16 June 2010

Ecology of Intelligence and Poverty

Filed under: Culture, Psychology — ktismatics @ 11:14 am

In a recent post I presented some demographic data pointing to college as a classist institution. The high and escalating price tag excludes kids who can’t afford it, preventing them from acquiring the sheepskin that grants access to higher-paying jobs. The ensuing discussion began to unpack the links between money, habitus, aptitude, education, and employment. This post looks at the relationship between parents’ socio-economic status, which is a composite measure of income and education, and kids’ intelligence/aptitude.

Empirical studies have consistently found that about half the differences in individuals’ intelligence can be attributed to genetic factors: bright parents tend to produce bright offspring. Parenting, in contrast, seems to exert surprisingly little effect on children’s intelligence. However, parents’ socio-economic status is empirically correlated with intelligence: on average, kids from more well-to-do families tend to score higher on standardized IQ and aptitude tests than do kids from poor families — see this graph. Can we infer that, on average, rich people are intrinsically brighter than poor people, resulting in brighter offspring who are better suited for higher education and more likely to benefit from it?

Psychologist Eric Turkheimer and his students/colleagues at the U. of Virginia (my doctoral alma mater) have conducted several studies exploring this question. Here I describe two of them, both involving statistical analyses of existing longitudinal databases collected from sets of twins.

In this study Turkheimer et al. evaluated the results of IQ tests taken at age 7 by identical (monozygotic) and fraternal (dizygotic) twins. The study estimated the proportion of differences in IQ accounted for by genotype (monozygotes share 100% of the same genes, dizygotes share 50%), shared environment (which consists largely of parenting — twins are raised by the same parents at the same time), and nonshared environment (other aspects of kids’ lives, including neighborhood, school, peers, parents’ social networks, etc. — and presumably also uncaused emergent intelligence if there is such a thing). Using some fancy multivariate structural modeling techniques, the researchers demonstrated that the relative importance of these three correlates of IQ varied as a function of socioeconomic status. For the highest-SES kids, genotype was by far the most powerful correlate of IQ, with neither shared nor unshared environment adding any predictive power. For the lowest-SES kids, both shared and nonshared environment were strongly and separately correlated with IQ, whereas genotype was unrelated to IQ. In other words, for low-SES kids IQ is almost entirely a function of environment, while for high-SES kids IQ is almost entirely a function of heredity. The researchers conclude:

“Additive models of linear and independent contributions of genes and environment to variations in intelligence cannot do justice to the complexity of the development of intelligence in children… [T]he developmental forces at work in poor environments are qualitatively different from those at work in adequate ones.”

  • “Genotype by Environment Interaction in Adolescents’ Cognitive Aptitude” (2006) — link available here

This study by Harden, Turkheimer & Loehlin replicated the 2003 study, using eleventh-grade twins’ SAT test scores as the measure of intellectual aptitude. Findings were similar to the prior study of 7-year-olds: the strong statistical impact of genetics on SAT scores appears only in the high-SES kids. In this study the shared environment accounted for practically none of the variance: this is consistent with other studies showing that whatever influence parenting has on younger children dissolves rapidly in adolescence. Unbundling the SES effect, the researchers found that the effect of parents’ income is stronger than their level of education. That is, kids’ genetic predisposition for academic aptitude is expressed more fully in rich families than in poor ones. The researchers conclude that their findings support a “social context as enhancement” hypothesis about genetic-environmental interaction:

“[D]ifferences among ‘normal’ environments are largely irrelevant for differences among children’s intelligence. Below a certain threshold of environmental quality, intelligence increases sharply with better environments… In contrast, above a certain threshold of environmental quality, the reaction plane is essentially flat: for any given genotype, better environments do not predict an increase in intelligence…, and for any given environment, genetic differences are equally well expressed… Thus any gene-environment interaction disappears above a threshold of environmental quality.”

