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The Behavioral Genetics Page
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This page is to make an easy to use (and easy to share) central repository for my posts on the science of behavioral genetics. This is fundamental reading for anyone interested in HBD – indeed for anyone interested in the human sciences in general.

All Human Behavioral Traits Are HeritableThis post introduces readers to the world of behavioral genetics and the concept of heritability. Introduces the “Three Laws” of behavioral genetics (the title of post being the First Law verbatim). Explains the usefulness of twin and adoption studies to partition the sources of human trait variation – that is, disentangle nature from “nurture.” Shows that the source of human differences can be broken into three basic parts, heredity, the shared (or common) environment, and the unique (or non-shared/unshared) environment. Details that all human traits show heritable contribution (hence the First Law). Also details the lack of impact from the shared environment for almost every human trait, which rules out nurture (as its commonly thought) and parenting of having a significant impact on life outcomes. Notes the huge impact the First Law has for human group differences. (Indeed, while not quoted in the post, by as John Derbyshire put it, “if dimensions of the individual human personality are heritable, then society is just a vector sum of a lot of individual personalities.”)

Environmental HereditarianismFollowing up my preceding posts on the matter, this one delves into the matter of the “environment” in more detail. Specifically, this touches on commonly claimed “environmental” influences and explains why they are mostly bunk. As well, this analyzes the “unique environment” and explains why it’s not really environment at all – but more accurately, the unexplained variance. Discusses some of the limitations of behavioral genetic tests that cause them to overestimate the effect of the environment and underestimate the effects of heredity – most significant among them being measurement error. Details the concept of developmental noise, noting how some of the variation between people (as seen in the differences between “identical” twins raised together) can be due to subtle disturbances during development (fingerprints being a simple example).

The Son Becomes The FatherFeatures the work of Gregory Clark, who (through surname analysis) shows that we evidence for heritability going back many centuries. I take the opportunity to detail larger, newer behavioral genetic studies that show high heritability for all human behavioral traits – including intelligence, personality, political views. As well, I detail evidence showing high heritability for major life outcomes, including criminality, marital stability, drug use, lifetime income, and life satisfaction. More importantly, I detail the lack of shared environment effects for all of these traits. This is key to silencing critics who would like to place “nurture” in an important role in shaping who we become.

More Behavioral Genetic FactsHere I analyze some of the last-ditch efforts to insert “environment” into life outcomes, demonstrating the usefulness of reared-apart twin studies, as well the impact that the informant/measurement method have on results. I also delve into the “extended twin design”, which looks at additional family members beyond twins to partition out further uncertainties in genetic vs. environmental influence (for one, this puts the kibosh on the notion that we choose spouses who resemble our opposite sex parent). I also touch on why peers are not likely a source of lasting influence. As well, I make a case against the importance of gene-environment correlations as a source of influence (itself a massive violation of Occam’s Razor).

* * *

In addition to these, be sure to read the papers collected at my HBD Fundamentals page, section On the science of behavioral genetics.

* * *

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  1. If Assortative mating is a confounding factor in analyzing heritability, would “assortative friending” be a confounding factor in analyzing what is called non-shared environment?

    Read More
    • Replies: @JayMan

    would “assortative friending” be a confounding factor in analyzing what is called non-shared environment?
     
    Not so much non-shared environment, but purported peer effects, definitely.
    , @Emil Kirkegaard
    Assortative mating makes H2 estimates smaller. This is because AM increases the genetic relatedness of DZs (for that trait) but not MZs (who cannot be more closely related). E.g. if there is .5 AM for the trait which is entirely genetic, then DZs will be related with a coefficient of .75 and MZs with 1.00 as normal. Since the equations assume that this number for DZs is actually .50, H2 is underestimated.

    I'm not sure if the numeric basis of the above is correct, but the general reasoning is like that.
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  2. @Ryan
    If Assortative mating is a confounding factor in analyzing heritability, would "assortative friending" be a confounding factor in analyzing what is called non-shared environment?

    would “assortative friending” be a confounding factor in analyzing what is called non-shared environment?

    Not so much non-shared environment, but purported peer effects, definitely.

    Read More
  3. @Ryan
    If Assortative mating is a confounding factor in analyzing heritability, would "assortative friending" be a confounding factor in analyzing what is called non-shared environment?

