syndrome. This person would correspond to our cultural stereotype of a geek who is clever but doesn’t have great social skills.


    My best guess would be that your idea of a Platonic, or ideal, human, with high symmetry (and therefore attractiveness) will have a highly balanced and symmetrical brain, and therefore a normal balance of these abilities, whereas someone with a non-symmetrical body, should have a non-symmetrical brain, which may lead to strengths in one area of cognition at the expense of others.

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  4. A small strength with a small deficit shouldn’t matter, but a large deficit in say, motor control or theory of mind (or IQ) will make you highly unattractive in terms of reproducing. Therefore, we might expect that the most successful people are those with the best *balance* of all cognitive abilities, not just working memory related IQ. Therefore we might expect the Platonic, or ideal human to be balanced and symmetrical, as you have suggested, but the selection pressure is really against too much variation from this norm (with a mild selection pressure for increasing abilities without upsetting the balance).

    If we had a battery of tests for all aspects of human cognition, lets call it H, we’d probably find that they don’t all correlate as well with each other as aspects of intelligence do. So whilst our g factor is quite high, our h factor would be lower. However, our h factor would correlate much more highly with attractiveness than our g factor, as our h factor represents a balance of cognitive ability, which is required for us to successfully survive and reproduce.

    I know that’s a lot of theory with very little evidence, but I’m feeling too lazy to reference this properly today. Does it make sense to you?

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    1. Thanks Tom for those interesting ideas. I get the gist of what you're saying, but would like to know if there is good evidence to support it. Especially the idea that increases in the size of one brain area must come at the expense of a reduction in adjacent areas (and thus related skills or faculties). More generally, the idea that our Platonic archetypal human would be optimal for many traits, as opposed to maximal seems likely.

      Regarding symmetry in the brain, we have to be careful though, as there are directional asymmetries in the brain that are part of the species-specific plan (though one can also detect fluctuating or random asymmetries beyond those, the degree of which correlates with fluctuating asymmetry in the body).

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    2. Baron-Cohen proposed (and tested) a more functional model in line with your suggestions Tom. He called it "empathising-systemizing quotient". In his research he revealed a weak negative correlation between them. So, there seems to be a (weak) trade-off phenomenon between these two.

      Another intresting finding was that autistic spectrum quotent scores of students were like:
      physical sciences>biological sciences> humanities.
      You can follow the article from here:
      http://docs.autismresearchcentre.com/papers/2006_Wheelwright_etal_BrainResearch.pdf

      He has many intresting work over segregation of ASD in families with engineer parents too.

      And I must say that determinants of population genetics would not allow to all individuals to share the same balanced mind (brain) structure as you proposed. Outliers (extreme empathisers and extreme systemizers in this case) would serve for different group and individual selective purposes.

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  5. One thing occurs to me as a confounding variable here - nonrandom mating. One would imagine that individuals with a higher g factor tend to mate with people who are generally fitter and more attractive (IQ correlating as it does with economic success). And probably the reverse. The result I guess would be a clustering of all sexually desirable traits in a manner very similiar to what you would get if they were mechanistically linked. Has anyone tried to control for this, e.g. by looking at parent's traits, or looking at sibling pairs?

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    1. Hi Dermot. You're right - there is significant assortative mating for intelligence (spousal correlation = 0.33). That is, not surprisingly, people marry other people of roughly similar intelligence to themselves. The same is true for physical attractiveness and height. But I don't think that's really a confound for the results cited above though - that would not cause a correlation between those factors in individuals if one did not really exist.

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  6. Is it known whether epigenetics has a role to play in development instability or fluctuating asymmetry? Genes are not always expressed depending on the way DNA is packaged and DNA packaging is involved in cellular differentiation. If somatic mutations were responsible for fluctuating asymmetry, wouldn't family affected with Li Fraumeni syndrome more likely to have fluctuating asymmetry?

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    1. If by epigenetics you mean "non-genetic", then, yes, clearly the noise in biochemical and molecular processes that leads to developmental instability is non-genetic (i.e., not caused by the DNA sequence itself). Whether differences in DNA packaging or methylation have any role in those mechanisms is not known - there might be variation in those processes too, but there is nothing to suggest that stable, heritable chromatin states are important. Somatic mutations may play a role but it's very hard to figure out how much - see here for more on that: http://wiringthebrain.blogspot.ie/2011/05/somatic-mutations-make-twins-brains.html

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  7. How would the current process of selecting against mutations that degrade intelligence relate to the process that caused the emergence of higher intelligence in the first place? Aren't we looking for a process that could equally explain the emergence of Homo sapiens and the inheritance of g today?

