The twins were all right

Twin studies have played a major role in investigations of genetic influences on intelligence and personality.  Generally, they suggest that genetic influences on those traits are quite strong,  a conclusion that a lot of people have had trouble swallowing.  As far as I can see, that gag reflex was entirely due to dislike of the implications of a strong genetic influence on human behavior, rather than any compelling argument or solid counter-evidence.

There have always been other approaches to investigating these questions, looking at adoption,  varied degrees of relatedness, etc. They produced generally similar results.  The latest and most powerful version is the Visscherian approach,  which uses overall genetic relatedness and is not affected by most of the possible problems with twin studies. Eric Turkheimer put it this way:  “Thanks to the Visscher program of research, it should now be impossible to argue that the whole body of quantitative genetic research showing the universal importance of genes for human development was somehow based on a sanguine view of the equal environment assumption in twin studies, putting an end to an entire misguided school of thought among traditional opponents of classical quantitative (and by association behavioral) genetics.”

I don’t think you get very far by talking about how the world ought to work.  Although you can sometimes get somewhere by deluding other people , and maybe that’s what some of these people were up to.  They were pretty successful: lots of people think that intelligence isn’t heritable.  For some reason, people with such views have lower-than-average birth rates, so maybe it is true, for them.

In general, though, I think people like Leon Kamin deceived themselves first. Anyone who grew up in the US and came to the conclusion that Uncle Joe was a great guy wasn’t exactly a fountain of common sense in the first place.



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52 Responses to The twins were all right

  1. Gary says:

    Approximately what percentage of intelligence does it seem is due to the genes?

    • JayMan says:

      The tiny sample size of MZA studies don’t lend themselves to slicing and dicing.

      Also, other methods that don’t use twins give heritabilities for IQ that approach 100%.

      All this was covered in my post linked here.

    • little spoon says:

      I also have this question. Although I am not sure the best precise way of phrasing it. What I mean is, in modern first world society, assuming all well fed families and excepting any extreme cases of child abuse or neglect, what is the average, upper and lower limit of the impact of environment on the expression of intelligence? That means the impact of environment on the observed real world usage of intelligence.

    • Anonymous says:

      h^2=80%. H^2=90%.

      • Richard Sharpe says:

        I suspect that to some extent IQ is buffered against environmental insults, but of course, if you starve a child to death its IQ will be zero.

        Also, if all the whole population goes through lean times, they all are likely to suffer a similar deficit in IQ. Since brain tissue is expensive to develop and maintain, what point is there in having more than needed to get your genes into the next generation? From that point of view, if selection is not selecting for more flexible intelligence, as it certainly seems not to be in some environments …

    • The fourth doorman of the apocalypse says:

      Approximately what percentage of intelligence does it seem is due to the genes?

      The answer is 100% and you asked a poor question.

      If an individual’s IQ is lower than it could have been in a better environment (ie, one where they are not being blasted with loud noises all the time and banged on the head with a ball-peen hammer) that only means that their genes are not robust enough in the face of environmental insults.

      • The fourth doorman of the apocalypse says:

        I can see how erudite you are.

      • gary says:

        @The fourth doorman of the apocalypse

        Look, you know what I mean. What percentage of the variation in intelligence can be attributed to the genes and what to the environment?

        My hunch is something like 2/3 genes and 1/3 environment.

      • The fourth doorman of the apocalypse says:

        My hunch is something like 2/3 genes and 1/3 environment.

        As Greg and others have pointed out, that environmental component is actually the error component in the equation, and it includes the non-additive genetic component. The non-linear component, if you will, ie, those gene X gene interactions that contribute in a non-linear fashion to IQ.

        However, the range of values for the additive component I have seen are from 50% to 80%.

  2. JayMan says:

    For extensive review of twin, adoption, and sibling studies, discussion of what they find, the concordance between the multiple methods including Visscher’s GCTA design, and a review of why the “environment” isn’t what we think, a must see is:

    More Behavioral Genetic Facts | JayMan’s Blog

    Yes, twin and other studies work, all behavioral traits are largely heritable, and the three laws of behavioral genetics are concrete. Indeed, it means when something deviates from this pattern, something is up.

  3. JayMan says:

    That’s complete and utter nonsense.

    Why does behavioral genetics bring out the crazies?

