Wednesday 29 January 2014

SES and heritability of intelligence


Many thanks to commenter Paige Harden for feeding in more recent, large sample studies on the heritability of intelligence and the influence of socio-economic status. To reiterate, socio-economic status conflates two sources of variance: genetic and environmental. Your status in society is usually a blend of your efforts and the opportunities open to you in a particular culture. It is not a pure measure of random allocation to occupations, nor even of capricious allocation, though in some non-democratic societies the way upwards is blocked for some groups, often for religious or clan reasons. In modern welfare-state open economies social status is much more likely to be affected by intelligence and personality. The one partial exception is that if you yourself have intelligence and diligence then you are likely to be wealthier than average, and can shield your children to some extent from the inevitable effects of regression to the mean. You can cushion their fall for a while, but others will rise above them.

In the papers below I have highlighted the sample sizes and the dates at which the original data were collected, since both are highly relevant to these sorts of investigations.

Childhood Socioeconomic Status Amplifies Genetic Effects on Adult Intelligence
Timothy C. Bates, Gary J. Lewis, and Alexander Weiss (2013)
Studies of intelligence in children reveal significantly higher heritability among groups with high socioeconomic status (SES) than among groups with low SES. These interaction effects, however, have not been examined in adults, when between-families environmental effects are reduced. Using 1,702 adult twins (aged 24–84) for whom intelligence assessment data were available (data collected 2004), we tested for interactions between childhood SES and genetic effects, between-families environmental effects, and unique environmental effects. Higher SES was associated with higher mean intelligence scores. Moreover, the magnitude of genetic influences on intelligence was proportional to SES. By contrast, environmental influences were constant. These results suggest that rather than setting lower and upper bounds on intelligence, genes multiply environmental inputs that support intellectual growth. This mechanism implies that increasing SES may raise average intelligence but also magnifies individual differences in intelligence.

Comment: Notice that these authors report IQ means by SES (fig 2, page 4) which allows readers to understand what is going on. There is a gradual rise in intelligence with social class, but nothing major, though there would be significant impacts on social class representation at IQ 130.

Studies of early childhood tend to exaggerate the apparent effects of home life on intelligence, but measures taken in late adolescence show far less of an influence. Nonetheless, here is a study showing a genetic influence on intelligence by 2 years of age.

Emergence of a Gene x socioeconomic status interaction on infant mental ability between 10 months and 2 years. Tucker-Drob EM, et al. Psychol Sci. 2011 Jan;22(1):125-33. doi: 10.1177/0956797610392926. Epub 2010 Dec 17.

Recent research in behavioral genetics has found evidence for a Gene × Environment interaction on cognitive ability: Individual differences in cognitive ability among children raised in socioeconomically advantaged homes are primarily due to genes, whereas environmental factors are more influential for children from disadvantaged homes. We investigated the developmental origins of this interaction in a sample of 750 pairs of twins measured on the Bayley Short Form test of infant mental ability, once at age 10 months and again at age 2 years. A Gene × Environment interaction was evident on the longitudinal change in mental ability over the study period (2001-2003). At age 10 months, genes accounted for negligible variation in mental ability across all levels of socioeconomic status (SES). However, genetic influences emerged over the course of development, with larger genetic influences emerging for infants raised in higher-SES homes. At age 2 years, genes accounted for nearly 50% of the variation in mental ability of children raised in high-SES homes, but genes continued to account for negligible variation in mental ability of children raised in low-SES homes.


Finally, in middle to upper class 17 year olds Harden, Turkheimer and Loehlin (2007) have done a study on a larger sample of children (839 twin pairs).Behav Genet (2007) 37:273–283 DOI 10.1007/s10519-006-9113-4123
Genotype by Environment Interaction in Adolescents’ Cognitive Aptitude. K. Paige Harden Æ Eric Turkheimer Æ John C. Loehlin

Abstract In a replication of Turkheimer, Haley, Waldron, D’Onofrio, Gottesman II (2003, Socioeconomic status modifies heritability of IQ in young children.
Psychological Science, 14:623-628), we investigate genotype–environment (G · E) interaction in the cognitive aptitude of 839 twin pairs who completed the National Merit Scholastic Qualifying Test in 1962. Shared environmental influences were stronger for adolescents from poorer homes, while genetic influences were stronger for adolescents from more affluent homes. No significant differences were found
between parental income and parental education interaction effects. Results suggest that environmental differences between middle- to upper-class families influence the expression of genetic potential for intelligence, as has previously been suggested by
Bronfenbrenner and Ceci’s model (1994, Nature-nurture reconceptualized in developmental perspective: a bioecological model Psychological Review, 101:568-586

In terms of sample size this is a better reference than his 2003 paper.



Authors should not keep quoting Turkheimer (2003) as the final word on the subject of SES mitigating genetic effects on the of intelligence. Harden, Turkheimer and Loehlin (2007) is the better reference in terms of sample size, though the data collection happened 50 years ago.

