[lbo-talk] twin studies & IQ

[Miles Jackson]

Doug Henwood wrote:
> On Nov 21, 2007, at 9:17 PM, Miles Jackson wrote:
> 
> 
>>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.
> 
> 
> Again, that tells us that genetic makeup has an influence on IQ test  
> performance. It doesn't tell us anything about heritability.

I'm not sure if I understand the distinction.  If something is 
influenced by genetics, then it is heritable by definition.  (Genetic 
transmission is the basis of heritability.)


>>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.
> 
> 
> Significant? If identical twins have an r2 of .50, then what do non- 
> twin sibs have? Parents and children? .20? That's getting down into  
> so-what territory, even for first-degree relatives.

Here's the chain of reasoning: if genetics has nothing to do with IQ 
performance, then the correlation in IQ scores between biological 
parents and the children they have no contact with should be zero.  The 
observed correlation is around .20.  Yes, I agree that this is a 
relatively weak predictive relationship; however, it does exist.  This 
does provide some evidence for genetic influence.

Miles