  4. Jim says:

    The molecular machinery of life is not perceptible to our senses so it is very difficult for most people to grasp anything about it. People hear the word “DNA” or “gene” but they have no intuitive model of these terms. So they just revert back to the “ghost in the machine”.

    • RS says:

      It’s no mystery that these molecules are not vivid to people. They never explain in high school, or early in college, the essence or secret of why life can come from ‘billiard balls’, dead atoms. (In compressed terms, it is just the induced-fit model of electro-steric complementarity — combined with constant energy input that prevents a static equilibrium, ie death.)

      Few smart people know that bio is just a cascade of one molecule changing its shape slightly (or sometimes dramatically) to fit with a second one, which also changes, which change renders that second one receptive to a third molecule (with the first one perhaps dropping off now), etc.

      Without that, you don’t understand why mRNAs and van der Waals forces don’t wind up at or close to rest — dead — just as a system of recoiling billiard balls ‘dies’ in a few seconds. If you don’t see clearly why ‘dead’ matter can be ‘alive’, it’s boring to hear about how mRNAs and van der Waals forces — not a thrill in themselves — support this central, essential miracle that no one gets into. I was unable to pay any attention. I never found calculus class boring because I could see what the red threads were : fairly hard to teach calc without them popping up. Evolution is one subject in which you can’t bury the central essence if you tried like heck, hence most smart people have at least the gist.

      I would show the ‘secret’ in the first minute of instruction.

  5. Patrick Boyle says:

    My mother was a twin but whether she and her sister were fraternal or identical twins we never quite knew. They looked very, very similar. All their lives they were being confused. They looked alike, talked alike and thought alike. But my cousin Willie – my aunt’s son – always claimed they were not identical. If they were identical then Willie was my half brother genetically. Maybe he didn’t care for that idea.

    Then when my mother died my aunt – her twin – showed me some early pictures I had never seen before. In high school, they didn’t look all that similar. They had lived together for years and they seemed to converge in appearance as they aged.

    So I wonder how accurate the early studies were in classifying twins. In 1875 when Galton did the first twin study he seemed to base his classification of twins on simple appearance. I’m guessing that the DNA tests we use today were only developed in the sixties. I gave up googling this particular fact. So Cyril Burt’s studies in the twenties for example all have another possible source of error – false identicals.

    If researchers misclassified twins it should tend to attenuate the genetic component, I would think. So the true results of those studies would have been even stronger than those reported.

    • dearieme says:

      If you accidentally misclassify fraternal twins as identical won’t you underestimate heritability of IQ, good teeth, height, and whatnot?

  6. This is from the paper you took that Turkheimer quote from (last sentence before conclusion,

    “… the very ubiquity of heritability has made it problematic to differentiate between heritable phenotypes that have genetic mechanisms and those that do not.”

    It seems to me that any phenotype that shows heritability using GCTA methods must have underlying genetic mechanisms. If not, how could such spurious results come about when using Visscher methods?

  7. The fourth doorman of the apocalypse says:

    Uncle Joe McCarthy or Uncle Joe Stalin?

    (I didn’t grow up in the US.)

  8. peppermint says:

    Why isn’t intelligence heritable? Because rarely is the child of a highly intelligent person even as smart, but dumb people are seen with smarter than them children all the time.

    Also, it’s racist.

  9. Matt says:

    The biggest stumbling block I see for people understanding the heritability of intelligence, or any traits is they assume that breeding is blending, and so think that the fact that kids aren’t always at the parental midpoint indicates large non-genetic factors.

    If they understood that breeding was a random sampling of 1/2 each of the parents’ genome, then they would see how children having quite different scores than the parental midpoint isn’t incompatible with a high degree of heritability.

    It’s something that takes about 30 seconds to explain (at most), yet no article in a mainstream publication for laypeople on the “controversy” of the heritability of IQ ever seems fit to see this and adjust for it.

  10. panjoomby says:

    “hastings1066” aka “anonymous” seems to be taking the internet a little too seriously.
    at least he’s not a twin 🙂

  11. Gottlieb says:

    The intelligence above all else is randomness. People confuse randomness with environment. Can happen in a couple of smart people, all probable phenotypes when combined, may result in different phenotypic expressions of high intelligence, but it can happen even if you have winning genes, they can combine in a way that produces a phenotype of a person, for example, which has a high iq but to intellectual issues to be stupid.
    The most important in the production of a being is the genetic combination of the parents, a priori. And I believe that we are not talking about” genes” but a variety of internal and combinable limited phenotypes of genes. People interested in evolutionary psychology, should give more priority to the analysis of combination of genetic predispositions as well as the conception of life, when the sperm fertilize the egg. These processes seem to be more interesting to explain the epigenetic conditions, because I believe that to develop any kind of condition, objectively or subjectively pathological, it is necessary that this predisposition has been favored in the conception of life and not in subsequent processes, such that act as secondary drivers of epigenetic tendencies.

