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IQ heritability

Twins and IQ heritability
Twins are a good starting point in the evaluation of IQ heritability

I have previously written about IQ heritability. It is the age old question of nature versus nurture. There are a reasonably large number of studies which examines IQ heritability. While the existing body of literature refutes anti-genetics ideology, it also lends support to environmental effects as well. So the extant body of literature of IQ heritability also suggests that the environment plays an important part in explaining some of the IQ variances observed between groups. The most interesting studies that currently exist are kinship and adoptive studies. For example some of the kinship studies look at fraternal vs. identical twins, which represent a great starting point to establish IQ heritability, namely how differences in IQ scores may be explained by genetic and/or environmental factors. This is so because it is not unreasonable to assume that fraternal twins (who are born together, but share only 50% of genes – the same as any normal siblings) are likely to benefit from the same environment as they grow up. On this basis, comparing the variability of fraternal twins IQ scores with identical twins (who share 100% of the same genes) reveals important and undeniable evidence in respect of the debate. These studies have shown that 50% of IQ is likely to be heritable, and initial studies from the late 1970s have been confirmed over the last few decades.

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.

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