Heritability of behavioral traits

By Razib Khan | June 28, 2012 12:48 am

As a father the content of my conversations with friends and acquaintances has changed somewhat. Whereas in my offline life discussions of behavior genetics rarely came up, now they loom large implicitly and explicitly. Though the vast majority of people I interact with have graduate degrees or are pursuing graduate degrees in the life sciences almost none of them are aware of the magnitude of the heritability of most bio-behavioral traits.

For those of you who forgot, heritability is a population wide statistic which assesses the proportion of variation in the population you can attribute to heritable genetic variation. So if heritability is 1.0 all of the variation is due genetic variation; offspring are just a linear combination of their parents. If heritability is ~0.0, then there’s basically no correlation between parents and offspring. Though, as I said, heritability is a population-wide statistic, it can be informative on an individual level. For example, the heritabiilty of height is ~0.90 in the Western world. To give you a sense of the expected height of the offspring of two individuals, just take the average (in sex-controlled standard deviation units) and shift it back toward the mean by 10%. There is going to be a lot of variation around this average. The rule of thumb seems to be that the standard deviation across siblings is roughly similar to the standard deviation within the population (though it seems to be a bit lower, with sibling I.Q. deviations being 2/3 of the magnitude of population-wide deviations).

Below the fold is a table reproduced from the paper Genetic Influence on Human Psychological Traits A Survey. Please do not read the table as a gauge of the “geneticness” of the trait. (whatever that means) Rather, it should give you a rough sense of the “pull” that biological inheritance will have on an individual. Biology may not be destiny, but it is definitely probability.

CATEGORIZED UNDER: Behavior Genetics
MORE ABOUT: Behavior Genetics
  • Thomas

    Razib, why might the heritability for intelligence increase with increasing age? Could this be in part due to more sophisticated testing data available as kids develop, or is this a more general trend for that trait?

  • Emma

    I love “Biology may not be destiny, but it is definitely probability”. A very neat summary!

    When teaching undergrads about heritability in humans, I observed that they usually found the high heritability of most behavioural traits both very surprising (and I think that they often were not convinced) and morally problematic, whereas the high heritability of physical traits is readily accepted.

    It is not clear for me what causes this problem specifically with behaviour : is it because behaviour is perceived as very plastic and thus “could not possibly be genetically determined”? Or is it because people feel that the fact that our behaviour is (partly) genetically determined undermines the possibility of free will?

  • http://Sjespositoweblog.blogspot.com S.J. Esposito

    Emma, for whatever it’s worth, I’d be totally shocked if the majority of undergraduates who object to a genetic basis of behavior do so on te grounds of it undermining free will. I’d stake my money on it being the former of the reasons you’ve stated. I think it’s very easy for people to refuse behavioral genetics because many people are under the assumption that they can simply change their behavior however they wish at any given time–of course, this is far and away an over simplification.

  • Karl Zimmerman

    2 -

    I disagree, I think it’s mostly about free will, even if it’s not recognized consciously.

    Human consciousness is not a decision-maker, but a storyteller and a rationalizer. We like to set up narratives where we are the masters of our own fate, and we like the mythos that we have a lot of independent agency.

    I think outside of undergrads who take a certain line of thought in the social sciences very seriously, people are almost as repelled by the mid-century blank slate idea that we lack free will due to environmental issues as genetic ones, and conditioning could make us into virtually anything.

    The predominant reason many people react with horror, or at least disbelief, when it comes to child-rearing is because it implies that parents lack agency over the lives of their children – that our “free will” in terms of childrearing is largely for naught, unless you do something to horribly mess up your child like raise them in a Skinnerbox.

    Personally, I find it mostly comforting, since I like myself and I have no issue with my daughter’s life turning out pretty much like mine. There are a few personality traits my daughter seems to have inherited from her mother which I can’t say I’d choose (high neuroticism, possibly OCD tendencies), but I think it’s balanced by her seemingly high conscientiousness, which means it will be much easier for her to apply her intelligence practically than it was for me.

  • pconroy

    @1,

    When teaching undergrads about heritability in humans, I observed that they usually found the high heritability of most behavioural traits both very surprising (and I think that they often were not convinced) and morally problematic, whereas the high heritability of physical traits is readily accepted.

    The reason for this false belief is more likely that they have been indoctrinated since birth in Feminist/Marxist ideology…

  • JR

    Interesting, Karl, conscientiousness, “balanced . . . easier to apply intellegence.” It would be interesting to see whether individuals who scored highest in concientiousness, were less likely to be effected to predisposition to certain other inheritable behavioral traits. Do those with greater conscientiousness have greater will over genetic pull?