    Assortative mating makes H2 estimates smaller. This is because AM increases the genetic relatedness of DZs (for that trait) but not MZs (who cannot be more closely related). E.g. if there is .5 AM for the trait which is entirely genetic, then DZs will be related with a coefficient of .75 and MZs with 1.00 as normal. Since the equations assume that this number for DZs is actually .50, H2 is underestimated.

    I’m not sure if the numeric basis of the above is correct, but the general reasoning is like that.

    Read More
  4. I’ve heard that identical twins are not 100% identical due to tandem repeats, signaling noise, epigenetic changes, etc. I remember reading that MZs are like 85% identical. Could this explain some of the problem?

    Read More
    • Replies: @JayMan
    No, MZ twins are not genetically identical, but it's far more than 85%. More like 99.9999999%.
    , @helena
    The 'problem' for Liberals is that they simply don't believe that behaviour can be genetic or inherited.

    The 'problem' for 'The Wider Population' is, Lay Epidemiology - people see with their own eyes that children can be very different from their parents and that is because Statistics are Averages. Not all Individuals sit on the Line of Best Fit.

    [I do know that when I did my PhD, I asked a member of the Statistics Dept. what sample size I should use and he just Laughed at me and said 'No social survey is robust because the sample sizes needed for true accuracy are so large'. I don't know if that applies to heritability studies, presumably less so than to Attitudinal Surveys.]


    Jayman - I've been thinking a lot lately about the flower diagram. My impression is that society is turning towards Group B thinking and behaving, I assume because fewer people in The West nowadays have analytical thought structures. Business, for instance, is Group B friendly, it's not really an Analytical activity. Do you have any thoughts on this?

  5. @expeedee
    I've heard that identical twins are not 100% identical due to tandem repeats, signaling noise, epigenetic changes, etc. I remember reading that MZs are like 85% identical. Could this explain some of the problem?

    No, MZ twins are not genetically identical, but it’s far more than 85%. More like 99.9999999%.

    Read More
  6. @expeedee
    I've heard that identical twins are not 100% identical due to tandem repeats, signaling noise, epigenetic changes, etc. I remember reading that MZs are like 85% identical. Could this explain some of the problem?

    The ‘problem’ for Liberals is that they simply don’t believe that behaviour can be genetic or inherited.

    The ‘problem’ for ‘The Wider Population’ is, Lay Epidemiology – people see with their own eyes that children can be very different from their parents and that is because Statistics are Averages. Not all Individuals sit on the Line of Best Fit.

    [I do know that when I did my PhD, I asked a member of the Statistics Dept. what sample size I should use and he just Laughed at me and said 'No social survey is robust because the sample sizes needed for true accuracy are so large'. I don't know if that applies to heritability studies, presumably less so than to Attitudinal Surveys.]

    Jayman – I’ve been thinking a lot lately about the flower diagram. My impression is that society is turning towards Group B thinking and behaving, I assume because fewer people in The West nowadays have analytical thought structures. Business, for instance, is Group B friendly, it’s not really an Analytical activity. Do you have any thoughts on this?

    Read More
    • Replies: @JayMan

    Jayman – I’ve been thinking a lot lately about the flower diagram. My impression is that society is turning towards Group B thinking and behaving, I assume because fewer people in The West nowadays have analytical thought structures. Business, for instance, is Group B friendly, it’s not really an Analytical activity. Do you have any thoughts on this?
     
    The big question about that test is if it is clannish people in general or East Asian-specific. Only more tests of more peoples around the world will tell.
  7. @helena
    The 'problem' for Liberals is that they simply don't believe that behaviour can be genetic or inherited.

    The 'problem' for 'The Wider Population' is, Lay Epidemiology - people see with their own eyes that children can be very different from their parents and that is because Statistics are Averages. Not all Individuals sit on the Line of Best Fit.

    [I do know that when I did my PhD, I asked a member of the Statistics Dept. what sample size I should use and he just Laughed at me and said 'No social survey is robust because the sample sizes needed for true accuracy are so large'. I don't know if that applies to heritability studies, presumably less so than to Attitudinal Surveys.]


    Jayman - I've been thinking a lot lately about the flower diagram. My impression is that society is turning towards Group B thinking and behaving, I assume because fewer people in The West nowadays have analytical thought structures. Business, for instance, is Group B friendly, it's not really an Analytical activity. Do you have any thoughts on this?

