IQ variations across time and space : the why and wherefore?

Mind Hacks has two posts on IQ: one focusing on IQ variants across time and discussing Flynn effect and the other focusing on variation across space (different population groups!) and discussing variation in IQ of identical and fraternal twins and taking help of adoption studies with special focus on economic background of biological and adopted parents.

I’ll discuss the second posting first which is based on this NYT article.

This article mentions a few observations based on meta analysis of data related to twin studies and also a study of adopted children -raised either in environments (adopted homes) that are of same socio-economic status as that of their biological parents or in different socio-economic environments.

Some of these observations are (first six are from adoption studies and the seventh is from twin studies. :

  1. Children of well-off biological parents reared by poor/well -off adopted parents have Average IQ about 16 point higher than children of poor biological parents
  2. Children of well-off biological parents reared by well-off adopted parents had average IQ scores of 119.6
  3. Children of well-off biological parents reared by poor adopted parents had average IQ scores of 107.5 – 12 points lower
  4. Children of poor biological parents reared by well-off adopted parents had average IQ scores of 103.6
  5. Children of poor biological parents reared by well-off adopted parents had average IQ scores of 92.4
  6. In another study, the average I.Q. scores of youngsters (from an orphanage at ages 4-6)placed in well-to-do homes climbed more than 20 points, to 98 – a jump from borderline retardation to a whisker below average , when measured after 9 years of placement in the well-off home. That is a huge difference – a person with an I.Q. of couldn’t explain the rules of baseball, while an individual with a 98 I.Q. could actually manage a baseball team – and it can only be explained by pointing to variations in family circumstances.
  7. In a meta-analysis, it was found, that among the poorest families , for those twins raised in the poor families, the I.Q.’s of identical twins vary just as much as the I.Q’s of fraternal twins; while in rich families the IQ’s of Identical twins are more identical than is the case for the IQ differences in fraternal twins.

First let us discuss the Twin studies (observation 7). If some trait A is found to co-occur say 80% of the times in identical twins (which have identical genotype) that are raised apart and if the same trait A is only found to vary 40 % of the times in fraternal twins (that have only 50 % of genes in common) that are raised apart; then one can conclude that this trait A is highly heritable and genetics dependent, with environmental influence limited to say affecting only say 20 % variation in the trait.

The premise is that if it is conclusively proved that if two organisms (identical twins) which contain more similar genes (double the number in comparison to fraternal twins) than a control pair of organisms (the fraternal twins); and effect of environment is subtracted (by letting the two organisms live in dissimilar environments – one adopted, while the other in biological home atmosphere); and if it is found that some trait A is found to concur more in these organisms (identical twins) compared to the control pair (the fraternal twins), then that trait must have a genetic component and is heavily influenced by genetic factors as opposed to environmental factors. So far so good.

In the normal twin studies, the adopted twin generally belongs to the same socio-economic status as the one reared by the biological parents.

The normal observation that identical twins belonging to well-off/middle class families have IQ rates similar as compared to fraternal twins, thus indicates that for children from well-off background (biological/adopted), the IQ (observed phenotype) is mostly due to genetic factors (underlying genotype) and environmental factors are not a big determinant.

The paradoxical observation that identical twins belonging to poor families have IQ rates as varying as compared to fraternal twins, should indicate that for children from poor background (biological/adopted), the IQ (observed phenotype) is mostly due to environmental factors and genetic factors (the underlying genotype ) are not a big determinant.

How do we conciliate the two observations. The paradox becomes a non-issue when one shifts focus from either-or thinking in terms of gene-environment influences and moves towards an interactionist view point viz. Nature via nurture as outlined by Matt Riddley amongst others and using genotype-phenotype distinctions. As per this viewpoint, any genotype is a potentiality and only if proper environmental factors are available can it lead to the desired (adaptive) phenotype. In absence of the required environmental factors, the genotype may not lead to the phenotype or may lead to sub-optimal phenotype expression. In a typical example, a fish may not show the color that the genotype codes for, if the environment under which it is developing provides little incentive to exhibit that color for reproductive/survival fitness. In less dramatic example, one may have genotype for having a more than average height, but if proper nutrition during a critical phase of development is not provided, then that height may not be exhibited.

