It’s often been assumed that the best studies demonstrating the heritability of IQ are of identical twins raised apart, but it turns out these do nothing to silence the deniers of HBD (behavioral genetics). HBD deniers will argue that identical twins raised apart are not really raised apart because many grow up in the same towns or are raised by families of similar socio-economic status. You can try to counter these arguments but doing so becomes quite technical because you have to discuss esoteric concepts like correcting correlations for range restriction and distributional skew, and even if you do, some will still insist that environment can not be measured.
Thus a simpler way to demonstrate the heritability of IQ is to forget about identical twins and simply look at a study where people have been adopted from a very young age from a bad environment to a good one. If the adopted children are found to have IQ’s resembling non-adopted children from bad environments, then clearly IQ is highly genetic, but if adopted children are found to have IQ’s resembling non-adopted children from good environments, then clearly IQ is highly environmental. It’s so simple; even a child can understand this.
Now when such studies are done, they usually show that while genetics is important, the social environment also has a big impact, raising a child’s IQ sometimes more than 10 points beyond what it would have been had the child been raised by his biological parent(s). The problem is that virtually all of these studies involve children, and thus can not confirm or debunk the HBD claim that how you were raised has virtually no effect on your adult IQ for the vast majority of Americans (extreme exceptions are always plausible).
It is interesting that there are so few studies on the IQ’s of adopted adults despite the fascination with this topic among so many scholar and despite all the published studies on the IQ’s of adopted kids. My guess is that many scholars are deliberately studying only adopted children because these studies produce more politically correct results that advance their careers; they likely avoid studies of adopted adults because these would show IQ is highly genetic, and most academics (even tenured ones) are simply not allowed to say that.
Now the only study I could find where kids were adopted from a deprived environment to a privileged environment and then tested near-adulthood, was the Minnesota Trans-racial Adoption Study. In this study you had black kids (and kids from other backgrounds) adopted from a very young age into white upper-middle class environments. There can be no doubt that black Americans typically experience oppressed culturally deprived environments, particularly in the 1970s, and there can be no doubt that the black kids in the study were adopted into privileged environments (raised by professional whites with college degrees) so it’s interesting to see how much adoption raised their IQ’s beyond the national black average of about 85 (on the test used at that time). It’s also interesting to note how much the IQ’s of “half-black” children (who theoretically have a mean IQ of 94; half way between the black mean of 85 and the white mean of 102; U.S. norms) benefited from adoption. The children were tested first at age 7 and then at age 17. Because the IQ tests used had old norms which give inflated results, the scores were corrected for the Flynn effect (see the wikipedia article):
Non adopted, with two white biological parents (n=101):
Age 7 corrected IQ: 110.5, Age 17 corrected IQ: 105.5
Adopted, with two white biological parents (n=16):
Age 7 corrected IQ: 111.5; Age 17 corrected IQ: 101.5
Adopted, 1 white & 1 black biological parent (n=55):
Age 7 corrected IQ: 105.4; Age 17 corrected IQ: 93.2
Adopted, with two black biological parents (n=21):
Age 7 corrected IQ: 91.4; Age 17 corrected IQ: 83.7
So we see that age 7, adoption into a privileged home raised the IQ’s of black kids by 6 points above what you’d expect from black kids raised by their biological parents (IQ 91 instead of 85) and adoption raised the IQ’s of “half-black” kids by 11 points above what you’d expect (IQ 105 instead of 94) had they been raised by their biological parents. Even the IQ’s of white kids adopted into these privileged homes score about 10 points higher than they would have (IQ 112 instead of IQ 102) if raised by average white parents instead of the upper-middle class white parents who adopted them. So clearly, adoption has a sizable impact on childhood IQ, but notice how the effect completely vanishes in adulthood for all three demographics.
Now obviously this is only one study and the results need to be replicated before we can take it as gospel, but the fact that the results were internally replicated for three different adopted sub-samples (blacks, half-blacks, and whites) is very convincing to me.
With respect to race differences, the fact that the “half-black” kids scored intermediate between the white and black adopted kids is especially interesting, especially because the “half-black” kids had white biological mothers and then were raised in white adopted homes, so they had both a white prenatal environment as well as a white family environment, yet still scored at age 17 exactly as a purely genetic hypothesis would predict.
Although I find this study to be very convincing evidence of the truth of HBD, it’s important to emphasize that not only has it not been replicated, but it’s actually been contradicted by several other trans-racial adoption studies that did NOT find any correlation between IQ and race, however those studies had problems:
For example Tizard (1974) compared black, white and mixed-race kids raised in English residential nurseries and found that the only significant IQ difference favored the non-white kids. A problem with this study is that the children were extremely young (below age 5) and ethnic differences in maturation rates favor black kids. A bigger problem with this study is that the parents of the black kids appeared to be immigrants (African or West Indian) and immigrants are often hyper-selected for IQ (see Indian Americans).
A second study by Eyferth (1961) found that the biological illegitimate children of white German women had a mean IQ of 97.2 if the biological father was was a white soldier and 96.5 if the biological father was a black soldier (a trivial difference). Both the white and mixed kids were raised by their biological white mothers. One problem with this study is that the biological fathers of both races would have been screened to have similar IQ’s because at the time, only the highest scoring 97% of whites and highest scoring 70% of blacks passed the Army General Classification Test and were allowed to be U.S. soldiers. In addition, 20% to 25% of the “black fathers” were not African-American or even black Africans, but rather French North Africans (non-white caucasoids or “dark whites” as they are sometimes called). In addition, there was no follow-up to measure the adult IQ of the children.
A third study by Moore (1986) included a section where he looked at sub-samples of children adopted by white parents. He found that nine adopted kids with two black biological parents averaged 2 IQ points higher than 14 adopted kids with only one biological black parent but the sample size was quite small, and again, no follow up when the kids were older.