A popular formula for estimating heritability (H^2) is as follows:

H^2 = 2[(correlation between MZ twins raised together) – (correlation between DZ twins raised together)]

Some have criticized the formula for assuming that MZ twins and DZ twins raised together have the same environment, but some of this criticism has been dismissed by blogger “Wax Empirical” who writes:

The vast majority of this research finds little to no evidence that twin studies are biased in this regard. Most twin studies have cited this research in support of what is known as the equal environment assumption or EEA for short. The equal environment assumption is perhaps a misnomer because it doesn’t mean that we assume that MZ twins share environments that are as similar as DZ twins. Instead, the equal environments assumption simply states that environmental similarity between twins does not have much of an impact on trait similarity.

One major way that MZ twins raised together differ from DZ twins raised together is that the former generally share the same placenta and the latter virtually always grew in different placentas. So even if one agrees that raised together twins of both types share the same cultural environment, the prenatal environments are clearly different.

As commenter Jorge Videla has mentioned, more sophisticated research done in Canada and France has studied identical twins grown in different placentas. I will focus on the Canadian research by Rose, Uchida, and Christian (1981) since it is based on adults, which is the age when heritability is maximized. A sample of 15 pairs of two placenta MZ twins and 28 DZ twins (all aged 20-44) were tested on Vocabulary (the most g loaded measure of Verbal IQ) and Block Design (the most g loaded measure of true Spatial IQ); all pairs presumably raised together.

The results showed that for Vocabulary, the two placenta MZ twin pairs correlated an astonishing 0.95, while the DZ twins correlated 0.55. Applying the formula cited at the start of this post, a heritability of 0.8 is obtained.

By contrast for Block Design, the two placenta MZ twin pairs correlated 0.48 and the fraternal twins correlated 0.44. Applying the same formula, a heritability of 0.08 is obtained.

I don’t know if the composite score of both subtests combined was obtained, but I guess we can crudely average the two heritabilies and guess that Full-scale IQ might have clocked in with a herirtability of only 0.44.

So maybe us Jensenists were wrong to think IQ has a sky high heritability of 0.8 in adults; only verbal IQ has such high heritability. But the sample size of this study is not huge, and the IQ test given was abbreviated (only two subtests) so only tentative conclusions are warranted.

But one thing HBD deniers are incapable of understanding is that relatively culture fair tests really do exist. HBD deniers make the mistake of thinking that because culture reduced tests are not strong measures of genes, they must be measuring culture. But what this placenta research shows is that it’s not differences in cultural environment that are driving down the heritability of culture “fair” tests like Block Design, but rather differences in prenatal environment. In other words, culture reduced tests really are measuring biological intelligence, not acquired cultural skills as critics claim, but biological IQ != genetic IQ.

This is why I have always favored Richard Lynn’s brilliant nutrition theory as the single biggest explanation for the Flynn effect. As Lynn brilliantly noted, it is the culture reduced tests of spatial reasoning that show the biggest Flynn effect and these just happen to be the same subtests that are more impaired by prenatal malnutrition. By contrast, the culture loaded measures of verbal knowledge show smaller Flynn effects, and seem unscathed by prenatal malnutrition.

But for decades, most experts in the field have ignored Lynn’s great insights and instead violated Occam’s razor by looking for cultural explanations for why culture reduced tests were showing large Flynn effects! Is that dumb or what?

To be sure, some culture reduced tests that require a lot of persistence and concentration (i.e. matrix reasoning) probably can be strongly influenced by cultural attitudes, but fun culture reduced tests involving playing with blocks and wooden puzzles, really are culture fair (assuming everyone understands the instructions and has basic motivation) in all but the most extreme of cases. But that doesn’t mean they’re necessarily heritable!

Now some folks might be getting excited at the possibility that adult IQ has a heritability of only 0.44 because it implies all the races are equally intelligent, right? Wrong! Races that have been separated for tens of thousands of years have had plenty of time to evolve significant genetic differences in IQ, even with such a low heritability. Instead a low heritability makes it even harder to close the genetic gap between races, because even generations of selective breeding will not bring low IQ races up to the level of high IQ races.

Sadly, it also means low IQ white trash might be able to feel genetically more intelligent than high IQ Ivy League minorities, because the IQ differences between them are environmental, while the racial differences between them might reflect genetic ability.

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