Recently I posted about a popular formula for estimating heritability (H^2):
H^2 = 2[(correlation between MZ twins raised together) – (correlation between DZ twins raised together)]
This formula assumes that since MZ twins share twice as many unique genes as DZ twins, then if a trait is heritable, MZ twins should correlate much more strongly than DZ twins do.
Implicit in this formula is the assumption that since MZ twins reared together and DZ twins reared together, both shared the same home, and were both born to the same mothers, then the only reason why MZ twins would be more similar in a trait is because they are more genetically similar. Thus the greater the similarity of MZ twins compared to DZ twins, the more heritable the trait is thought to be.
However a major flaw with this model is that MZ twins are not only more similar than DZ twins genetically, but at least before birth, they are more similar environmentally, because MZ twins typically grow in the same placenta while DZ twins almost never do.
One way to correct for this environmental confound is to compare the correlation between DZ twins with the correlation between MZ twins who grew in different placentas (dichorionic MZ twins).
I recently blogged about an IQ study that did exactly that and what they found was that on tests like vocabulary, even when using dichorionic MZ twins, you still get the sky high heritability estimates that traditional twin studies found. However on a culture reduced test of spatial reasoning (Block Design) heritability suddenly dropped to less than 10%.
However that study had a very small sample size, and commenter “Lion of the Judah-sphere” was immediately skeptical.
Well I recently found a similar study but with an absolutely enormous sample size, and the results are very different. In this study, when you take the difference between the correlations between dichorionic MZ twins and DZ twins (see table 2 at the end of chapter 3 of this document), and apply the formula cited at the start of this post, you get a heritability of 0.48 for vocabulary and an astonishing 0.8 for block design. For overall verbal IQ, you get a heritability of 0.72, and for overall non-verbal IQ, you get a heritability of 0.6, and for full-scale IQ, you get a heritability of 0.76.
And this study was conducted on children, so a study done on adults would be expected to yield even higher numbers.
Of course the formula I cited at the start of this post assumes the Phenotype = Genotype + Environment model so disputed by anti-hereditists, however the most formidable attack on twin studies is that they don’t control for prenatal factors. This criticism has now been rebutted, and it’s a great day to be a hereditist.