    Jayman – I’ve been thinking a lot lately about the flower diagram. My impression is that society is turning towards Group B thinking and behaving, I assume because fewer people in The West nowadays have analytical thought structures. Business, for instance, is Group B friendly, it’s not really an Analytical activity. Do you have any thoughts on this?

    The big question about that test is if it is clannish people in general or East Asian-specific. Only more tests of more peoples around the world will tell.

    Read More
  8. @Santoculto
    I NO HAVE FREE WILL JAYMAN,
    I NO HAVE FREE WILL
    I NO HAVE FREE
    I NO HAVE
    I NO
    I

    http://vignette2.wikia.nocookie.net/rare/images/c/c8/Dixie_Running_Artwork_-_Donkey_Kong_Country_2.png/revision/latest?cb=20120424032446

    I COULD….

    Read More
  9. Reading this site for the first time I am struck by the twin studies quoted showing the overwhelming influence of genetics over nurture. These studies do, however, ignore the important influence of the environment from conception till birth and just after. It is now clear that epigenetics can have a major impact by determining which genes are switched on and so able to exert their full impact.

    Read More
    • Replies: @JayMan

    These studies do, however, ignore the important influence of the environment from conception till birth and just after.
     
    Not really. Standard twin studies can answer questions there too, showing, for example, that hormonal exposure in utero is in fact heritable.

    It is now clear that epigenetics can have a major impact by determining which genes are switched on and so able to exert their full impact.
     
    That's technically a tautology. What, exactly, do you mean by "epigenetics"?
  10. @David Cockburn
    Reading this site for the first time I am struck by the twin studies quoted showing the overwhelming influence of genetics over nurture. These studies do, however, ignore the important influence of the environment from conception till birth and just after. It is now clear that epigenetics can have a major impact by determining which genes are switched on and so able to exert their full impact.

    These studies do, however, ignore the important influence of the environment from conception till birth and just after.

    Not really. Standard twin studies can answer questions there too, showing, for example, that hormonal exposure in utero is in fact heritable.

    It is now clear that epigenetics can have a major impact by determining which genes are switched on and so able to exert their full impact.

    That’s technically a tautology. What, exactly, do you mean by “epigenetics”?

    Read More
  11. I was thinking specifically of the Dutch study showing children starved in the first trimester were more likely to develop metabolic syndrome that siblings which were not. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2579375/
    This was a demonstrable impact of environment on methylation of IGF2 leading to later consequences. It may well have been the case that some of these children were less impacted than others and that could have a genetic basis but the overall effect remains.

    Read More
    • Replies: @JayMan

    I was thinking specifically of the Dutch study showing children starved in the first trimester were more likely to develop metabolic syndrome that siblings which were not.
     
    Sure that's believable. That said, see Greg Cochran on it:

    https://www.youtube.com/watch?v=w3310KWlDXg&feature=youtube_gdata

  12. @David Cockburn
    I was thinking specifically of the Dutch study showing children starved in the first trimester were more likely to develop metabolic syndrome that siblings which were not. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2579375/
    This was a demonstrable impact of environment on methylation of IGF2 leading to later consequences. It may well have been the case that some of these children were less impacted than others and that could have a genetic basis but the overall effect remains.

    I was thinking specifically of the Dutch study showing children starved in the first trimester were more likely to develop metabolic syndrome that siblings which were not.

    Sure that’s believable. That said, see Greg Cochran on it:

    Read More
  13. I listened to Greg Cochran with interest and he accepts that whole body effects like starvation could have genetic impacts.
    I should have thought it was possible however to argue that the effect of the hunger winter was in fact to kill off all those fetuses which did not have the ‘hunger protection’ gene, thus selecting for fetuses which could survive starvation.
    In this case the doubling of schizophrenia would be an unfortunate linked effect.

    Read More
    • Replies: @JayMan

    I should have thought it was possible however to argue that the effect of the hunger winter was in fact to kill off all those fetuses which did not have the ‘hunger protection’ gene, thus selecting for fetuses which could survive starvation.
     
    Probably not. One generation of selection generally doesn't make much of a difference as per the breeder's equation.
  14. @David Cockburn
    I listened to Greg Cochran with interest and he accepts that whole body effects like starvation could have genetic impacts.
    I should have thought it was possible however to argue that the effect of the hunger winter was in fact to kill off all those fetuses which did not have the 'hunger protection' gene, thus selecting for fetuses which could survive starvation.
    In this case the doubling of schizophrenia would be an unfortunate linked effect.

