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

Race Differences in Intelligence 183
had to estimate the proportions of MZ and DZ twins in her white
and Negro samples from the percentages of like-sex and unlike-sex
twin pairs. The MZ and DZ twin correlations, in turn, are based
on these estimates (along with the correlations for like and unlike
sex pairs) and are not the directly obtained correlations between
test scores for MZ and DZ twins. This estimation procedure used
by Scarr-Salapatek greatly enlarges the standard errors of the
estimated twin correlations and, of course, of the heritability
estimates derived therefrom. This serious statistical inadequacy of
the study is not emphasized in Scarr-Salapatek’s report, which
contains no statistical tests of significance of any of her h2 values,
few, if any, of which would have proven sufficiently reliable
statistically to permit rejection of the null hypothesis or of one or
another of the alternative hypotheses under consideration. For
this reason, the present account of Scarr-Salapatek’s findings refers
only to her particular sample and procedure. The study permits
no strong inferences concerning the heritability of IQ in white
and Negro populations. Unless such studies in the future are done
with sufficiently fastidious methodology and large enough samples
to permit strong statistical inference with respect to clearly
formulated models, the mounting data will serve merely as a kind
of Rorschach inkblot into which researchers project their particular
biases and discern any interpretation that suits their fancy. Hunches
may be gleaned, but hypotheses remain untested.
Scarr-Salapatek searched through the records of a total of
250,258 children from kindergarten to twelfth grade in the
Philadelphia schools and found 1,521 twin pairs (493 of opposite
sex and 1,028 of same sex). The racial distribution of the twin
sample was 36 percent white and 64 percent Negro. Groupadministered
IQ and scholastic achievement test scores were
available. Scarr-Salapatek’s heritabilities in the white sample, both
for IQ and scholastic achievement, are lower than the median
values typically found in other studies of white populations in
England and the United States. The heritability for both IQ and
achievement is slightly lower in Scarr-Salapatek’s Negro sample
than in the w'hite, but the white and Negro heritabilities are about
the same within social classes. In the Negro and in the white
samples heritability increased as a function of social class, but
the trend toward lower heritabilities for low SES holds only for
the intelligence measures, while the achievement tends to go in