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From: hes@ecsvax.UUCP
Newsgroups: sci.bio,sci.misc
Subject: Re: Knowledge and the Academics (really heritability & IQ)
Message-ID: <3422@ecsvax.UUCP>
Date: Wed, 17-Jun-87 18:44:43 EDT
Article-I.D.: ecsvax.3422
Posted: Wed Jun 17 18:44:43 1987
Date-Received: Fri, 19-Jun-87 05:36:46 EDT
References: <16224@brahms.Berkeley.EDU> <160200002@inmet> <2172@mmintl.UUCP> <6566@diamond.BBN.COM>
Organization: NC State Univ.
Lines: 36
Xref: utgpu sci.bio:390 sci.misc:295
Summary: heritability - includes variation in genetic and environmental components

In article <6566@diamond.BBN.COM>, aweinste@Diamond.BBN.COM (Anders Weinstein) writes:
> As I understand it, "heritability" is a rather bogus statistic from which to
> draw political or sociological conclusions. Heritability indicates the extent
> to which genes account for variance *within a population*. By itself, it
> doesn't license any meaningful conclusions about the causes of differences
> *between* populations (which is what is usually at issue in IQ debates).

Many misunderstandings arise because the array of environmental conditions
are not stated.  The above is not wrong, but it does not deal with the
environment array.

> Example: Take two identical samples, A and B, of genetically diverse corn,
> and grow each in a controlled environment. Deprive one group, B, of some
> essential nutrient. Both populations will exhibit variation in height. In each
> group this is due entirely to genetics, so height is highly heritable in
> each. But the fact that the average height in B is much lower than that of A
> is *not* due to genetics; it is entirely environmentally caused.

But the two samples of corn are part of the same population.  The problem
actually is that the heritability statistic applies to a given array of
genotypes (ok, a population) *and* a given array of environmental conditions
(often confusingly called simply an "environment").  In the example the
heritability could have been calculated for an overall environment which 
included variation in the essential nutrient - and then the herit. would have
been smaller than calculated for either of the two environmental situations
alone, and it *would* be appropriate to consult it in trying to understand the
origin of the difference between the corn plants in the two groups.  In this
case the herit. value would be intermediate and it would tell one that the
difference could be genetic, environmental or some combination (which usually
is the case in real life.)

(In all the above when I say "array" of genotypes or environmental conditions,
I really mean to refer to a frequency distribution, so both the effects and
their relative degrees of occurence are both important.)

--henry schaffer  n c state univ