    Also, I found it interesting that genetics has a greater pull over one’s predisposition to Schizophrenia than it has over a predisposition to depression, panic disorder, and anxiety disorder.

  • marcel

    For those of you who forgot, heritability is a population wide statistic which assesses the proportion of variation in the population you can attribute to heritable genetic variation. So if heritability is 1.0 all of the variation is due genetic variation; offspring are just a linear combination of their parents. If heritability is ~0.0, then there’s basically no correlation between parents and offspring.

    2 questions about this.

    1) Is heritability necessarily genetic (or even epigenetic). In his Teaching Company course, “Biology and Human Behavior: The Neurological Origins of Individuality, 2nd Edition”, Sapolsky has an example (details of which I may make mistakes in recounting) in which because of prenatal influences, 1 part of a fetus’s brain develops so that the resulting individual is more anxious than the norm. When she in turn becomes pregnant, this leads her body to produce hormones that generate a similar effect on the fetus she is carrying. Heritability, but at least as Sapolsky presents it, not due to genetics, and I do not recall any mention of epigenetics; rather a difference in brain structure in one generation, due to environment, that reverberates down the generations.

    2) This is more nitpicky, but I am asking because I am not sure of the answer. Even if heritability is 0, does that mean that correlation between parents and offspring is 0? Can’t there be non-zero correlation due to environmental factors? If not, why not?

  • Emma

    > Karl Zimmerman and S.J. Esposito : yes I think the free will issue may play a role, even if it is probably not formulated in these terms! My impression is any amount of genetic determinism is frequently perceived as being as if something external to the self was imposing things on the self. (by the way, apologies for the clumsy english…)

    I was quite convinced by the way B. caplan sumarized the implications of behavioral genetics results regarding child rearing (in Selfish Reasons to Have More Kids) : that we have indeed little influence on how our children turn out, but that we have plenty of influence on the way they spend their childhood with us. So let’s enjoy the present with our kids and not focus too much on the (largely non influencable) futur!

  • https://plus.google.com/109962494182694679780/posts Razib Khan

    #1, the general idea is that as you age “random noise” tends to become less of an issue. for example, stupid and smart people sort environments through gene-environment correlation.

    #7, re: heritability, epigenetics. i’m pretty sure epigenetics is significant, but also overemphasized. second, heritability does take into account maternal effects (one reason some designs look only at fathers).

    re: correlation. heritability in the technical sense is the proportion which is genetically mediated. so yes, it “corrects” for non-zero correlation in its design (e.g., sibling design, who share the same environment).

  • gcochran

    Almost nobody pursuing higher degrees in the life sciences knows this, but every peasant did, back in the Middle Ages. I guess you can’t fight progress.

  • marcel

    Razib:

    Thank you for addressing my questions.

    Your responses have a near telegraphic style of abbreviation to them, so they are less clear to me than they would be if I knew more (but then I would probably not be asking these questions). Ta-Nehisi Coates has an approach that he calls “Talk to me like I’m stupid.” I’d rephrase that as “Talk to me like I’m ignorant,” (wihch in this area is truer than I would like, which is one reason that I follow this blog) and ask you to try again.

    For example, you wrote, “I’m pretty sure epigenetics is significant, but also overemphasized.” Do you mean in the example I mentioned, or generally? I suspect so, since in that example, I do not recall Sapolsky’s mentioning epigenetics. So you are saying that epigenetics is another significant but nevertheless overemphasized channel of heritability?

    2nd: “maternal effects” = prenatal environment?

    I had a 3rd question about “re: correlation…” but I think I cracked the code as I typed the question. “no correlation between parents and offspring” means no correlation between p’s. and o. over and above what can be explained by variation in environment, so in “it “corrects” for non-zero correlation in its design”, the word “it” refers to heritability studies.

    Thanks for claryifying the technical meaning of heritability. I was only aware of a more colloquial usage in which any correlation due to shared biology (as in the example I cited) is described as an inherited feature (and therefore, the feature is heritable).

  • https://plus.google.com/109962494182694679780/posts Razib Khan

    Do you mean in the example I mentioned, or generally? I suspect so, since in that example, I do not recall Sapolsky’s mentioning epigenetics. So you are saying that epigenetics is another significant but nevertheless overemphasized channel of heritability?

    epigenetics is now being promoted by some (mostly outside academia) as an anti-darwinian/central dogma neo-lamarckian dynamic. it is real, but i think it in relation to standard genetic inheritance it remains secondary. though this is an average, there are no doubt traits which have low heritability (probably more because of lack of genetic variation than low environmental noise i’d bet) where epigenetics matters more.

