A term I sometimes use on this blog is “genetic IQ”. An aggressively anti-HBD commenter on this blog named “Mugabe” feels the very idea of “genetic IQ” reveals how confused us HBD people are. I think it depends how HBD people define the term genetic IQ. If an HBD person defines genetic IQ (as I might have in the past) as the IQ people would have if everyone experienced the exact same environment, then this might show flawed thinking because it fails to consider the possibility that John might be smarter than Ted in environment A, but Ted might be smarter than John in environment B, so who has the higher genetic IQ? My answer might be whichever geneotype averages higher IQ phenotypes holding a wide range of environments constant, has the higher genetic IQ.
Mugabe has suggested that an ideal measure of heritability would be the IQ correlation between identical twins randomly assigned to different wombs and homes across the developed world. I think he’s on the right track, but if it were possible, I would do it very differently.
The ideal study (though obviously not practical) would be 100 groups of identical octuplets, where one member of each brood is randomly assigned to a womb and home in America, and each of her seven identical siblings are randomly assigned to wombs and homes across the entire world (not the developed world, only). At age 40, the 100 American assigned fetuses and each of their seven identical international siblings take a large battery of culture reduced IQ tests, and the IQs of the (now adult) American fetuses are correlated with the average of their seven siblings raised all over the world.
The average IQ of the seven clones would correlate extremely highly with the genetic IQ of each American because the seven identical siblings are raised in random environments around the World, so environmental effects, whether cultural, economic, prenatal, or nutritional, tend to cancel out when you average the IQs of the seven siblings. Thus knowing how well the IQ of Americans correlate with the average IQ of their seven international clones tells us how well IQ correlates with genetic IQ in America. Squaring this correlation might give us a much more meaningful measure of heritability than conventionally defined.
Now such an elaborate study might produce the same sky high heritability figures that conventional twin studies report, or it might produce a value much lower. It just depends on whether most of the genetic effects on IQ are dependent on the environment or independent genetic effects.