Saturday, February 07, 2009

Skin color and IQ


I've copied the correlation table and the results from a 2008 study in Personality and Individual Differences (Templer, Donald. 45, 440-444). From the author's conclusion:
The finding that skin color shows substantial correlations with life expectancy, birth rate, and infant mortality confirms the importance of this variable, as originally identified by Templer and Arikawa (2006). It merits further research given the suggestion made by Jensen (2006) that pleiotropy(genes having more than one effect) may underlie both IQ and skin color. Skin color and IQ have been found to be correlated in several studies of people of mixed-race ancestry Jensen, 1998; Rushton, 2008; Rushton & Jensen,2005).

The nature of a latent common factor such as the r–K dimension requires inference and interpretation. Life history theory provides a central organizing theoretical principle for generating specific predictions as to which traits should be correlated and it suggests physiological processes such as brain functioning that provide a substrate (Jensen, 1998; Rushton, 2004). This latter approach was adopted by Figueredo et al. (2006) who proposed an integration that traced the development of a life history from genes to brain to the various associated components of the reproductive strategy. They explored the psychometrics and behavioral genetics of the K factor, and specific areas of the human brain, including the frontal lobes, amygdala, and hippocampus. They then applied life history theory to predict patterns of development within the brain that are either paedomorphic (i.e., development begins later, proceeds at a slower rate, and has an earlier cessation) or peramorphic (i.e., development begins early, proceeds at a faster rate, and has a later cessation). The theory of r–K reproductive strategies may eventually provide an evolutionary understanding of many seemingly disparate results in personality research.


67 comments:

  1. It merits further research given the suggestion made by Jensen (2006) that pleiotropy(genes having more than one effect) may underlie both IQ and skin color.

    4-6 genes control 90% of the variation in IQ (here are some of them, SLC24A5, SLC45A2, OCA2/HERC2/, KITLG and TRYP1). the best we know from the research on IQ loci is that likely the genes of largest effect don't control more than 1%, at most, of the variation each, within the population. i assume that you can connect the dots here. the genetic architecture of the two traits is radically different. rushton obviously can't make the connections since i've pointed this out to him over a year ago, so word should spread through the psychometric community (i've long abandoned any hope that that psychologists who consider their discipline a branch of biology would do anything more than superficially follow the human evolution & genetics).

    ReplyDelete
  2. 4-6 genes control 90% of the variation in IQ

    meant skin color. obviously.

    ReplyDelete
  3. Razib, your writing is so pithy that I'm having trouble chewing. Can you rephrase the last sentence in your first post?

    Am I correct in paraphrasing: the most important IQ-affecting genes control no more than 1% of the variation apiece?

    Is it just that one can't make simple statements about the correlation between a "many gene" trait and a "few gene" trait (I'm sure there are more precise terms for those)? That may be true for deep logical reasons, but I haven't worked out what they are. By asking of course I risk an even greater mouthful of pithiness. :)

    And then about the psychologists ... what were you hoping they would do? I haven't figured out which category you were putting Rushton in....

    ReplyDelete
  4. the most important IQ-affecting genes control no more than 1% of the variation apiece?

    yes, that's what the assumption is since they aren't popping up in association or linkage studies. 1% is the *high bound*. IQ is like height, likely lots of small effect genes. people having been looking for large effects for decades and simply not found them (see plomin et al).

    Is it just that one can't make simple statements about the correlation between a "many gene" trait and a "few gene" trait

    people see the correlation between skin color and IQ. then they logically wonder: perhaps genes x, y & z have an effect on *both* IQ & skin color in terms of variation within the population. so if you modulate gene x, you get darker skin and lower IQ, or the inverse. the two traits would correlate because of a common set of causal factors.

    that's the theory. but as it happens we pretty much know most of the genes that cause 90% of the variation in the skin color trait. there are fewer than 10 of large effect. so, for example, one gene, SLC24A5, is responsible for 30-40% of the skin color difference between europeans & africans (another gene, KITLG, might account for 25%, etc.). the reason these skin color genes popped up in association studies so quickly is that they have huge effects within the population. so we have

    1) pretty certain knowledge that a maximum of a dozen genes are probably responsible for more than 90% of the skin color human variation in world populations (different genes make east asians light than europeans, so there's duplication here, so a dozen is probably a good number, though probably 4 genes account for 3/4 of the variation).