    2nd: “maternal effects” = prenatal environment?

    that’s the biggest maternal effect. there can be others (in many species fathers don’t live with/provide for offspring, so mother-offspring have a stronger correlation on many traits than father-offspring).

    means no correlation between p’s. and o. over and above what can be explained by variation in environment, so in “it “corrects” for non-zero correlation in its design”, the word “it” refers to heritability studies.

    the gist is that in theory the numbers above give you genetic heritability, heritability as understood in quant gen. it does capture cultural/environmental heritability. one way you can ascertain genetic heritability is to compare monozygotic vs. dizygotic twins. same environmental heritability, different genetic heritability. a new way, which confirms twin studies, is to compare siblings who differ genetically in their relationship (expected value = 0.5, but standard deviation is 0.03), and see how it predicts phenotypic difference.

  • Karl Zimmerman

    It should be clear, but when i replied, 3 was 2. I dunno how that first post came in. I guess the format picks up local time and sorts posts according to that, rather than a universal time?

    6 – I’m fairly sure this is in part because Schizophrenia, unlike the others, is really hard to over-diagnose. It’s pretty easy to see how neurotypical people going through bad times/stress might be wrongly pegged as depressed or anxious. There’s been a lot of discussion in the mainstream press recently how many children are diagnosed as bipolar, even though consensus is bipolar traits cannot be determined until adolescence.

    Razib -

    I understand what you mean with intelligence. Kids forced, for example, into “enriched” environments by well-meaning parents who are themselves not that bright will as adults not choose an enriched environment, and thus slowly forget what was drummed into them. Conversely, bright people raised in an environment where they were deprived will tend to glom onto learning later in life.

    One thing I do wonder, however, is if there is a 100% correlation between generalized intelligence and, for lack of a better way to describe it, enjoying learning?

    I can definitely see how a naturally curious stupid person could become averse to use this trait, due to their likelihood of being confused, or being embarrassed when their mistakes are pointed out to peers.

    On the other hand, I can easily see someone being smart and incurious. Indeed, I’ve met many such people – particularly those of moderately above-average intelligence who seem to look at smarts as mostly a tool to get a given job done, not something to otherwise be enjoyed. One would assume environment would play a much larger role in their ultimate intelligence.

  • https://plus.google.com/109962494182694679780/posts Razib Khan

    #13, i agree with your observations. i had a stupid friend in high school whose curiosity about science diminished over time because he had a difficult time grasping it eventually, and he was smart enough to know he had a difficult time. conversely, i know people who ‘test well’ who managed to pass coursework sleepwalking. hell, there were courses where i was like this on occasion :-) [e.g., molecular genetics!]

  • Euler

    Razib, #12:
    epigenetics is now being promoted by some (mostly outside academia) as an anti-darwinian/central dogma neo-lamarckian dynamic. it is real, but i think it in relation to standard genetic inheritance it remains secondary. though this is an average, there are no doubt traits which have low heritability (probably more because of lack of genetic variation than low environmental noise i’d bet) where epigenetics matters more

    As I understood it, epigenetics is important in humans for things like differentiating cells (e.g. making neurons different from muscles) and and responding to the environment, but as far as I knew, there are no well established cases where epigenetic changes are inherited, other than imprinting where the father and mother contribute different epigenetic patterns to the offspring. I know there is one study that looked at the effect of famine in Sweden and found patterns consistent with epigenetic inheritance, but isn’t it a bit early to conclude that these “neo lamarckian” effects in humans are real without qualification? I’m not aware of any other study that confirmed these effects, and can we be so sure there are not alternative explanations, like changes in parenting behavior due to the famine? I would be very interested if you knew of any other evidence for this.

  • Gil

    @15 I don’t have a citation onhand, but isn’t variable DNA methylation considered a well established example of epigenetic inheritance in humans?

  • Joseph Zavala

    I believe the documentary “Zeitgeist – Moving forward” explains in my opinion the best evidence for answering the questions discussed here. These noted Dr.s in this video explained their points of view backed with statistics to explain this issue. The info is at the very beginning of the video which can be viewed on You Tube. I found it quite compelling and made me look back at my life and why I am who I am….

