Genetics, Race and IQ

Brink Lindsey has an great article discussing race, genetics and IQ.  It's hard to excerpt, but here is a bit of it:

A study of twins by psychologist Eric Turkheimer and colleagues that similarly tracked parents' education, occupation, and income yielded especially striking results. Specifically, they found that the "heritability" of IQ - the degree to which IQ variations can be explained by genes - varies dramatically by socioeconomic class. Heritability among high-SES (socioeconomic status) kids was 0.72; in other words, genetic factors accounted for 72 percent of the variations in IQ, while shared environment accounted for only 15 percent. For low-SES kids, on the other hand, the relative influence of genes and environment was inverted: Estimated heritability was only 0.10, while shared environment explained 58 percent of IQ variations.

Turkheimer's findings make perfect sense once you recognize that IQ scores reflect some varying combination of differences in native ability and differences in opportunities. Among rich kids, good opportunities for developing the relevant cognitive skills are plentiful, so IQ differences are driven primarily by genetic factors. For less advantaged kids, though, test scores say more about the environmental deficits they face than they do about native ability.

I have been struggling to articulate my issues with IQ for a long time.  I have always been frustrated with the nature vs. nurture arguments on intelligence, because I have always thought the answer is both.  But Brink's article get's me thinking along the lines of this simple model:


In this model, intelligence is not a product that works straight out of the box, so to speak.  It's an engine with some inherent potential that requires a lot of fine-tuning and a long break-in period to reach that potential. Let's say in the US suburbs our kids have a development percentage of 0.9 (we have to leave room for future Flynn Effect -- it would be awesome if it turned out we were only at 0.5).  I assume education is an exponential rise to a limit, where early gains are easy but incremental gains at the margin are harder and harder to achieve.


If this is the case, then US suburban kids are probably pretty tightly clustered around that 0.9 (say from 0.88 to 0.92).  This cluster seems tight but again remember in an exponential rise to a limit, the effort and expense to take a kid from 0.88 to 0.92 might be very very large**.  In this situation, measured IQ is going to be driven mainly by genetics, with a wide bell curve in native intelligence dwarfing the effect of a much tighter bell curve around development.  Small improvements in educational development in this model both come at a high price and have little effect on measured IQ.

In a different sort of society, say in rural Mexico, kids might be much lower on the development scale, say around 0.6, due to cultural factors, educational opportunities, even diet.  In this case, large changes can occur in measured intelligence even from small changes in education (the steep part of the curve) and difference in education and development might be at least as important as the genetic contribution.

** Postscript:  Some may object that differences in education seem to be much larger than these in US schools, but we have to make sure we are talking about the same output.   Here we are solely talking about the ability to improve IQ as measured by IQ tests.  There are many other things education does than just polish native intelligence and cognitive ability.  It teaches skills.  For example, it teaches one to write.   I would agree that there are huge differences in schools in their ability to produce kids that can write good 5-paragraph essays, or complete a calculus problem, or understand how to analyze a historical document.


  1. Frink:

    The problem with this post is that IQ scores stabilize by the age of 8. The author makes the rather large and naturally biased leap that differences in socioeconomic status must translate into educational opportunities. Since post 2nd grade education has no impact on an individual's IQ score, perhaps there is another factor at play here.

  2. herdgadfly:

    A 2001 study conducted by Paul Thompson at UCLA showed that identical twins have almost identical brain functions - 95 to 100% of the time. Fraternal twins were the same 60 - 70 % of the time. That is very strong evidence for intelligence originating in our genes.

  3. WGR:

    And then here was this piece published in the Telegraph (UK) last week:
    It suggests that the very bright are no longer reproducing at the same rate as the less bright, causing a slow downward engrenage in average heritable IQ.

    Also, for what it's worth, I stumbled across the following chart doing an online search. I can't vouch for the accuracy of the data, but the author(s) claim it's a demographic/ethnic breakout of the 2009 PISA scores—the school achievement tests, administered by the OECD, for 15-year-olds in the 34 OECD countries (plus 31 other countries or regions). These are the scores we always hear about when people compare American schools/academic achievement to schools/academic achievement elsewhere in the world. It looks like many of our schools might be doing a pretty good job.

  4. gblack:

    That fits a bit better from the anecdotes I've heard. I remember hearing a story of a lady from a bad neighborhood that cleaned the houses of rich folks. She observed they read more and watched less TV. So she did the same and her kids succeeded. So many times it seems a little effort for the disadvantaged, equals success.

  5. jay:

    There's another selection factor too. On average, people with higher IQ and perhaps some other social and work skills tend to make more money. Neighborhoods often tend to stratify by income, leading to a higher percentage of high IQ people (and their offspring) in those neighborhoods. It should not be a surprise, then, that even without discrimination factors, schools in these areas would have higher performing student bodies. Children of successful people would tend to be more successful, influenced by both genetics and the home cultural environment.

  6. ErikTheRed:

    There is some recent research that suggest epigenetics (genes that are altered / re-coded in response to environmental forces, often up to the age of five or so) may play a meaningful role in IQ level - and that certain psychological / stress-related environmental factors can be damaging to IQ. (sorry, too lazy to dig up the links; Google is your friend).

  7. Craig Howard:

    The author of the cited article waltzes perilously close to the "IQ tests are racist" meme that got the analogies section removed from the SAT. He believes that different groups of people have different definitions of intelligence. Really? I'd have thought intelligence a pretty solid concept. Of course, I thought marriage was, too.

  8. skhpcola:

    Link for that, please. My (admittedly cursory) search of reputable sources could not find the evidence for your assertion.

  9. Frink:

    Try a google search. It will bring up numerous links that claim IQ stabilizes between 6-8, and is a good predictor for adult IQ.

  10. big bang dissertation:

    Very comprehensive thoughts about education. The way how you describe the kids learning development is such a good thing to remember because it ideally brings good aspects of education, and it needs to be determined by individual.