    2) at a bare minimum it is likely that you will need 100 genes to account for 90% of IQ variation.

    i loaded the die on both counts, increasing the number of skin color genes as much as possible, and dropping the IQ genes number as much as i could (1% is a large effect QTL in IQ, so probably more like 500 or 1000 is reasonable). to make the math transparent, let's assume that 10 skin color genes account for some % of IQ variation. assuming that all 10 are huge effects, i.e., 1%, you can't really account for much of the IQ variation from skin color genes (no more than 10%). there just aren't enough of them. if the genetic architecture was similar between the two traits IQ genes would have popped up as quickly as skin color genes.

    And then about the psychologists ... what were you hoping they would do? I haven't figured out which category you were putting Rushton in....

    they should stop making this dumb causal speculation. rushton mooted that if perhaps skin color genes and IQ genes are in common that would explain the correlation in a forum where we are both members last year. i laid out the evidence above. he never responded. differential psychologists fancy themselves evolutionarily informed, but my personal experience is that they don't actually bother to follow the latest work on evolutionary genetics, and selectively import models to forward some pet theory they have. when i pointed out to rushton some problems with what he was doing he responded to the effect that he couldn't be bothered to keep up with the latest evolutionary genetics.

    shorter: psychologists should stick to psychology, not evolutionary theory. ignore all this life history crap they talk about, i'm sure they don't understand it.

    ReplyDelete
  5. shorter:

    sum(1000 variables of equal weight) = variance in trait X

    sum(10 variables of equal weight) = variance in trait Y

    the 10 variables in the latter are a subset of the 1000 former.

    correlation between X & Y = 0.9, so that 81% of the variation in one can explain the other.

    does this entail that that 81% is likely due to the 10 variables out of 1000?

    ReplyDelete
  6. people see the correlation between skin color and IQ. then they logically wonder: perhaps genes x, y & z have an effect on *both* IQ & skin color in terms of variation within the population... that's the theory.

    rushton mooted that if perhaps skin color genes and IQ genes are in common that would explain the correlation in a forum where we are both members last year.


    That is exactly what I didn't know. Such speculation is indeed odd ... I sort of thought correlation was enough for most people, and the causality angle (on this particular subject) was mostly a strawman. To wit, that the only people talking about "if skin color genes effect IQ" were the anti-psychometry types.

    This is, of course, totally my fault, since upon re-reading I see that Inductivist has already mentioned exactly this assertion by Rushton!

    My worry, of course, is that people are going to use your handy dismissal of the causality as a way of attacking the correlation. But I worry too much.

    ReplyDelete
  7. My worry, of course, is that people are going to use your handy dismissal of the causality as a way of attacking the correlation.

    scientists who are moderately familiar with the state-of-the-art in human evolutionary genomics will do exactly that. which is why it is best to not say such obviously uninformed things.

    ReplyDelete
  8. yes, that's what the assumption is since they aren't popping up in association or linkage studies. 1% is the *high bound*.

    Assuming these studies have been done on admixed populations.

    ReplyDelete
  9. Anonymous2:26 PM

    Nice post, kind of drawn out though. Really good subject matter though.

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    Do you even speak English? Seriously, wall of text crits me for 99999k.

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  11. IQ and skin color. It is clear that darker skin leading to very low vitamin D level in mothers blood has devastating effect on newborns brain.
    Check http://www.endphysics.com/burkabrain.html

    ReplyDelete
  12. Chinese are fair and do not cover their skin and eat fish (omega 3) Their written language also have an impact on IQ as shown in some studies.
    Check: http://www.endphysics.com/burkabrain.html

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