[lbo-talk] twin studies & IQ

[Miles Jackson]

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? That's a parent- 
> child relation, not a sib-sib relation. What's the correlation  
> between parental and child IQs?

One typical comparison is between fraternal twins (50% genetic sim) and 
identical twins (100% genetic sim).  In each case, we're talking about 
same age sibs growing up in the same family environment at the same 
time. IQ performance tends to be more similar for the identical twins.

In the adoption studies, there is a significant correlation in IQ 
performance between biological parents and children they have put up for 
adoption and never interacted with.  As enthusiastic as I am for 
sociostructural explanations, I have a hard time explaining this without 
granting that genetics has some impact on IQ performance.  I rush to add 
that this is not evidence in favor of the dubious claim that 
"intelligence is genetically determined".

Miles