Race-Related IQ

Toby Bradshaw toby at u.washington.edu
Fri Dec 2 14:12:10 EST 1994


In article <gfb1.27.2EDF4B76 at psu.edu>, Guy F. Barbato <gfb1 at psu.edu> wrote:

>Sorry, but you can reject the notion of 'heritability as being useless for any 
>purpose but selective breeding' all you want...it is nonetheless true.

Assertion does not constitute proof.  Perhaps you have more to offer.

>the concept of heritability is used to define the proportion of additive 
>genetic variance in a *population* for the sole purpose of defining *breeding 
>value*.

Perhaps you can point to a post where I, or anyone, mentioned narrow-sense
heritability.  For broad-sense heritability, additive variance is
irrelevant.

>to suggest that heritability is intrinsic or even relevant to 
>educational strategy is also fallacious.

Strawman.  Perhaps you can point to a post where I claimed it did.

>each child enters school with 
>certain attitudes and abilities determined by his family and environment, 
>and to some undetermined (and undeterminable) degree due to genetics. 
                               ^^^^^^^^^^^^^^
Why is this so?

>however the school tries to optimize the individual childs' content of 
>instruction to accomodate individual differences for some expected outcome.

That is the rosiest scenario I can imagine, and quite at variance
with my own experience with schools, but fantasy can be healthy :)

>if you still want to calculate heritabilities, great.  BUT remember that 
>heritability estimates are unreliable when environmental factors are 
>nonrandom across families, which presupposes genotype-environment 
>correlations and underestimates the environmental contribution to group 
>differences

Part of any good experimental design is to assure that as many of
the assumptions of the chosen analysis are met as well as possible.
Has this been done for heritability of human intelligence or IQ?
I have no idea.  Perhaps you can show, rather than assert, that
such an experiment is impossible.

>in fact, if one wants to pursue the concept of the inheritance or the genetic 
>nature of intelligence (IQ test-determined, or some other concept), it seems 
>to me that by focussing on heritability one completely ignores non-additive 
>gene action (i.e., dominance and epistasis).

If I were discussing narrow-sense hertiability, you might have a point.
You might want to re-read the earliest posts of mine in this thread.

>I could create a lovely just-so 
>story, revolving around the fitness value of intelligence, and therefore, most 
>of the genetic variation *should* be non-additive!!  Interestingly, twin 
>models used to calculate heritability of IQ specifically overestimate 
>dominance and epistasis.....Curiouser and curiouser.

Are you claiming that the twin models overestimate heritability due to
non-additivity, or that only the *proportion* of nonadditive genetic
variance is biased upward relative to the total genetic variance?  Unless
you're interested in breeding, the partitioning of *genetic* variance
components isn't necessary to determine the degree of genetic control
of a trait.

>ANYWAY, the genetics of individual differences (in humans) is an interesting 
>academic problem which is probable insolube without the randomization of 
>environments....or maybe a selection experiment   <ok...it was a bad joke>

You might want to check into the trisomy 21 literature, which seems
to explain individual differences in human intelligence
in a very limited sense, and for which negative selection is performed
routinely.

-Toby Bradshaw
toby at u.washington.edu




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