These findings and their interpretations are fascinating in their own right. They unfold within a broader ecological model of development that’s more conceptually and methodologically sophisticated than traditional schemes in which nature, nurture, and culture are treated as separate and independent components that can simply be added together. There are practical implications as well, but I’ll leave those either to discussion or to a separate post.



  1. Earning ability is highly correlated with intelligence and IQ is strongly inherited. So it is only to be expected that dumb kids tend to be found in low-SES households, where they are begat by dumb parnets and inherit the parental low intelligence. It isn’t the low-SES that causes the dumb kids, it’s the low-IQ genes…

    Comment by Big Don — 16 June 2010 @ 6:43 pm

    • Well Big Don, I guess you didn’t understand my post — perhaps it was over your head?

      Comment by ktismatics — 16 June 2010 @ 6:58 pm

    • ‘Dumb parnets’ was worth the whole feces, I never heard such a hilarious phrase.

      Comment by Quantity of Butchness — 16 June 2010 @ 6:59 pm

    • I’m from a poor family but my sister just graduated from medical school and I have an extremely high GPA at a major University as senior chemistry major… I would like to know where you read that earning power is linked to intelligence because I’ve never heard that before in my life! In fact I think that you made that statement up and you don’t have any numbers to prove what you said because you’ve never read that from a credible source before…. and just for your information earning power is linked to opportunity more than anything else followed by contributions from intelligence and work ethic… for example just look a George W. Bush or Donald Trump… both of those men have high magnitude of earning power but they obviously don’t possess a high degree of intelligence so maybe their financial success has been a result of opportunity granted to them by their environments…. anyway your comments are extremely ill-informed and one dimensional and I suggest that in the future you look up these things called “facts” before making such ridiculous and declarative statements like the one that you posted here… it just might prevent you from looking so dumb in the future or from getting your butt handed to you by another 19 year old if you ever do decide to make another post in your lifetime…  also my parents are poor but just like my sister they aren’t dumb… what do you do??? Do you have two kids that are doctors because my parents have one doctor already (my sister) and unless I get hit by a bus before I graduate then I’m going to be one too!

      Comment by The Anti-Big Don (which implies that i have a functional brain...) — 31 May 2011 @ 2:20 pm

    • Congratulations to you and your sister, Anti. If you stop back I’d be curious to hear whether you plan to enter a subspecialty and, if so, whether you’re doing it out of interest or for the money. Doctors are generally smarter than average and make considerably more income than average, so there’s that counterbalance to the dumb politicians and tycoons.

      It’s long been the case in this country that people with college degrees earn considerably more in a lifetime than those without it, but are college educated people necessarily smarter? Increasingly college is accessible mostly to those who can afford it. As you say, Anti, “earning power is linked to opportunity more than anything else followed by contributions from intelligence.”

      So is wealth associated with intelligence? No, says this study. According to the data, intelligence is related, though not strongly, to income, but is unrelated to wealth.

      Comment by ktismatics — 31 May 2011 @ 7:53 pm

  2. The take away from the study seems to be that, under near ideal conditions, the playing field gets levelled at least to the extent that genetic potential (which varies in individuals) is free to express itself in everyone. I don’t know whether that’s encouraging or depressing, considering how far from ideal living conditions are for most people.

    But then, I should probably read the study in full before I try to extrapolate too much…

    Comment by anodyne lite — 16 June 2010 @ 6:48 pm

  3. Yes, that’s the gist, Anodyne. In the second study, the environmental variance didn’t decrease, which means that the increase in the proportion of variance accounted for by genetics was driven entirely by an increase in overall genetic variance. I.e., what you said: the genetic expression of intelligence is suppressed by poverty. Increases in income lift this suppression, so that latent genetic intellectual potentials became actualized. We could legitimately infer that those same low-SES kids who scored poorly on IQ and SAT in these studies would have done considerably better if their “parnets” had had more money. I don’t think it’s possible to determine the magnitude of the potential effect from the results reported in these articles, but the studies do throw some serious doubt on Big Don’s hypothesis.

