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Brain size and socioeconomic status

A new drug, Numarol, is currently being trialled which increases the surface area of the brain in children. Numarol causes children to have bigger brains, do better in cognitive tests and generally improves their life prospects. One critic of Numarol recently pointed out it would be very expensive, and only the rich would be able to afford it. Its release would likely create a significant difference in brain size between the highest and lowest socioeconomic groups. Numarol would create a world in which biological inequalities are forged from economic ones.   The rich would not only have bigger houses, better cars, and better healthcare than the poor, their children would also have bigger brains. Such a world would be abhorrent.

But we already live in this world.  Numarol is fictional, but the rich do have children with bigger brains than the poor.   Social inequalities have already been written into our biology. 

This is the lesson from one of largest studies of brain morphology and structure in children. Brains of children from the lowest income bracket — less than US$25,000 — had up to 6% less surface area than those from children whose parental income was more than US$150,000.   These differences in brain size where then found to be associated with differences in performance in a number of cognitive tests measuring working memory, vocabulary, and reading ability. These associations were independent of age, sex, parental education level and genetic ancestry (which was assessed through a whole genome analysis).

The relationship between family income and brain size was more pronounced among children in the poorest families where “income disparities of a few thousand dollars were associated with major differences in brain structure, particularly in areas associated with language and decision-making skills”. (1)

The mechanisms through which parental income affects brain size and development are not known. Environmental influences, like nutrition, parental engagement and stress in the household environment may play some role.  However a separate study found differences in brain size and structure even between one month old infants of different socioeconomic groups.(2) This is likely too soon for many environmental factors to have an effect. It could be that uncorrected-for genetic factors are responsible. On this view children in lower socioeconomic groups have smaller brains because they have inherited particular genetic variants from their parents. But this idea clashes with other data which shows that providing extra money to poor families has an immediate positive effect on cognitive development in children. (3) The fact that environmental interventions seem to mitigate the neurological differences between the rich and the poor suggests that the origin of this difference is not genetic.

Whatever the mechanism, if there is a direct causal relationship between parental income and cognitive development this has important implications for ethics and political philosophy. Low socioeconomic status may have far wider consequences than is currently recognised. When governments cut welfare privileges to the poor, this may not only harm the welfare recipient, but also their future children in very direct ways. Similarly if we want to increase equality of opportunity, we may need to look beyond increasing access to social services like good schools.  By the time the children from poor socioeconomic groups go to school, they may already be at a biological disadvantage. The game would already be rigged to favour the rich.

Most of us would be deeply troubled by a drug which was only available to the rich, and allowed them to give their children a biological advantage over the poor. The realisation that such an advantage already exists should be shocking. If we value equality of opportunity, we may need to place a much greater value on income equality.

1.   Reardon, Poverty shrinks brain from birth, Nature News, 30 March 2015,  <>

2. ibid

3. Fernald et al, Role of cash in conditional cash transfer programmes for child health, growth, and development: an analysis of Mexico’s Oportunidades, The Lancet, 371: 828–837 <>

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9 Comment on this post

  1. What if it were established that family income correlates with general intelligence and that providing money to poor families works only when the poor families are outside the economies of the developed countries? It certainly isn’t inconceivable that genetics predicts both intelligence and income. Then what? What are the ethical consequences of that sort of situation?

    1. Thanks for your response Russ. I think that would be ethically significant. If it were found that income inequalities do not have a material effect on intelligence in the next generation (because it’s purely genetic), this may reduce the disvalue we should place on such inequities. Conversely if it is robustly shown that one’s socioeconomic group is affecting intelligence in the next generation- this should increase this disvalue we place on income inequalities.

  2. Anthony Drinkwater

    Thanks, Chris.
    I like your Numarol example, but I don’t really think that the results of this study change much.
    Even if the conclusions turn out not to be robust, there is a plethora of other studies showing how inequality persists through generations, whether accompanied by physical changes in the brain or not.
    Odd how neuro-science imaging should be seen to be necessary to prove the point !
    (But I suppose that if it makes people think seriously about inequality for the first time, why not ?)

    1. Thanks for your response Anthony. I think the idea of young children being at a biological disadvantage to other children, before they reach school, just because their parents are poor may help people think more seriously about financial inequality. But on reflection I’m inclined to agree with you – it’s not clear why changes to the brain should be intrinsically important in this regard.

