IQ heritability: what the existing literature says.
Adoption studies are also particularly interesting in assessing IQ heritability. That is, if environment/nurture is a leading explanatory variable for the variability in IQ scores, then one would expect that adoptive children’s IQs should converge towards the performance of any siblings residing in their adoptive households, including any genetic children of the adoptive parents or the adoptive parents themselves. The Texan adoptive study for instance, found that at age 8, the correlation between IQ scores between the child and adoptive parents was 0.13, compared to 0.32 as compared to their genetic mothers. When the children were re-tested by the age of 18, the IQ correlation coefficient with adoptive mothers and fathers had fallen to 0.02 and 0.10 respectively, while IQ correlation coefficients with biological mothers has increased to 0.48. The conclusions here are that genetics prevail over the long run, giving credence to the notion of IQ heritability.
I will write further on this topic in my next posting.