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    1. Thanks for that question. I would say no - we should not think of those as the same set of genes or even look for ones affecting the same set of processes, necessarily. There undoubtedly was some series of mutations in the lineage leading to modern humans that led to our current level of cognitive capability. These were probably highly unlikely events, but provided a strong selective advantage once the cognitive niche opened up. Whatever they were and whatever processes they affected, those mutations should now be fixed. But the systems can be degraded in so many thousands of different ways that there may be very little overlap between that set of genes and the ones in which variation can affect inheritance of g today. Indeed, the main point of the article is that there may be a very general degradation in g due to aggregate mutational load, which simply impairs the ability of the system to produce an optimal phenotypic outcome.

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  8. It's fun to make a living out of statistical correlations, I know - but the section of this essay on the correlation between g (a frequently challenged concept itself) and general indicators of fitness sounds as convincing and fact-based as phrenology to me. I've known way too many men and women who had very high IQ scores and who died young. And others with very high IQ scores and at least average life spans who were not "fit" in oodles of ways. I'm not buying it, statistically significant as it may be.

    Other than that - an interesting essay.

    I think we're a very long way from finding out what intelligence is, and how it works, but it's good to keep trying, we'll get there eventually.

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    1. Thanks for that. Generally speaking, correlations across large numbers of people, especially ones that are partial as in the cases cited above, merely say that there is a statistical relationship between the two variables. There will always be exceptions, like the ones you mention. The exceptions may make more of a psychological impression, like the inevitable granny who smoked 40 cigarettes a day and lived til 100. Doesn't mean they don't cause cancer. That's why stats beat anecdotes.

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  9. Hmmm. I wonder if this might help explain the Flynn Effect? Perhaps the increase in IQ over time is the result of better health? (Not that there is a reduction in deleterious mutations over time, but that their effect is lessened in a healthier environment.)

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    1. Yes, I think the Flynn effect is all about nutrition and education. (And it's a dramatic illustration of why you can't interpret differences in IQ between countries or even between socioeconomically disparate groups within a country as having any kind of genetic basis).

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  10. I think the first step towards figuring out these things is to stop equating intelligence with a highly flawed visual test and quit reifying these results as some singularly defined, innate quality of mind, i.e., 'g'.

    In fact, I thought it was settled years ago that IQ and g are nonsense; are we really still hanging on to these like some sort of throwback Freudian psychoanalist?

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    1. Well, flawed as IQ tests may be, the general factor g is very robust, quite stable, highly heritable and somewhat predictive of a lot of life outcomes. It does not define every aspect of a person's performance on tasks tapping "intelligence" however - it is an underlying factor that has an impact across many tasks, but does not explain all the variation in them - instead, it explains the correlation between them. Nothing Freudian there, as far as I can tell.

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  11. A related question is what is the range of symmetry fluctuation in the brain, or specific parts of it (shape, density, function) for humans ? Do we know for sure what part is a species-specific plan and what is open to individual variation ? Is it the same in other primates ? There is no clear case to my knowledge that the degree of symmetry or asymmetry in brain organization is correlated to intelligence for a given species.

    On a more cultural level, body plan asymmetry and light handicaps can be a social and intellectual challenge stimulating the use of one's own capacities.

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    1. Great question. The species-typical patterns of brain asymmetry are pretty well defined. It is possible to look at fluctuating asymmetry against that background - more random asymmetries in structures that are normally symmetric or actually any deviation from typical asymmetry. The following paper analysed that: Robert J. Thoma, Ronald A. Yeo, Steven W.
      Gangestad, Jeffrey D. Lewine & John T. Davis (2002): Fluctuating
      asymmetry and the human brain, Laterality: Asymmetries of Body, Brain
      and Cognition, 7:1, 45-58
      http://dx.doi.org/10.1080/13576500143000122

      They found FA in brain correlated with FA in the body.

      As for other species, there are lateralities in brain structure, though they tend to be less pronounced than in humans. For a great review by Corballis, see: http://www.ncbi.nlm.nih.gov/pubmed/19064358

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  12. How can you control for developmental factors when attributing the correlation between symmetry and intelligence to genetic factors. For example, an attractive person would have been given different opportunities from a young age than an unattractive person. She would have more confidence and achieved more education.