  3. I think the causality is really critical here. The way it looks to me is that over the last forty or so years there has been a huge amount of research on child development. Lots of middle class parents have sought this out and incorporated it into the way they bring up their kids. So have many parents in lower socioeconomic groups, but many parents on low incomes have not. This asymmetry in uptake of information has led to an asymmetry in child development by the time kids walk in the school gate for the first time. But note that no one has done anything wrong here. Good on the parents who have made best use of the available information about how to bring up bright kids. Other parents have chosen not to do so. You could argue that this creates some serious principal-agent problems (I think that is clear) but attempts to address these by chastising the poor often smack of middle class hectoring – I’m not sure there’s a general solution to this sort of principal-agent problem (if there is, no society has yet found it (the whole “nudge” thing is kind of promising, but not as promising as some people make out)). On this account your analogy that “Most of us would be deeply troubled by a drug which was only available to the rich, and allowed them to give their children a biological advantage over the poor” is mistaken. The drug is freely available, but poor parents obtain prescriptions for the drug at much lower rates than do middle class parents. Availability is not asymmetric; uptake is. And that makes a massive difference, morally, since there is no injustice involved.

    If – and I find this harder to believe – the uptick in assortative mating (which is usually supposed to have followed women’s increased participation in the labour force (this is unambiguously a good thing)) is beginning to have a negative externality in the form of an emerging difference in the genes of young kids, then I think this is more problematic. Again, it’s hard to see how this amounts to injustice – it’s more just an unintended consequence of legitimate individual* decision-making. You could argue that the state should try to do something about the unintended consequence, when it aggregates in a way that undermines social cohesion. But the state already does – most OECD countries (the USA is an exception) spend more per pupil on schools in poor areas than they do on schools in rich areas. (Reactionaries could argue that this preferential treatment is unjustified – the state should spend the same on every child – and (in light of the evidence Chris has assembled) inefficient, since the marginal gain from an additional dollar spent on a naturally bright kid will (probably) be higher than the marginal gain from an additional dollar spent on a less talented kid.)

    *I take it no one seriously believes the state should get involved in match-making…
    **Or you could argue that the state shouldn’t get involved in projects as untidy and political as “social cohesion”.

    1. Good points Dave. I agree that its important to get the causal picture right here. If its just due to information and parenting practices you are right but I find this hard to believe. An alternate hypothesis is that stress experienced by women while pregnant is having adverse effects on development in-utero. Hence there are adverse effects on development before the children are born (this explains why differences can be seen from one month). Another hypothesis is that children in poverty are actually inheriting epigenetic factors from their parents (again due to stress), which could be adversely affecting their development. In these latter cases in certainly seems like an injustice to me (like the drug case). I agree assortative mating seems unlikely

    2. “there is no injustice involved”
      No individual has set out with the intention to harm any other individual in that scenario, true. But there is still an injustice if causes completely out of the childs control or responsibility bring it about that the child has comparatively lower cognitive capacity as it starts out its life. Since each new child born into society has a justice claim against that society (and indirectly against all its citizens) to equal opportunity, a fair share of resources and social equality.

      1. Ed wrote: “Since each new child born into society has a justice claim against that society (and indirectly against all its citizens) to equal opportunity, a fair share of resources and social equality.”

        On the whole I just don’t see those things as matters of justice. I accept that equality of opportunity might well have a justice component, but it seems to me primarily a procedural requirement – treating everyone the same before the meritocratic sorting algorithm kicks in. But “fair share of resources” and “social equality” strike me as being less related to justice. (Plus they’re hopelessly fuzzy notions – contested as well as a bit apple pie.)

        Part of my dislike of the social justice bandwagon is that justice strikes me as an inherently divisive idea: it portrays issues as a matter of the righteous smiting the unrighteous. That alienates those who wind up – sometimes through no fault of their own* – on the side of the unrighteous. If (as I would argue) improving the lot of the poor is primarily a duty of benevolence or charity or kindness (etc) then you cast the middle classes as helpers, rather than villains. Not only is this, I think, a much simpler and better way to think about matters of equality, but it’s also much more likely to work.

        *As in the example of middle class parents above. And this sensation of being called unjust without having committed an injustice is both confusing, and (potentially) an injustice in itself (if you accept that calling people unjust when they haven’t done anything wrong is poor forum, justice-wise).

  4. One has to consider the following simple hypothesis.

    (a) There is a natural distribution of intelligence.
    (b) Smarter people on average make more money than less smart people.
    (c) Intelligence has a genetic component.

    Given this, one would expect there to be a correlation between lower income and lower intelligence (in an otherwise homogeneous population). The proviso says we leave out isolated populations that test poorly for intelligence until they are given a more Western education.

    Of course even if this holds, it talks only about averages, not individuals. We still want to give every child the best opportunity to grow and develop. We also want to encourage everyone, rich and poor, to adopt what is being called the “growth hypothesis,” the idea that hard work makes a significant difference in educational and financial success — and that native talent isn’t the only thing that matters.

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