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

222 Educability and Group Differences
be used in this determination. Those blood groups which show
the largest differences in frequency between West Africans and
Caucasians are, of course, the more useful, and the best blood
groups for this purpose are those which are totally absent in one
or the other ancestral population. The Duffy blood group gene
Fya is frequently used because it is virtually absent in the West
Africans from whom the original slave populations were derived
and it occurs in over 40 percent of Caucasians. For this reason
Fya has been called the ‘Caucasian gene’. In this case M = NJC.
SKIN COLOR AND IQ
Skin color is inherited and Negro-Caucasian differences in skin
color are attributable most probably to only 3 to 4 pairs of genes
(Stern, 1970).1 Various genetic models for skin color postulate
anywhere from 2 to 8 loci to explain the distribution of skin colors
found in hybrid groups. Although skin color is definitely related
to degree of African-Caucasian admixture for the average of
groups having different degrees of admixture, skin color is not a
highly reliable index of Caucasian admixture in individuals
(Harrison et al., 1967; Stern, 1970). When so few genes are involved
in a characteristic, the individual variability of the characteristic
among persons having exactly the same ancestry is great. The
offspring of true mulattoes (who are the offspring of Caucasian
and African parents), for example, show a wide range of skin color
even within the same family. Estimates of the correlation of skin
color in Negroes with amount of Caucasian ancestry are about
0*30 to 0-40. Thus, in terms of measurement theory, where the
reliability of a measurement is the square of the correlation between
true score and the observed score, the reliability of skin color
(‘observed score’) as an index of Caucasian ancestry (‘true score’)
would be at most about 0-402 or 0*16. If now we hypothesize that
there is a correlation between Negroes’ IQs and the amount of
their Caucasian ancestry and that this correlation is slightly higher
than for skin color (since more genes are involved in intelligence),
say about 0-50 as an upper limit of the correlation, the reliability
of IQ as an index of Caucasian ancestry would be about 0-502 or
0-25. The highest correlation that can be obtained between two
measures is the square root of the product of their reliabilities.
So the highest correlation we could expect to find between IQ