Educability-and-Group-Differences-1973-by-Arthur-Robert-Jensen

200 Educability and Group Differences
SE2 = * Tmz /(^ + tmz)2 + (1+rpz)2
1 ~ rnz V N
where rMZ and rDZ are the MZ and DZ twin correlations and N is
the total number of twin pairs (MZ+DZ pairs).
20. It is not entirely clear if Lederberg extends this alienation hypothesis
also to IQ and other ability differences. He writes:
‘intelligence’ undoubtedly does have a very large and relatively
simple genetic component. In fact, the genes are all too visible:
they control the color of the skin. In our present milieu, these
genes may lead a student with the highest intellectual potential to
turn his back on the hard work of learning physics, chemistry,
and mathematics (which will measure out as intelligence by
middle-class standards) in favor of black studies that he hopes
may meet his more urgent needs in other spheres. (Lederberg,
1969, p. 612)
21. The degree of assortative mating in any single generation affects
the total genetic variance in the population; positive assortative
mating increases the total genetic variance. The degree of assortative
mating, however, does not influence the within-family variance
(i.e., the difference among full siblings) in any one generation, so
that the proportion of within-family genetic variance is decreased
by positive assortative mating and the correlation among siblings
is increased. The additive genetic variance within families is
strictly a function of the heterozygosity (i.e., number of pairs of
dissimilar alleles) of each of the parents and not of their genotypic
similarity or dissimilarity. However, within-family genetic variance
for polygenic traits may be decreased after continued assortative
mating for several generations. (For a good discussion of the
quantitative genetics of assortative mating, see Crow and Felsenstein,
1968.)
22. Sitgreaves assumes h1 = 0-80 in both populations, and assumes a
true-score phenotypic variance for IQ of 200, so that (0-80) (200) =
160 as the genetic variance (VG) in each group.
23. It is not clear why Sitgreaves refers specifically to Negroes in the
South - perhaps to emphasize environmental disadvantages - but
the fact is that 15 IQ points is an underestimate of this group’s
deviation from the white national mean of 100. The best estimate
of Stanford-Binet IQ in Negro school children in the South gives a
mean of 80-7, SD = 12-4 (Kennedy, Van De Reit, & White, 1963).
24. Light and Smith obtain this 1 percent interaction by noting that
the median correlation between identical twins reared apart is 0-75,