Returning back to our discussion, it is apparent that IQ , though highly heritable ( and being genotype based), remains as a potentiality and only if environmental factors ranging from nutrition to socio-economic factors resulting in environmental influences like number of words exposed during childhood, results in appropriate IQ scores and intelligence (observed phenotype) only when such environmental influences as measured by socio-economic atmosphere during childhood are present during critical stages of development. Thus, while children of and raised by well-off parents could have a high correlation between genotype and phenotype ( and thus show high correlation in IQ across identical twins vis-a-vis fraternal twins), the same would not be true for children of poor parents where environmental factors will limit the observed IQ scores ) and thus, though the genotype of identical twins is similar than fraternal in this case too, the variation would be greater as the genetic influence has been subdued by environmental (negative) influence.

Now to explain the first observation, viz. that children of biological parents have average IQ higher than children of poor/working parents can be explained by the fact, that as a group, the well-off parents would have higher IQ than poor/working parents- as intelligence would be one of the major factors predicting who would be well-off and who would be poor in a fair world. Thus, it is no surprise that their children, would also have higher intelligence as compared to poor children- as the rich parent’s child would on an average get better IQ genes than a poor children would get from its poor (and less IQ) parent.

The observations 2 and 3 taken together corroborate the fact that IQ flowers only under the right environment. When 2 child start with similar average IQ potentialities (as they are from well-off parents), they nevertheless end with different final exhibition of intelligence (as measured by IQ scores) based on the limiting influence of environment on the genes.

The observations 4 and 5 taken together yield to similar interpretations.

The observation 6 is a stark example of how providing a proper environment can lead to drastic improvements in the exhibited phenotype and lead to the phenotype attaining the maximum potentiality present in its genotype.

It is clear that affirmative action is needed to ensure that environmental influences do not lead to sub-optimal flowering of intelligence. These affirmative actions should be based on reducing poverty and focused on that alone. Other options like Mandal commission reservation of jobs (after the child has already got a sub-optimal IQ due to early socio-economic environment) are clearly counter-productive and unfair. Poverty is the only evil to be tackled.

Returning to the first post on Mind Hacks related to Flynn effect,based on this American Scientist article. To me, it seems apparent, that biological evolution is very slow in comparison to social and environmental evolution that we humans have managed to achieve. I believe that based on our current genotypes for intelligence, we have achieved a plateau in terms of providing the maximal socio-economic environmental conditions necessary for full flowering of intelligence. Thus, we seem to be reaching a plateau in terms of increases in IQ score from one generation to the other. The Flynn Effect, in my opinion, was not a change in genotype, but in exhibited phenotypes, due to availability of proper environmental conditions.

For IQ to change within generations due to underlying change in genotype is assuming heavy and continuous selection pressure on those genes responsible for IQ. I believe that IQ (and intelligence) would keep on improving, as it may be part of runaway selection due to other-sex mate preference (reproductive advantage) – like that of evolution of beauty – or peacock’s tail – or it may keep evolving as intelligence does confer survival advantage too, but such increases would not be as dramatically observable as the Flynn effect.

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3 thoughts on “IQ variations across time and space : the why and wherefore?

  1. Alvaro

    Sandy, you can check a very interesting research study on the impact of working memory training on IQ: Wajima, Kayo, and Sawaguchi, T. 2005. The effect of working memory training on general intelligence in children. Society for Neuroscience Abstracts. Abstract 772.11.

    I am trying to contact you to have a good chat. You can check what we do at


  2. Sandy G

    Hi Alvaro,

    You can write to me at sandygautamATyahooDOTcom and I would love to know more about you and your work. Unfortunately, I couldnt locate the SfN article/abstract on the web, but got to know the major findings of that stuudy by following a blogpost on the same.

  3. Evil Monkey

    I’m concerned that “genetic” effects could in fact be misattributed; nutritional quality in utero could very well account for the same patterns seen in these studies. Was that controlled for?

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