  • Joseph Zavala

    On a side note on this subject: I am 6 feet tall but, I have small feet for someone my size. After looking back of my life as a child, I realized that it was because I used to wear shoes to small for my feet because they where what I liked but, they didn’t have the sized I actually should have worn. Because of this, I believe I stunted my growth and thus the smaller than normal feet size….

  • Euler

    @16
    There is genetic imprinting, but that is not a Lamarckian effect. In that case, each human has one chromosome from their father with a paternal imprinting pattern, and one from the mother with a maternal pattern. When they form gametes, these patterns are wiped away, and all the chromosomes get a paternal pattern in men, and all get a maternal pattern in women. It has nothing to do with any experiences of either parent, so there is no “inheritance of acquired characteristics”. The only change to the classical Mendelian pattern of inheritance this causes is that some traits can skip generations as a result of being silenced silenced epigenitically, but this depends only on the sequence of mothers and fathers the genes go through, and not on what happens during the organisms life.

  • Ed

    I definitely agree that intelligence is highly heritable. I do think this passage from Wiki is relevant though:

    “If the Flynn effect has ended in developed nations, then this may possibly allow national differences in IQ scores to diminish if the Flynn effect continues in nations with lower average national IQs.[5]
    Also, if the Flynn effect has ended for the majority in developed nations, it may still continue for minorities, especially for groups like immigrants where many may have received poor nutrition during early childhood or have had other disadvantages. A study in the Netherlands found that children to non-Western immigrants had improvements for g, educational achievements, and work proficiency compared to their parents although there were still remaining differences to ethnic Dutch.[16]
    There is a controversy regarding whether the US racial gap in IQ scores is converging. If that is the case then this may or may not be related to the Flynn effect. Rushton and Jensen argue against expecting the Flynn Effect to narrow the US black-white IQ gap since they see that gap as mostly genetic in origin and there is evidence from mathematical analyses that what causes the Flynn effect is different from what causes the black-white gap.[51] Flynn has replied that he never claimed that the Flynn effect has the same causes as the black-white gap but that it shows that environmental factors can create IQ differences of a magnitude similar to the gap.[52]
    The Flynn effect has also been part of the discussions regarding Spearman’s hypothesis, which states that differences in the g factor are the major source of differences between blacks and whites observed in many studies of race and intelligence.”
    http://en.wikipedia.org/wiki/Flynn_effect#The_rise_in_IQ

    Once you have equal (or even close to equal) environments the rest would be 100% genetic. Few would argue that Blacks are better athletes than Asians, yet when you even suggest that Asians might just be naturally more adept at math, it’s offensive. Of course there are athletic Asians and smart Black people though. You have to judge people as individuals.

  • http://jayman.blog.com/ JayMan

    @#20:

    A good way at looking at that aspect of the Flynn effect was described by Jason Malloy (on Gene Expression classic) a few years back:

    Based on the IQs of transracially adopted black children, Lynn decides that the 1 SD IQ difference of American blacks (same as in Britain and the Netherlands) is 100% genetic, given the lack of any convincing environmental theory or data for the gap. Based on this he decides that poor nutrition primarily is depressing the African (and mostly identical black Latin-American/Caribbean) IQ about 13 points. Indeed, incredulity that African IQ could be any lower than African-American IQ is belied by known drastic comparative disadvantages of Africans on variables known to affect IQ. These include things such as higher lead exposure (which can lead to IQ reductions of 4-7 points) and micronutrient deprivation, such as iodine deficiency (reductions of 10 points). Indeed, critics are incredulous over the wrong gap! – after all, it is the 15 points between American blacks and whites that is hard to account for, not the 15 points between American blacks and Africans.

    Few would deny that the average IQs of populations in impoverished Third World nations are lower than they would be if they didn’t live in the deprived environments that they do. What you see is these individuals benefiting from First World environments and their IQs actually reaching something closer to their genetic “potential” levels – e.g., sub-Saharan African immigrants to the First World (assuming that these immigrants were representative of their home populations, which they’re typically not) with an average IQ of 70 having children with an average IQ of 80-85. In other words, that these immigrants’ IQs improve in the First World doesn’t mean that they will one day match First World levels – and so far, they haven’t.

  • http://jayman.blog.com/ JayMan

    #2:

    It is not clear for me what causes this problem specifically with behaviour : is it because behaviour is perceived as very plastic and thus “could not possibly be genetically determined”? Or is it because people feel that the fact that our behaviour is (partly) genetically determined undermines the possibility of free will?