To my mind the best study by far in terms of 1) sample size 2) contemporary data collection and 3) age sample (young adults being the “finished product” as far as establishing their own occupations and social status is concerned) is the Bates, Lewis, Weiss (2013) paper.

My thanks to my commenters and email correspondents. Can we try ensuring that authors include the Bates, Lewis and Weiss (2013) paper when they talk about the effect of socio-economic status? Also, can you remind them that SES is not a pure measure of social unfairness, but contains significant elements of intelligence and personality, which are themselves partially under genetic influence?


  1. I routinely encounter shocked faces whenever I argue in class at the IoE that social class is not purely an environmental variable, but is itself significantly determined by heritable factors such as intelligence. One would almost think I had let fly a particular foul and unpleasant-smelling belch. Clearly this represents a significant and unwelcome challenge to the suffocating soft-left consensus that percolates our disgusting Brutalist piece of architectural refuse like a plague-filled miasma. Yet the miasma leads to routine errors in published papers: the ghost of the sociologist's fallacy stalks most work done at the IoE that relates to intelligence, an unwelcome revenant.

    The mindset that refuses to see how individual choices and differences contribute to outcomes should by all rights have died 30 years. Yet here we are.

    1. I am sorry that the IOE seems to have a house style which brushes aside genetic factors. The architecture probably doesn't help, but the problem may be no more than cultural lag in academia. The genetic age has barely begun, and many people are having problems adjusting to it.

    2. Maybe they're a bit dim?

    3. I wonder if that mindset could be helpful on occasion. Suppose you wanted to use IQ in some study, but your audience hates and fears that variable. Could you instead use the fitted value (the predicted values) of a regression of IQ on outcomes variables such as income, education, no. of illegitimate children, etc., call that IQ proxy "Overall SES" and get the same results?

    4. Interesting. I had considered whether "predicted values" or "predicted inherent correction factor" might do the trick, but in the end decided to eschew such evasions.

  2. Thanks James! A key next step, in my opinion, is understanding what components of the "SES" amalgam raise IQ and how. If identified, they might turn out to be cheaper than we think to copy. Intuition suggests that it is parental education that turns genes into IQ variance, others (greg Duncan) suggest it is family income that drives up scores, i.e.

    Alternatively, exposure to school which bootstraps key skills like numeracy and reinforces the value of learning might be (a big part) of the answer.

    PS: People often forget (and don't cite) those who started this work: Sandra Scarr and David Rowe.
    PPS: "Turkheimer (2007)" is Harden, Turkheimer and Loehlin (2007).

    1. The Duncan Magnuson Brookings link is "not found". Can you suggest another, because a search does not throw up the paper.

    2. link is fine: just make sure not to copy the full stop at the end....

    3. Tim,

      In Sandra Scarr's, Bronfenbrenner, & Ceci's, and Turkheimer's model SES modifies heritability. What you found is quite different. You don't have a SES x h^2 interaction, but rather, as noted in the paper, a SES x A (genetic component) one. Yet, adding to the existing confusion, you go onto state: "This finding supports the bioecological model of intelligence, in which rich environmental support is posited to maximize genetic effects" Read Bronfenbrenner, & Ceci's: "Nature-nuture reconceptualized in developmental perspective: A bioecological model." Bioecological models are generally Environment x h^2 models, not Environment x A ones. Since you are not using "bioecological model" in the original way, it's not clear what you mean. Perhaps you were thinking of Jensen's never cited (1968) threshold hypothesis, a variant of Plomin's genetic amplification model, or even a COV(GE) model?

    4. Oops. I looked at the paper. You did find a positive SES x h^2 interaction. I was thinking of Hart, S. A., Soden, B., Johnson, W., Schatschneider, C., & Taylor, J. (2013). They interpreted their found increased genetic variance as support for a bioecological model, despite their ratio of genetic to phenotypic variance not increasing.

    5. @Anonymous Correct :-)

  3. Dear Timothy, Thanks for your comments. I have altered the post to include the full 2007 reference. I will try to post later on which of the components of childhood experience boost ability. I also agree that Sandra Scarr did a lot in this field (and David Rowe) and regret she is no longer researching. My current problem is to figure out which bits of the whole spectrum of experience actually makes a difference. More reading required on my part.

  4. Grant et al. did not find any SES-heritability interaction for g. Their study is based on a big American sample, so it's surprising that the results are so different from the Bates study. Everybody were of the same age (about 20) in the Grant study, while there's a big age range in Bates et al. I wonder if age somehow confounds the results.

    Turkheimer has recently written review article on this stuff.

  5. Stuart Ritchie write to me saying:

    Just to complicate things, here are four studies showing NO moderation of the heritability of cognitive abilities by socioeconomic status:

    Nagoshi & Johnson (2005):

    Kremen et al. (2005):

    van der Sluis et al. (2008):

    Hanscombe et al. (2012):

    My thanks to him and to all of you for these extra references.

