The heritability of IQ from an adoption study

September 13, 2021 • 11:15 am

One of the big hot-button issues at the intersection of science and politics is the degree to which human behavioral traits, particularly IQ, are based on genes. To be more specific, the one question that can meaningfully answered about genes and IQ is this one: What proportion of the variation among individuals in a population for any human trait (not just IQ, but also propensity to smoke, risk-taking, neuroticism, etc.) is based on variation among those individual’s genes?  The figure showing that degree of genetic determination of variation in a population is called the heritability of the trait, symbolized by  h², and ranges from 0% (or 0.0), meaning that none of the variation you see in the trait is based on variation in genes (it’s due to environmental factors) up to 100%, or 1, meaning that all the variation we see in the trait is due to variation among individuals in their genomes.

I don’t want to go back again and explain heritability, what it means, and the difficulties with measuring and interpreting it, because I did that in a recent post, and if you want to know more, you might want to read that one first.

That earlier post also summarizes a new practice of finding an individual’s “polygenic score” (PGS) for a trait by correlating the variation in many DNA markers in an individual with that individual’s trait of interest, usually as an adult. This gives you a way to suss out suggestive information about an individual’s future behavior when it’s just born, for you can sequence the DNA of a newborn and. combining that with previous data on the PGS correlation with, say, educational attainment (degree of school completed) to get an idea of how far that individual will go in school. This is of course not absolutely predictive, of course, and has only one possible useful consequence that I can think of—more about that later—but it’s also a way to locate genes that influence variation in a trait. (We now know of 1271 regions in the genome that are correlated with educational attainment; each of these, of course, can have only a tiny influence on variation in a population.) Right now, the correlation between the “polygenic score” of an individual and his/her educational attainment is about 14% (it varies between 12% and 18% in the study below).

The use of PGS values in studying human behavior was the subject of the New Yorker’s recent profile of behavioral geneticist Kathryn Harden, which is a very good piece.

These PGS correlations are much lower than the traditional heritabilities that measure the contribution of the genome as a whole—via correlation between the performance of relatives with known relatedness)—with heritabilities of human traits, which, as I noted in the previous post, often range between 25% and 70%. The heritability of IQ in the American white population, which (for reasons that are debated) increases with age, ranges from 40% to 75% for adults, but I’ll use a conservative general estimate of 50% or 0.5. That means that about half of the variation in IQ in the white population is due to variation among individuals in their genes. While everyone is interested in IQ because, in good American fashion, it’s substantially correlated with conventional measures of “success”, remember that there are lots of interesting traits whose genetic variation we’d like to know about—my previous post listed about 30.

Today’s paper, which you might be able to access by clicking on the screenshot below (or get the pdf here), is a new study of the heritability of IQ that uses adoption instead of the “twin studies” described in my previous post. Twin studies have small samples because they estimate heritability of IQ using either identical or fraternal twins reared apart or together, and the “apart” number is quite small. I’ll describe the adoption method below:

I’ll try to briefly summarize a complicated paper.

The principle is that if genes play a big role in variation of IQ, then if you take a child from its parent and have it brought up by an unrelated family, the IQ of the child, when it grows up, should correlate very closely with its biological parents and not very much with that of its adoptive parents. If, on the other hand, the correlation with the adoptive parents is quite high and with the biological parents low, we can guess that variation in the environment plays a much bigger role than variation in genes in IQ: in other words, the heritability is low. (This assumes that environmental factors that affect IQ are uncorrelated between biological and adoptive families, an assumption that was validated in this study.)

And these adoption studies exist. This one, in fact, was quite large. The researchers had access to 486 families who had given up a child for adoption or taken in one. 95% of the parents were of non-Hispanic white Caucasian ancestry, but 21% of the adopted kids were white, 66% Asian (probably inter-country adoptions), and 13% were of other ethnicities. This somewhat weakens the measurement of IQ as a statistic within a population, but it turns out that there was no difference in heritability whether you used the white or the Asian children.

The mean age of the adopted children was 4.7 months (oldest was 2 years old), assuring that there’s not much cultural influence on their intelligence before they’re adopted out. When older, these children, along with as many adoptive and biological parents who would assent, were given IQ tests. The adopted children were IQ tested at both 15 and around 32 years of age. The children’s IQ’s were then matched (when data from both parents existed) with the “midparent value” (average of mom and dad’s IQs, separated by “biological” or “adoptive” families), or with only one parent if that was all that was available.

The upshot is what you’d expect with a high heritability of IQ: the IQs of children at either 15 or in their 30s was much more highly correlated with the IQs of their biological parents than with their adoptive parents. Below are the charts showing the correlations with biological parents (left) and adoptive parents (right). Actually, they use “g” as the term for IQ, and they also have these data for a number of different sub-tests, which you can find in the paper.  The red lines are the regressions of offspring on parents at age 15, the blue lines for the 30-ish followup IQ tests. (Each dot is presumably one family.) The correlation of adults with their biological families (not the regression slope) is 0.42, a highly significant value, while with the adoptive parents it’s 0.10—close to zero but probably still significant (they don’t say), meaning there may be a slight rearing effect: the IQ of your adoptive parents could slightly affect the adoptive child.