    It would also explain free will, the main belief of behavioral environmentalists. Unlike the extremism of certain people, free will exists, but it is limited by genetics itself. Human genetics, aiming to be adaptive at the individual level, requires flexibility and internal predispositions, eg for coevolutionary interactions with pathogens as well as the individual response to events that change the genetic-environmental provision as multiculturalism. If we not had this internal availability of choices, we perish immediately that the environment, social or natural, suffer sudden changes. What happens with most of the non-human species. This individual adaptability seems to be a human skill.

    • Jim says:

      Gottlieb – I have only the vaguest idea what the hell you talking about.

      • Gottlieb says:

        Sorry, I think I was very anxious. I can not help it.
        I’ll summarize in a few words.

        First part

        People confuse internal genetic malleability of human beings with environmental factors.

        second part

        All subsequent predispositions of any order or nature born or pre-determined in the design oe conception of life, when the sperm fertilizes the egg.
        This depends on the phenotypic combination of genes from parents. This leads us to conclude that the conception is a random limitedly event.

        The first event of life is the conception. ‘She’ is the source of all other predispositions. Hormonal, environmental events such as pollution, smoking during pregnancy etc … are subsequent factors. If there is a predisposition for certain behavioral phenotype, it has been created at conception.
        If there are children who are born” perfect” of a pregnancy based on bad habits of the mother, so there is no causality between bad habits and reduced fitness. There may be that women who smoke during pregnancy exhibit fragile genetics and their children, depending on the pattern of mating, may inherit this tendency. In other words, the most important is the phenotypic combination of father and mother that occurs during conception, when the sperm fertilizes the egg.

        I think the third part was understandable, but if you have any questions you can ask.

        One of the fundamental cores, the primary source, the holy grail of environmentalists is the idea of free will. All your assumptions, theories (all of them that emphasize environmental factors) and actions derive from this source. And as you know, the way they interpret the nature x nurture dichotomy is misguided.
        Deny free will would be the initial, most obvious and pragmatic response as a means to counteract decades of pseudo-scientific catechism. However, when I least I’m not here to serve the interests of conservative groups, but to the interests of the truth of what is.
        If there were no free will, we would perish instantly that our environments were changed. This happens with most non-human species. We are individual variables, most non-human species are collectively variables.

  12. Test Subject says:

    Of course you may believe that you are such a genius that your opinions are 100% correct and that no other opinions matter, and for all i know that could be true, However if this is true why do you post comments at all ?

  13. Julian says:

    ***In general, though, I think people like Leon Kamin deceived themselves first. ***

    In William Wright’s book “Born that Way” Kamin makes an interesting admission when the author asks him whether he thinks intelligence has no inherited component:

    “Kamin smiled mischievously “of course there is a genetic component” he said, “but our techniques of measuring it are too flawed.” (page 221)

  14. The fourth doorman of the apocalypse says:

    I would guess that those things that remain constant across human social environments would be candidates for selection to select for hard-wiring, while those things that are not constant must be handled with learning.

    Detecting cheaters would seem to be constant across human social environments and the need for language would seen to be constant. However, the form that each language takes is different.

  15. Pingback: The twins were all right | West Hunter | Gene-Culture Homepage

  16. neilfutureboy says:

    If Uncle Joe was so bad it should be easy for nice people to say what he should have done, nicely, in his circumstances, that would have worked.

    Is it paranoid to think they are plotting against you when they are plotting against you?

  17. JayMan says:

    So, you heard it here: new genomic study looking at American Blacks and assessing trait heritability by looking at amount of European ancestry claims to find much lower heritabilities for many traits than found from twin studies:

    Leveraging population admixture to characterize the heritability of complex traits : Nature Genetics : Nature Publishing Group

    I don’t believe their conclusion. Adoption and reared-apart twin studies produce the same results as classical twin studies. As well, previous GCTA studies have found an additive heritability of certain traits (height, BMI) in line with those from twin studies.

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