    What I’m left wondering is how wide the critical window is for the expression of genetic intelligence factors. So, e.g., if you moved some poor high school kid into a residential high-end private college on full scholarship, would this economic upgrade allow the kid’s genetic IQ potential finally to kick in? Or is it too late? Now I don’t think one needs to be exceptionally bright to get a college degree, and I think money more than brainpower is the big obstacle blocking the path through college for most low-SES kids. Still, a little extra boost in latent IQ wouldn’t hurt.

    Comment by ktismatics — 16 June 2010 @ 9:34 pm

  4. It’s possible that the genetic capacity for doing well on IQ and SAT tests manifests itself differently in poor kids than in rich ones. This would assume that these tests measure a general but very plastic form of intelligence — sort of the equivalent of intellectual stem cells. This plastic intelligence would equip its impoverished possessor to adapt to life as a poor person differently from how this “plastic IQ” equips rich people. Maybe the sorts of abstract intellectual ability measured by IQ and SAT aren’t of much use for those kids whose material lacks require more concrete survival skills. Abstract intellect might even be harmful to survival, so it’s important for the organism to shut that innate capacity down. On average, poor children are smaller physically than rich children. This of course is because malnutrition stunts growth, but it’s also the case that big bodies continue to require more nutrition than small ones. The big-bodied poor are less likely to survive in a world with resources that can fuel only small bodies. Even if there is a genetically plastic intellectual capability, I suspect that concrete survival intelligence also requires the expression of some additional set of genes beyond those that are useful in answering IQ and SAT tests.

    General intelligence/aptitude tests aren’t culturally biased in the sense that the right answers for rich people are different from those for poor people. There really are right and wrong answers to the questions in these tests, independent of who comes up with the answers. Instead, the implication is that the kinds of questions being asked on these tests are biased. IQ/SAT tests constitute a representative sampling of a problem space that’s typically explored by rich people and their children but that’s alien territory for poor people.

    Empirically, differences in IQ/SAT among poor kids are correlated largely with differences in nonshared enviironment — neighborhoods, schools, peers, etc. This implies that better schools really do make a difference for the poor. For poor kids, good schools would be like protected artificial islands of abstract intellect floating on a sea where sheer survival dominates. For poor kids, attending such a school is like spending many hours a day, many days a year in a theme park. It’s an entertaining but unreal world. The kinds of things you can do on the artificial island, the kinds of skills you can master, are very different from those you need to survive in the sea that surrounds the island. To think that your island skills will help you swim is to entertain a potentially fatal delusion.

    For rich kids, schools and the abstract abilities they teach are just part of the ordinary world. The teachers are like their parents, the abilities they hone in school are the ones they’ll use themselves as rich grown-ups. You move from family to school to college to well-paid job as a series of ordinary life stages. In this world abstract intellect does give its bearer an ecological advantage: better school performance, access to better colleges and grad school which generate even more money and status, attractiveness and social access to high-IQ potential mates who can increase the potency of the intelligence genes you pass on to your children, and so on. For poor kids to thrive in the rich man’s world, he needs to rely not on the natural unfolding of genetic and socio-economic potential, but on artificial skills developed in an artificial environment, their innate genetic capabilities having remained unexpressed. It’s a harder row to hoe for sure.

    Of course now I’m way beyond the implications of Turkheimer’s research.

    Comment by ktismatics — 17 June 2010 @ 6:32 am

  5. There’s a paradoxical sort of mind-body thing going on here. One usually thinks of the rich as sufficiently buffered from bodily concerns that they can devote themselves to the life of the mind. However, it’s primarily the rich whose innate biological capacities for abstract thought kick in; i.e, the bodies of the rich immerse them the life of the mind. The poor who would excel in abstract thinking have to rely largely external non-bodily resources: schools, peers, will power…

    Comment by ktismatics — 17 June 2010 @ 8:54 am

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