(Caption from paper): Fig. 2. Scatter plots and associated regression lines for measures of cognitive ability g taken at intake and follow-up 3 for both biological (left panel) and adopted (right panel) offspring and their rearing parents. Intake measure of g is full-scale Wechsler IQ score, and follow-up 3 measure is ICAR-16 score. All parent-offspring pairs are included, which means that the data points are not independent. All values are standardized.

Now the figures given are the correlations, not the slopes of the midparent-offspring regression lines, which are the heritabilities—what we want to know. The authors have calculated that heritability, but in a slightly more complicated way, and I give it below for adult data, along with its equivalent for environmental effects: the proportion of variation in IQ due to variation in an adopted child’s rearing environment. Here are the figures (“CI” is the confidence interval).

Heritability of IQ was estimated to be 0.42 [95% CI 0.21, 0.64]

Proportion of variance in IQ attributable to environmentally mediated effects of parental IQs was estimated at .01 [95% CI 0.00, 0.02]

The heritability is close to the 50% figure I’ve taken to be a conservative value for IQ, and it’s still substantial: 42% of the variation in the population (although there are really more than one populations conflated here), is due to variation in genes.  The 95% confidence interval for IQ heritability is 21% to 64%, so the chances are 95% that the true value of heritability lies within that range. Ergo, IQ is pretty damn heritable.

In contrast, the proportion of variation due to parental environment (adoptive parents) is estimated atjust 1%, with a 95% confidence interval of 0% to 2%.  There seems to be a very small effect of rearing environment on IQ compared to genes.

Why do we want to know this? I’m running out of gas, as this paper was complicated, so I’ll devote a very short post to that either today or tomorrow.

h/t: Luana

26 thoughts on “The heritability of IQ from an adoption study

  1. One can argue from first principles that there must be a significant genetic contribution to intelligence. Begin by assuming that modern humans are more intelligent (almost regardless of definition) than our last common ancestor with chimps. The differences between humans and chimps can have evolved only if genes play a major role.

    1. Wait a minute, aren’t chimpanzees social animals? Surely, if we were to read to them every day from the texts of Judith Butler, make sure that they never, never engage in cultural appropriation, and arrange for the chimps to form little D/E/I committees of their own, wouldn’t they evolve culturally into hominids? Something like “the New Soviet Man”, that some once expected to evolve in a galaxy far away?

      Actually, what strikes me about the Figure is that, although there is significant correlation between scores of offspring and biological parents overall, the points show a lot of scatter. I wonder: to what extent does the scatter reflect the ambiguity of the g score, and to what extent does it reflect variations in life experience (which could be termed , in a broad sense, the environment)? The latter, of course, is so heterogeneous that it can scarcely be analyzed, let alone controlled—although we always
      hope to improve definable bits of it.

      1. … if we were to read to them every day from the texts of Judith Butler …

        What kind of monstrous sadomasochists do you take us for, Jon?

  2. One of my favorite hypotheticals is dropping 100 Ashkenazi infants into Equatorial Guinea while also dropping 100 Equatorial Guinean infants into Brookline, MA. You would be surprised at how many believe IQ would be indistinguishable from natives.

  3. The fact that differences in IQ are strongly influenced by genes likely won’t surprise readers here (I presume we have few readers from the blank-slateist, social-constructivist social “sciences”?). More surprising is that even things that we might assume are strongly influenced by parenting and environment are not. A good example is the body-mass index of adopted children. That shows a much stronger correlation with the body-mass index of their biological parents than with their adoptive parents (e.g. ref).

  4. You can only work with the raw materials that you have. Our genes are our raw materials.
    The idea that our environment can have some meaningful impact is humanity grasping at a belief that we are more significant than we really are.
    We like to think that if we change our environment – something over which we can have some control – then we can steer our human course and prospects.
    And yet if we nurture a child with good food and healthy living that child will grow taller and stronger.
    This variation in outcomes might suggest that some genetic characteristics are more malleable than others.

    1. It is worth recounting here what Dr. PCCe said in his previous post (linked to in this one);

      Second, heritability says very little about how much a trait can be changed, for genetics isn’t destiny. Yes, the heritability of height may be 70%, but that doesn’t mean that we can’t make people bigger or smaller by feeding them a lot of good food, injecting them with growth hormones, or starving them. When people object to measuring heritability of IQ, for instance, they often mistakenly think that because IQ has a sizable heritability, which it does, it therefore can’t be changed. But that’s bogus; there are many possible interventions that can affect IQ.

      (emphasis in original)

      1. Yes, Jerry’s comments on polygenic score (PGS) and outcomes such as educational attainment are relevant to what you say. Very importantly that while scores on IQ tests and the like might have a large genetic contribution (Jerry’s example: 50%), IQ related outcomes such as educational attainment have much less of one (Jerry’s example: 14%).

