c60a-4er@tart7.berkeley.edu.UUCP (06/15/87)
A trap to beware of in any discussion of the heritability of IQ:
The definition of "heritability" commonly used by population geneticists
and such is not quite the same as the common meaning of the word. The
definition is this: heritability is the amount of the variation within
the population under consideration (with regard to a particular trait)
that can be attributed to heredity. The words "within the population
under consideration" are tremendously important. Within inbred mouse
strains, for example, size of body has a very low heritability--since the mice
are practically identical genetically, almost all the variation is due to
birth order, feeding, etc. However, in wild mice size is quite heritable.
The point is, that figure of 80% heritability of IQ means that within
the population under consideration, about 80% of the observed variation
appears to be genetic. It DOES NOT mean that, for example, no possible
environment will make more than a 20% difference in IQ--an obviously false
statement (consider massive brain damage as an environmental condition).
I realize that this sounds like a semantic quibble, but it's not.
It is very easy to draw nasty false conclusions from heritability statistics.
An example: mental retardation due to phenylketonuria was, until the
disease was understood, nearly 100% heritable--much more heritable than IQ.
However, appropriate treatment early in life will prevent such retardation.
Can we be sure that there is not some equivalent of "appropriate treatment"
(I don't mean drugs or surgery, I mean maybe better education and home environ-
ment) to benefit those with "heritably" lower IQ?
Mary K. Kuhner
"the dance of the nucleotides, the fourfold sarabande..."aweinste@Diamond.BBN.COM (Anders Weinstein) (06/15/87)
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). 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.
hes@ecsvax.UUCP (06/18/87)
A fairly readable, somewhat technical, and slightly dated reference is "Race Differences in Intelligence" by John C. Loehlin, Gardner Lindzey and J. N. Spuhler. 1975, W. H. Freeman and Co. It gives much background, definitions and surveys the literature. (Chap. 4, 27 pages, is on Heritibility.) Does anyone know if there is a newer edition? --henry schaffer n c state univ
hes@ecsvax.UUCP (06/18/87)
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
tim@ism780c.UUCP (Tim Smith) (06/26/87)
In article <16745@cca.CCA.COM> g-rh@CCA.CCA.COM.UUCP (Richard Harter) writes:
< Well now, the evidence is not nearly so strong as you claim.
< The principle results on identical twins were those of Cyril Burt's,
< which is where the conventional figure of 80% heritability comes from.
< However Burt's data and results were forged. (I don't know if this is
There was an article in a U.S. News and World Report a while back ( I
can't be more specific, since I read it in a waiting room somewhere )
on this. It said that a large study of twins and other siblings had
recently been completed ( I think they said that it was the largest
such study ever conducted ), and that the results were that such things
as intelligence and personality were very strongly determined by
heredity. For instance, introvert vs. extrovert was something like
70% determined by heredity, and intelligence was similar.
Does anyone have more information on this? Was the USN&WR article
accurate, or is this another case of a popular magazine screwing
up their science reporting?
Also, what does it mean to say that something is 70% determined
by heredity?
--
Tim Smith, Knowledgian {sdcrdcf,seismo}!ism780c!tim