    I would say it’s both, but the free will component is definitely an important one. What’s so utterly ironic about this is that eliminating heritability doesn’t restore free will, because then you’re left with environmental determinism: e.g.: “I’m a criminal because I come from a broken home.” That your brain is shaped by experience wouldn’t make your actions any more “free”; you’re still just a slave to the results of the things that impressed upon you. See here, my post on free will.

  • https://plus.google.com/109962494182694679780/posts Razib Khan

    third world is a stupid term now. wanna bet that vietnamese won’t converge? or haven’t? but still technically “third world.” should just disaggregate regions, instead of using retarded catchalls like ‘global north’ and ‘global south.’ first world & third world are cold war artifacts, useful in their time, but relatively anachronistic now.

  • http://jayman.blog.com/ JayMan

    Razib, excellent post! I will certainly use this as a reference. Though I see a few interesting things here that seem like openings for future study (such as the heritability of political orientation and religiosity; there’s an interestingly high shared environment component).

  • http://entitledtoanopinion.wordpress.com/ TGGP

    Karl Smith’s theory for why heritability increases with age is that adults have more agency and can better follow their preferences for intellectual stimulation (or lack thereof). He has a similar theory for how obesity can be heritable and yet greatly increase in prevalence over a single generation.

  • Douglas Knight

    TGGP, does Karl Smith define “adult” as 12 years old? or does he not look at the data?

  • Emma

    >Jayman

    “I would say it’s both, but the free will component is definitely an important one. What’s so utterly ironic about this is that eliminating heritability doesn’t restore free will, because then you’re left with environmental determinism: e.g.: “I’m a criminal because I come from a broken home.” That your brain is shaped by experience wouldn’t make your actions any more “free”; you’re still just a slave to the results of the things that impressed upon you.”
    I totally agree. Furthermore, IMHO the debate on free will is a false one, the only problem being how to define exactly the agent that makes choices.

    Also, I think that the problem that people have with genetics strongly influencing behaviour is in large part caused by the fact that currently, the popular view on the subject, the one in novels and movies for example, is predominantly the Blank state view. So it is the default one for many poeple, the one that they find natural.

    “See here, my post on free will.” The link does not work for me, could you post it again?

  • Emma

    >TGGP
    “Karl Smith’s theory for why heritability increases with age is that adults have more agency and can better follow their preferences for intellectual stimulation (or lack thereof). He has a similar theory for how obesity can be heritable and yet greatly increase in prevalence over a single generation.”

    My own prefered explanation is that heritability of IQ increases with age but genetic variance does not (and therefore no environment x genotypes interactions are needed). The increase in heritability would just come “mathematically” from the disappearing with age of the shared env variance. My limited understanding of the actual way heritabilities numbers are calculated prevent me to realize if this explanation is possible or not.

  • Miguel Madeira

    “One thing I do wonder, however, is if there is a 100% correlation between generalized intelligence and, for lack of a better way to describe it, enjoying learning? ”

    “enjoying learning” is very similar to what is measured by “Big Five – Openess” and by “Psychological Interests – Investigative”. The fact that the hereditability of these 3 factors is very different probably will mean that the correlation between the 3 will not be much close to 1.

  • http://jayman.blog.com/ JayMan
  • Matt

    why might the heritability for intelligence increase with increasing age?

    Possibly a dumb explanation, but would it not simply be that children converge more with parents (in terms of percentile) at the age their parents were measured, as they themselves approach that same age (and thus decreasing heritability as old people)?

    Also I think I can recall Deary making the claim that increasing convergence with parents is because of the increased importance of much more heritable repair mechanisms over time (whereas initial growth/development is less heritable).

  • http://facebook.com/DocLongLegs Andrew Selvarasa

    Razib, would you be able to recommend any books, blogs or articles that get heavily into behavioural genetics? I own THE NURTURE ASSUMPTION (2009 edition) and NO TWO ALIKE, and they are fantastic books, but I would love something more heavy on statistics and research.

    I’m currently reading THE SCIENCE OF EVIL by Simon Baron-Cohen, and it is elaborate in its explanation of the spectrum of empathy (or lack thereof).

    Thanks!

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This blog is about evolution, genetics, genomics and their interstices. Please beware that comments are aggressively moderated. Uncivil or churlish comments will likely get you banned immediately, so make any contribution count!

About Razib Khan

I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. In relation to nationality I'm a American Northwesterner, in politics I'm a reactionary, and as for religion I have none (I'm an atheist). If you want to know more, see the links at http://www.razib.com

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