Anytime HBDers show commenter “Charteruse” evidence that IQ is substantially heritable, he dismisses it because it doesn’t show independent genetic effects.  In other words, twin studies and adoption research has more or less proven that that genes cause IQ within Western countries, but they have not yet proven that genes cause IQ in a wide range of environments.

In other words, if you score low on an IQ test, and then you read that IQ is highly genetic, you do not have to feel genetically inferior, because it could be that your genes only impair IQ in Western societies.  It could be that those same genes, if reared in the rain forest or the Arctic, with different foods and experiences, might make you incredibly brilliant.

Now if you have Down’s Syndrome, you would likely have an extremely low IQ in any environment, regardless of whether you were raised in the Arctic, the United States, or the jungles of Africa, but if you’re just a regular low IQ person, we just don’t know, because all the heritability studies have been confined to a small range of environments.  For example, when psychologists study identical twins raised apart, the twins and their co-twins were raised in the same country, often in the same town, so we don’t really know how highly their IQs would have correlated if they had been separated into radically different environments.

The father-child IQ correlation

It’s well known that the IQ correlation between fathers and sons is 0.45.  Part of the reason the correlation is this high is because assortative mating, where men mate with women of similar IQ, which maximizes the genetic similarity between parent and child.  I calculate that if men mated with women at random, the parent-child IQ correlation would drop to 0.3.

We know from adoption studies that how you were raised has almost no measurable effect on adult IQ within  the range of most American homes, so if men just donated their sperm to random women, and did not even meet their kids, the father-child IQ correlation would still be about 0.3.  A 0.3 correlation implies that the sperm of a man with an IQ of 145 will likely produce a kid that is 21 IQ points higher than the sperm of a man with IQ 75, even though the mother is chosen at random and neither she or the kid have any contact with him.

But what if we had a random sample of 1000 white American men donate their sperm and had their IQs tested, and then we paid a 1000 female San women living a hunter-gatherer life style in Africa to have their babies and raise them to adulthood.  If at adulthood, we tested the offspring using a version of the WAIS adapted to San culture, would we still find a 0.3 correlation between the offsprings’ IQs and the IQs of the American fathers they never knew, or would we find that genes that enhance IQ in America do not correlate with high IQ in this radically different environment, or worse still, correlate negatively?  Would the offspring of IQ 145 men still be 21 IQ points higher than the offspring of IQ 75 men?

If the father-child IQ correlation could transcend such radical differences in environment between father and child, then we could say there are independent genetic effects on IQ within the normal range of IQ variation, but if not, we would be forced to conclude that the high reported heritability of IQ is misleading.

Perhaps it would be more ethical to do the study the other way around.  Have random women in America paid to have and raise the baby of random San men.  Either way, this is a study that could easily be done.

Discussion

My own view is that the cognitive difference between humans and chimps are caused by independent genetic effects.  That is, in almost any environment, humans will be smarter than chimps.  I also think the IQ differences between the three largest most well established races (Negroids, Caucasoids, and Mongoloids) are caused by independent genetic effects, because these are very ancient and have had tens of thousands of years to evolve under many different kinds of circumstances.

I am not however 100% sure that the IQ differences within macro-races (East Asians > Native Americans or  Ashkenazi Jews > Whites > Arabs) are caused by independent genetic effects, because some of these differences supposedly evolved so so recently (Ashkenazi Jews > Whites) and do not appear to be corroborated by differences in brain size, so  more evidence is needed.

I’m also skeptical that the IQ differences between individuals are caused mostly by independent genetic effects.  As commenters Charteruse and Swank have noted, if IQ was highly selected during human evolution, then there should be relatively little genetic variation among humans (since all the low IQ genes were weeded out), especially humans of the same ethnic group.  We know that in any correlation, when there is extreme range restriction, correlations tend to shrink, so the correlation between IQ and genes should be small among humans.

That’s not to deny that the 10 IQ point difference between Ashkenazem and Whites is genetic and could not have evolved in the last 800 years, however it might imply that the difference is not an independent genetic effect.  In others words, if Jews and Gentiles were raised together in a radically different environment like the arctic or the jungle, the IQ gap might vanish, or even reverse, but no such studies have ever been done so we don’t know.

But it’s worth noting that genetic mutations that enhance IQ seem surprisingly rare.  For example, we’ve heard of genetic disorders that impair IQ or genetic disorders that enhance traits like height, but the evidence for genetic disorders that enhance IQ is slim.

If IQ enhancing mutations are rare, then evolution can only raise IQ by acting on existing genetic variation, and because there’s already been so much natural selection for high IQ in humans, such variation can not be that great.

 

 

 

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