[lbo-talk] twin studies & IQ

[ravi]

On Nov 21, 2007, at 1:11 PM, Doug Henwood wrote:
> On Nov 20, 2007, at 2:40 PM, Miles Jackson wrote:
>
>> Based on twin and adoption studies, the heritability ratio
>> for IQ performance is .40 to .60, meaning about half of the
>> variation in
>> IQ scores can be attributed to genetic variation.
>
> Wait a minute. This says something about genetic composition and
> performance on IQ tests - specifically, people with identical
> chromosomes perform similarly, but well short of identically, on IQ
> tests. But what does it tell us about inheritance?
>

A good question. A couple of counter-arguments arise:

The genetic make-up of the twins is contributed two by two sets of  
alleles, not one, and traits such as analytical capabilities, even if  
genetic, cannot be determined by one unitary gene. The issue then  
becomes one of not just the correlation between the twins' IQ to that  
of their parents, but the comparison of each parent's IQ with the other.

As studies of embryonic development and learning have demonstrated,  
birth is not the beginning of the environmental influence on an  
organism. The shared embryonic developmental history of the twins is  
still a part of the environment (for instance, a recent study, that  
involved pregnant women in the Bronx who carried backpack monitors,  
found that there was a strong correlation between the level of  
pollutants and toxic material in the environment and the head size of  
the infants once born). Once again, the attempt to banish nurture from  
the equation, is premature.

	--ravi