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    1. It's quite true generally that initial differences can have cascading effects across life that can amplify them by changing the environment. (See here: http://wiringthebrain.blogspot.ie/2011/10/does-brain-plasticity-trump-innateness.html). I have to say I haven't really thought how this would apply to attractive people, but there is definitely an extensive literature on how they get an easier ride in many circumstances. (My guess is that this does not necessarily include in education, however).

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  13. I seem to remember a study done in Britain during or after WWII which found correlations between low brows,smaller heads correlating to low intelligence.

    The question I have is: Does intellect and symmetry run in the family or is it random? Moreover, what about the genetic family history of genuises / dumbells? Are these qualities passed on or are they independent? Just a thought...

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    1. Yes, both intellect and symmetry "run in the family" - or, more accurately, they are both heritable. And intelligence does correlate with head size and brain size, as a trend. Francis Galton famously and immodestly used his own family tree as evidence for "Hereditary Genius"! http://en.wikipedia.org/wiki/Francis_Galton

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  14. unfortunately assuming that all readers believe in evolution is a major negative. once should never assume readers starting points.

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    1. Well, if you are among the readers who don't believe in evolution, I can only say you have the right to your own opinions, but not your own facts. Evolution is like biology's Standard Model - every single observation made in biology fits within it and is consistent with it. We do not understand all the details, but evolution by natural selection provides the essential conceptual framework to make sense of the weird and wonderful world of biology.

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  15. Very, very interesting. Just one question. I thought that there was a strong correlation between myopia and high intelligence?

    I was a member of Mensa for a few years and can't remember my fellows as being any better-looking than the average. (I'm certainly not.)

    In my country, the major IQ stunter is malnutrition of the pregnant woman. Now we have an excellent system of social grants and I expect our national IQ to increase markedly over the next 20 years.

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  16. For more on the impact of mutational load on susceptibility to randomness and the genetics of developmental robustness, see this short (5 min) presentation from Ignite 9.0, which I gave at Dublin's Science Gallery in June 2012: http://www.youtube.com/watch?v=KDFh74eENuw

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  17. Yes indeed, an excellent thought provoking idea that makes sense. It certainly generates a new direction of studying the converse - that could result in added knowledge about the reverse of 'intelligence' - to provide greater understanding about both intelligence and stupidity.

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  18. This is a very interesting essay and thanks for the work that you do. However, I question the connection between facial symmetry and intelligence. Einstein lacked the Sylvian fissure and had extra glia cells but wasn't particularly attractive. I agree with Tom, and aside from the species-specific asymmetries, e.g. a larger left hemisphere, that an individual's specific cerebral differences will reflect varying strengths and weaknesses. This is especially apparent in autistic savants, Asperger's syndrome, etc.

    Left- or mixed-handed people often show variable traits for intelligence. I'm left-handed, presumably right-hemisphere dominant for language judging by the way I hold a pen and my linguistic fluency versus total absence of visuospatial reasoning. Yes, my left eye, left hand and left foot are larger than the right side and definitely dominant. I guess I'm predisposed to find asymmetries more interesting than symmetries, even if they are less attractive. I also believe that we may be genetically predisposed to be readers vs. non-readers and that the former are more likely to perform better at school, at least in language-based vs. quantitative-based testing. I've written about atypical lateralization here: http://rightmindmatters.blogspot.com/2012/01/born-that-way.html.


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    1. Thanks Carol for your comments. The observed correlation between symmetry and intelligence is pretty weak, so, even if it is a real effect (the hypothesis I present here), it is not a complete correlation. There will obviously be lots of independent variation in both parameters and many individuals who seem to be exceptions. I won't say exceptions to the rule - to the trend, maybe.

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  19. Is it assumed, or proven, that in modern societies certain maladaptive traits are inexorably reduced through differential reproduction? Our societies have constructed social systems which support the tendency of some groups to reproduce (and therefore amplify the frequency of their particular mutations) far beyond their expected level of success without those systems. Religions have typically promoted reproduction as a good in itself, without regard to the fitness of the individual's involved and created support systems to enable them to do so.
    An example is the current differential reproduction between the worlds wealthiest and poorest regions. Note that I am specifically not stating that the wealthy regions necessarily contain more intelligent individuals than the poorest. Rather, that the differential rates of reproduction exist without regard to underlying "fitness", and instead are mainly due to cultural factors. Have we been able to stymie human evolution or at least divert its course through our cultural institutions?

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    1. This is a really interesting question. Clearly, modern culture shelters individuals from selective forces that they would have been exposed to in the past. This may indeed have some effect on the frequencies of modestly deleterious variants, as any change in environment which alters selective pressures can. Selection can obviously work through survival or reproduction or both and sometimes decreased survival could be balanced by increased reproduction, or vice versa.