  6. Intuition suggests that it is parental education that turns genes into IQ variance, others (greg Duncan) suggest it is family income that drives up scores, ...
    Alternatively, exposure to school which bootstraps key skills like numeracy and reinforces the value of learning might be (a big part) of the answer."

    These are three very different suggestions. The third works (I suppose) by the idea that learning and practising intellectual skills modifies the brain for the better. But how do parental education, or income, work their magic?

  7. I don't have a huge amount of time, so I can't leave a thorough response at the moment, but I may leave one later. In either case, these studies will certainly be featured in my upcoming post on the environment.

    My single biggest objection was, as you pointed out:

    "To reiterate, socio-economic status conflates two sources of variance: genetic and environmental. Your status in society is usually a blend of your efforts and the opportunities open to you in a particular culture. It is not a pure measure of random allocation to occupations, nor even of capricious allocation...In modern welfare-state open economies social status is much more likely to be affected by intelligence and personality."

    SES is itself a variable under partial genetic control. In broadest terms, the more you restrict the sample in question by heritable phenotype (SES here), the more homogenous your sample becomes genetically, and of course the environmental effect is going to appear to become more important. As I said elsewhere, studying gene-environment interactions is poisoned for that reason, because you could be (and likely are) looking at gene-gene interactions.

    An additional problem that works itself into the Bates study was the broad range of ages in the study. Bates did address this, but nonetheless, generational effects could contaminating the results here.

    As I suggested there, finding a purely g-loaded measure, say of reaction time, could help disentangle the result (this is because parental education/SES could be artificially inflating performance on the non-loaded parts of the IQ test).

    However, with older individuals considered, SES might truly be an important source of phenotypic variance, because in pre-welfare state times, malnutrition and other physical maladies one encountered in lower classes could have been a significant factor. This would reduce the heritability estimates.

    Then there is plain sampling issues. The sample in these studies are too small to make reliable statements about potential interaction effects.

    Finally, as mentioned over at Steve Hsu's, Plomin did a study looking for gene-environment interactions, where he reviewed some of the literature on the matter. No such effect could be consistently found, and he didn't find one in his paper. A key problem, of course, is that his study was on children. Finding fluctuating gene vs shared environment contributions is to be expected.

  8. With the greatest possible respect to Dr Bates, it is his field and not mine, I think he is being a little premature. Considering all the literature in this field it doesn't seem that a clear, perhaps definitive is a better word, picture has emerged. Other people have made interesting points that complicate the SES/heritability issue. I just wanted to raise one more small possibility. There are papers that suggest the various ACE ratios vary between children of different IQ levels at different times, that is vary in the development period not the outcome. A paper by Angela Brant discuss the literature 'the developmental etiology of high iq (Behav Genet 2009 39) there is another similar paper but I can't seem to find it right now

    1. Dear Sarahj

      A recent paper of hers is "The Nature and Nurture of High IQ
      An Extended Sensitive Period for Intellectual Development" (2012) Abstract

      IQ predicts many measures of life success, as well as trajectories of brain development. Prolonged cortical thickening observed in individuals with high IQ might reflect an extended period of synaptogenesis and high environmental sensitivity or plasticity. We tested this hypothesis by examining the timing of changes in the magnitude of genetic and environmental influences on IQ as a function of IQ score. We found that individuals with high IQ show high environmental influence on IQ into adolescence (resembling younger children), whereas individuals with low IQ show high heritability of IQ in adolescence (resembling adults), a pattern consistent with an extended sensitive period for intellectual development in more-intelligent individuals. The pattern held across a cross-sectional sample of almost 11,000 twin pairs and a longitudinal sample of twins, biological siblings, and adoptive siblings.

      I don't think that this is necessarily in conflict with SES findings. However, it does back up the general finding that the picture becomes clearer if you measure effects in young adults. I will see if I can get others to comment on this issue.

  9. "We found that individuals with high IQ show high environmental influence on IQ into adolescence (resembling younger children), whereas individuals with low IQ show high heritability of IQ in adolescence (resembling adults) ..."

    Does that just mean that bright people keep developing intellectually for longer, whilst their dimmer brethren stop developing sooner? Or is there some distinct point emerging?

    1. What I was trying to suggest is that its well known that the ratio between genetic/environment factors changes as children grow into adults and that change is basically an increase in genetic vs environmental variance. It has been suggested that at any given age, high vs low IQ children, might develop slightly differently and thus (at that point) have greater of lesser G/E ratios. I raised this as a possibility because the orginal paper by Eric Turkheimer uses essentially the ratio differences between high and low SES children to discover the effect. Now I was not suggesting that this might explain away SES/IQ heritability issues. I just think that there are a number of unresolved issues in this area and I wanted to suggest another, simply possible, factor that might explain some of the variance. When the original study was published Turkhiemer believed that it functioned as a cutoff effect but he now argues it is better understood as a gradient. In some ways I am happier with the former rather than the latter. Although if the latter is true it's great for society in general.