        Personally I’m hoping Part II will talk more about them. It seems to be intuitively correct to me: there were a lot of smart people in my grad and undergrad classes. Not all the smartest went to grad school, and not all the smartest succeeded when they did. PhD’s can often be as much about perseverance, work ethic, and the ability to consistently work towards a multi-year goal, as they can be about smarts. Sometimes the rabbit gets tired or bored, while the tortoise does not.

  5. Of course genes play a large role in the variability of intelligence. How else does one suppose that we evolved our relatively massive, energy intensive brains from smaller-brained hominid ancestors?

    The minute you invoke evolution by natural selection to explain a feature/trait, you are by definition saying that at least some of the variability in the evolved trait is heritable, and therefore genetic.

  6. Heritability is a time and place limited estimate. Evolution of X by natural selection implies there was genetic variation for X during the evolution of X; it does not imply that there is standing genetic variation for X now, after X has evolved. Indeed, it is often said that natural selection “consumes” genetic variation– reducing it. So, there is no necessity that a trait which has evolved by natural selection has a non-zero heritability now. Variation in the trait might be heritable now, but that must be demonstrated now.


  7. When we look at the fossil record of our bipedal ancestors, what we see is a pattern where smaller brained ancestors, associated with less sophisticated tool use (if any signs of tool use) are consistently replaced by bigger brained species with fancier tools. The implication as to what causes that is sort of chilling. We see similar things happening all the time in the modern age, where a species becomes displaced in an area by more competitive species.

    1. Is it so consistent? Though they weren’t our ancestors (and not even really not us), Neanderthals had bigger brains than we do. Maybe they don’t count though, as they aren’t our ancestors.

    1. There has been a lot of ink spilled studying IQ and stuff like wealth and crime, but I am not aware of much literature on links between executive function and crime rates/delinquency rates or behaviors like saving money. Given the amount of concern about economic inequality and crime and punishment, it would be very interesting to look at the role executive function plays, which intuitively is presumably correlated with these kinds of behaviors.

  8. Most people recognize the blank slate is a problem scientifically, but when you start to see the paltry effect sizes for most environmental impacts on behavioral traits, it is very difficult to entertain a more sociological explanation for human differences. You don’t have to be a genetic determinist, but you can say “genetics are really important, ultimately we don’t know why but we can clearly rule out x.”

  9. Brave authors who will probably be on the receiving end of attempts to cancel them and their careers.

    If the takeaway numbers are 42% to genes, and 0.01% to adoptive parental effects, it is important to remember that it has been suggested for a long time that peers mediate environmental effects on IQ achieved far more strongly than the home environment, and presumably this is a good chunk of the missing ~58%, with some just being random chance, head injuries etc etc. Nonetheless, 42% is very significant, and the fact this figure is derived by a different method and yet still agrees with twin studies suggests it will turn out to be reliable. One might be forgiven for smiling a little at the memory, if my memory serves, that Murray and Herrenstein were castigated for saying, at a rough guess, heritability might be 50%.

  10. Together, these findings provide further evidence for the pre-dominance of genetic influences on adult intelligence over any other systematic source of variation.

    Many believe that the reason that the average African-American IQ score is lower than the average American white IQ score has to do with “systemic racism.” For purposes of this question let’s ignore the fact that no concrete current examples of systemic racism have been supplied by these people, or that no rational explanation is provided as to how the supplied examples would produce the effects attributed to them. Nevertheless, such a person might ask how the findings of this study show that the effect of systemic racism (an environmental influence) is less than the effect of the genetic influences. According to such a person, systemic racism is like lead in the water of black people stunting them intellectually. If the average child in such an environment ends up with an IQ that is 15 points lower than it would otherwise be, that is probably a greater effect than the difference in IQ between children and adopted parents that this study attributes to birth parents.

    Maybe this is just a restatement of the point made in the earlier post describing difficulties with measuring and interpreting heritability that:

    …measuring heritability within a group tells us very little, if anything, about the genetic basis of difference among groups. That’s because there may be environmental differences between groups that affect the character in profound ways and make extrapolations from one group to another useless.

    In fact, apparently the adopted children in this study were biologically 95% of non-Hispanic white European ancestry, making it difficult to measure environmental influences not impacting them. What answer should be given to the person objecting to the conclusion that this study demonstrates that there exists a “pre-dominance of genetic influences on adult intelligence over any other systematic source of variation”?

    1. First of all, you didn’t read carefully; 95% of the adoptive PARENTS were white, not the kids, which were largely Asian.

      Second, this is a study of heritability within a population (well, white + Asian, but the Asians showed no difference from the whites). It again says nothing about the causes of IQ differences between groups.

      1. Thanks for the response. Given the inflammatory nature of the question of IQ heritability in today’s racial climate I would expect authors to be very careful when stating their conclusions in this area. The phrase “these findings provide further evidence for the pre-dominance of genetic influences on adult intelligence over any other systematic source of variation” seems carelessly overbroad and unnecessarily open to being misinterpreted by lay audiences. Am I wrong?

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