    I should have thought it was possible however to argue that the effect of the hunger winter was in fact to kill off all those fetuses which did not have the ‘hunger protection’ gene, thus selecting for fetuses which could survive starvation.

    Probably not. One generation of selection generally doesn’t make much of a difference as per the breeder’s equation.

    Read More
  15. Hi Jayman,

    I followed link to an old comment of yours on Steve Hsu’s blog. With the user “ben_g” commenting saying that there is some study which looked into the effects of the shared environment on IQ, and posted a blog post of his discussing it. In particular:

    [S]hared environment shows a decrease from childhood (33%) to adolescence (18%) but remained at that modest level in young adulthood (16%).

    This of course is in direct contradiction to your often statement that c^2 = 0. Is it because the study only examined adults up to a certain age, so perhaps the shared environment effect would go to zero only at an even later age? But I remember the plots for IQ showing c^2–>0 at about 12 years of age. Or is there some flaw in the Hawthorn study that I’m just not seeing? You never replied to him over at Steve’s blog, so I apologize in advance if this is something you’ve already covered before. But I’d appreciate it if you could comment and clarify this seeming contradiction here.

    Read More
    • Replies: @JayMan
    The Polderman et al 2015 mega-meta-analysis of all twin studies can give some idea here. First, some basic caveats. It's important to know that there will be study-to-study variation, both because of random error and systematic measurement issues. Meta-analyses can correct for the former (along with check for publication bias) but can't adjust for the latter.

    In any case, a review of the findings for twin studies on intelligence can be seen here, on their interactive page.

    http://match.ctglab.nl/#/

    It shows that the C component for cognitive abilities is very small in adulthood.

    Of course, this doesn't take into account measurement error or the effect of assortative mating (which can inflate the C term at the expense of A).

  16. @reezy
    Hi Jayman,

    I followed link to an old comment of yours on Steve Hsu's blog. With the user "ben_g" commenting saying that there is some study which looked into the effects of the shared environment on IQ, and posted a blog post of his discussing it. In particular:


    [S]hared environment shows a decrease from childhood (33%) to adolescence (18%) but remained at that modest level in young adulthood (16%).
     
    This of course is in direct contradiction to your often statement that c^2 = 0. Is it because the study only examined adults up to a certain age, so perhaps the shared environment effect would go to zero only at an even later age? But I remember the plots for IQ showing c^2-->0 at about 12 years of age. Or is there some flaw in the Hawthorn study that I'm just not seeing? You never replied to him over at Steve's blog, so I apologize in advance if this is something you've already covered before. But I'd appreciate it if you could comment and clarify this seeming contradiction here.

    The Polderman et al 2015 mega-meta-analysis of all twin studies can give some idea here. First, some basic caveats. It’s important to know that there will be study-to-study variation, both because of random error and systematic measurement issues. Meta-analyses can correct for the former (along with check for publication bias) but can’t adjust for the latter.

    In any case, a review of the findings for twin studies on intelligence can be seen here, on their interactive page.

    http://match.ctglab.nl/#/

    It shows that the C component for cognitive abilities is very small in adulthood.

    Of course, this doesn’t take into account measurement error or the effect of assortative mating (which can inflate the C term at the expense of A).

    Read More
    • Replies: @reezy
    Thanks for the clarification. Will definitely read over the meta-analysis study you linked.
  17. @JayMan
    The Polderman et al 2015 mega-meta-analysis of all twin studies can give some idea here. First, some basic caveats. It's important to know that there will be study-to-study variation, both because of random error and systematic measurement issues. Meta-analyses can correct for the former (along with check for publication bias) but can't adjust for the latter.

    In any case, a review of the findings for twin studies on intelligence can be seen here, on their interactive page.

    http://match.ctglab.nl/#/

    It shows that the C component for cognitive abilities is very small in adulthood.

    Of course, this doesn't take into account measurement error or the effect of assortative mating (which can inflate the C term at the expense of A).

    Thanks for the clarification. Will definitely read over the meta-analysis study you linked.

    Read More
  18. […] are heritable, then society is just a vector sum of a lot of individual personalities.” See my Behavioral Genetics Page for […]

    Read More
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