Cognitive Daily

I’m still on vacation, but I’ve got just enough time to pop in with a quick link to a nice discussion about the link between heredity and IQ. While some have argued that as much of 75 percent of the variability in IQ is hereditary, more recent research suggests a more complex interaction. The key, as always, are studies on adoption of identical twins:

Regardless of whether the adopting families were rich or poor… children whose biological parents were well-off had I.Q. scores averaging 16 points higher than those from working-class parents. Yet what is really remarkable is how big a difference the adopting families’ backgrounds made all the same. The average I.Q. of children from well-to-do parents who were placed with families from the same social stratum was 119.6. But when such infants were adopted by poor families, their average I.Q. was 107.5 — 12 points lower. The same holds true for children born into impoverished families: youngsters adopted by parents of similarly modest means had average I.Q.’s of 92.4, while the I.Q.’s of those placed with well-off parents averaged 103.6. These studies confirm that environment matters — the only, and crucial, difference between these children is the lives they have led.

Read the whole article. And don’t forget that IQ isn’t all that matters.

(via Kevin Drum)


  1. #1 small and gray
    July 26, 2006

    One thing to keep in mind is that, while the Turkheimer et al. study is very nice, it’s not quite as revolutionary as the NYT article makes it out to be. It’s been appreciated for a long time that there’s a much larger shared environmental component to IQ in children than in adults. What tends to happen is that as children near adulthood, the genetic contribution increases to 50-80%, and the shared environmental contribution decreases to almost nothing. Given that Turkheimer et al. studied 7 year-olds, their work fits nicely into this framework.

    What would shake up the literature quite a bit would be a reliable finding that a substantial gene x parental income interaction persists into adulthood. The NYT article points to one or two studies to this effect that were just published or are ongoing, but they’ve flown under the radar compared to the Turkheimer paper. It’ll be interesting to see how this latter literature develops.

  2. #2 Caledonian
    July 26, 2006

    It’s also worth keeping in mind that IQ isn’t affected by frontal lobe damage, although those areas of the brain are essential for complex thought. People with severe frontal damage act like “zombies” — but their IQs don’t drop.

    It follows that having a high IQ doesn’t guarantee effective thinking ability.

  3. #3 J. Todd Ormsbee
    July 26, 2006

    Just a question for clarification: So given the comments here and the article, if I’m understanding correcctly, the genetic or heritable contribution to intelligence would become statistically visible when comparing children of the same socio-economic (or environmental) conditions? And would it also turn out that the relative bell curves would have similar distributions, although the environmental factors would move them up and down the IQ scale?

    Another question, since IQ testing is so culturally dependent (I recently took an IQ test where some of the logic questions could only be answered by a North American anglophone), is there a way to test thinking cross culturally that provides reliable data for cross-cultural comparisons?

    And if such a test were available, would it also reveal comparable bell curves when adjusted for socio-economic/environmental factors?

  4. #4 adoptee
    August 15, 2006

    I was adopted when I was five years old and my IQ was supposedly 135. My family was middle class, but both my parents did not continue into higher education and my siblings who were ‘naturally born’ from my parents had a similar educational background. They were probably in the not ‘well to do’ category.

    Just curious, if this article is correct, does that mean my IQ would have been 16 points higher?

  5. #5 Joe Feagin
    August 18, 2006

    As a social scientist who has looked at various so-called “IQ tests” and related tests for 40 years, I do not understand how anyone can take them seriously as measures of “general intelligence” or “IQ.” They are very culture bound and culture specific. Anyone who examines them critically can see that they are only (and often mostly paper and pencil) measures of certain learned knowledge and skills. If properly named, “skills tests,” then many debates over IQ really become debates over inheritance of “learned skills abilities” or racial differences in “learned skills” abilities. “IQ” is a social (Ideological) construct of no use except to get people off in the wrong direction on many issues of human differences.

    Joe Feagin, Sociology, Texas

  6. #6 Caledonian
    August 20, 2006

    I’d like to see an example of these ‘culturally-biased’ test questions.

  7. #7 MK
    January 21, 2010

    “As a social scientist who has looked at various so-called “IQ tests” and related tests for 40 years, I do not understand how anyone can take them seriously as measures of “general intelligence” or “IQ.” They are very culture bound and culture specific.”

    The trouble with this argument is that there are many neurological correlates. See the paper by Paul Thompson & Jeremy Gray.

    Also, see this recent study by Thompson:

    “The UCLA researchers took the study a step further by comparing the white matter architecture of identical twins, who share almost all their DNA, and fraternal twins, who share only half. Results showed that the quality of the white matter is highly genetically determined, although the influence of genetics varies by brain area. According to the findings, about 85 percent of the variation in white matter in the parietal lobe, which is involved in mathematics, logic, and visual-spatial skills, can be attributed to genetics. But only about 45 percent of the variation in the temporal lobe, which plays a central role in learning and memory, appears to be inherited.”

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