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  20. I tend to agree. Have you read much from Cochran on this topic? He's posted a decent bit on this this year, on the concept itself and some implications. Specifically, paternal age looks like an enormous contributor to genetic load, with cultures who have practiced... how did he put it... geriatric polygamy? having a much higher load than others. Heat may (or may not) also be a major contributing factor.

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    1. Sounds really interesting - do you have any refs?

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    2. Yeah, sorry it took me a week to get back to you--

      Here's a fairly general post on genetic load:
      http://westhunt.wordpress.com/2012/04/10/more-thoughts-on-genetic-load/

      Here's some speculation on heat and genetic load:
      http://westhunt.wordpress.com/2012/07/14/too-darn-hot/

      And some general posts on paternal age and genetic load:
      http://westhunt.wordpress.com/2012/09/16/paternal-age-and-the-force-of-mortality/
      http://westhunt.wordpress.com/2012/09/17/gambia/
      http://westhunt.wordpress.com/2012/10/29/talkin-bout-their-generations/

      I'm convinced that variation in IQ is highly correlated with variation in genetic load; I think perhaps the majority? of variation in genetic load can be explained through historical mating dynamics... but perhaps not all. What are your intuitions here?

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  21. a absentee process, where genetic changes bootstrap on cultural growth in a way that reinforce their own adaptive ness,

    acknowledgement of dissertation

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  22. A very intelligent speculation, using that phrase in the technical sense of "agreeing with what I said earlier". To wit,
    "It is not the case that some people are XTs, some 286s, some 386s, some 486s and some Pentiums-that there are high- and low-powered systems. We all have essentially the same machine, and use essentially the same operating system; some of us, however, have programming viruses that have introduced bugs into the system - particular problems, with particular solutions."
    at http://home.vicnet.net.au/~borth/DOWN1.HTM.

    Mind you, I'm fairly sure I'm willing to push the point further than you are.

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  23. Please follow this article for bottleneck hypothesis for homo sapiens. http://whyevolutionistrue.wordpress.com/2011/09/18/how-big-was-the-human-population-bottleneck-not-anything-close-to-2/

    All humans around the world are sharing the same around 1.000 anchestors lived 100.000 years ago, despite the population was much higher prior to that date. This finding implies a huge selection pressure for humans. This pressure might have arised from climate changes around the Rift Valley or else. Intresting point is, phenomenon called "great leap forward" appeared occured just after that bottleneck incident, and humans probably developed linguistic and theory of mind facilities etc. I can assume that although homo genus was spread over the world millions of years ago, only descendance of these 1.000 humans emerging from Africa had that sociological, technological, and cultural boost that brougth us today by means of some genetic change occured under that unique selective pressure.

    I assume that that bottleneck effect was a consequence of a (couple of) mutation(s) that conferred to increase in the intelligence. Microcephalin and ASPM are two candidate genes for this explanation, both adressing same bottleneck effect around 40.00 years ago. This study revealed no effect of different allelles on general IQ scores. http://hmg.oxfordjournals.org/content/16/6/600.long

    This migth be concluded as another negative result. But another study here revealed polymorphism of these genes effected that populations language (namely linguistic tone). http://www.pnas.org/content/104/26/10944.full
    Let us not forget the FOXP2 which was introduced as "language gene" http://www.pnas.org/content/92/3/930.abstract

    So, this genes might contributed to language acquistion, and indirectly to developing better intelligence. (again adressing the deaf-dumb argument)

    I have digressed with anthropological, linguistic and evolutionary material, but still hope makes a sense as a different perspective. And sorry for my own linguistic constraints as a non-native speaker.

    ReplyDelete
  24. A comment from a behaviorist that uses mouse genetics. The model proposed in this article stress the importance of negative over positive effects of genetic variants on intelligence. How this fits with the handful of mutant mice showing enhanced learning and memory (see for instance Lee and Silva, Nat Rev Neurosci 2009)? Some of these mutants point to genes whose impairment causes intelectual disability in humans.

    ReplyDelete
    Replies
    1. There are indeed a few cases where mutation of a gene leads to an "improvement" on learning and memory tasks in animals (mice and also flies). Whether the more rapid learning of particular tasks corresponds to a general increase in the animal equivalent of "intelligence" is not at all clear, however. There may be very good reasons why we don't normally do one-trial learning for everything we encounter - that may be a very sub-optimal strategy and highly non-adaptive.

      Delete
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