[Please post all off-topic comments in the most recent open thread.  They will not be posted here]

The famous Bouchard twin study found a potent 0.75 IQ correlation for MZ twins reared apart.  Note that the phenotype correlation for MZ twins reared apart is a direct estimate of broad sense heritability (H^2).

But even MZ twins raised apart may spend their early years together, grow up in similar homes, have contact in later life, and be self-selected for similarity.  For this reason, back in August 2014, commenter Mug of Pee cited a little known critic of twin studies named Susan Farber. Mug of Pee wrote:

Farber investigated and found the right figure for IQ’s h^2 in the US is more like 20%. http://www.nytimes.com/1981/03/01/books/nature-vs-nurture-a-natural-experiment.html

Clicking on the above NY Times article, the source for the 20% figure seems to be this paragraph (emphasis mine):

 Defining ”reared apart” poses another great difficulty for researchers of twins. Different studies have used different criteria – such as age of separation, frequency of encounters between the twins or knowledge of the other twin’s existence. Dr. Farber, in her original and synthesizing role, has turned this confusion into an advantage. She devised a mathematical index with which she could measure the degree of separateness and used this information to correct the correlations found between the I.Q. test scores of twins reared separately. So corrected, the calculated correlation between twins’ I.Q. scores fell from a modest degree of within-pair similarity (accounting for about one-half of the variance) to a much lower degree of similarity (accounting for one-fifth of the variance). In other words, on the average, the more separately the twins were reared, the greater the difference between their I.Q. scores.

Presumably, the statement “one-fifth of the variance” is where Mug of Pee got his 20% heritability statistic, but 20% seems to actually be a squaring of the correlation between MZ twins apart (to get the percentage of variance explained).  Taking the square root of 20% suggests that the corrected IQ correlation for MZ twins reared apart is 0.45.

This is much smaller than the 0.75 heritability found in the Bouchard study, but it’s still pretty high when you consider that heritability itself is a square of the genotype-phenotype correlation.  Thus square rooting 0.45 implies a 0.67 correlation between genotype and IQ (among people raised in random homes).

So even after one of the biggest critics of twin studies corrects the data in a very biased way (according to her critics) genotype still predicts IQ about as accurately as SAT scores do (at least in countries like the U.S.)

Advertisements