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

Race Differences in Intelligence 161
had annual earnings only three-quarters as high as those of white
men with the stated average characteristics of the Negro men.
TWIN DIFFERENCES AND RACE DIFFERENCES
Social scientists who are most critical and disapproving of any
suggestion of the possibility that genetic factors are implicated in
racial differences in intelligence apparently believe there is an
important argument in their favor to be gleaned from comparisons
of identical twin differences and race differences in IQ. For
example, in a widely published statement by the Council of the
Society for the Psychological Study of Social Issues (1969)1 we
read: ‘In an examination of Jensen’s [1969a] data, we find that
observed racial differences in intelligence can be attributed to
environmental factors. Thus, identical twins reared in different
environments can show differences in intelligence test scores which
are fully comparable to the differences found between racial
groups.’ This argument has been emphasized and elaborated upon
by a number of psychologists (e.g., Gottesman, 1968, p. 28;
Deutsch, 1969, p. 549; Kagan, 1969, p. 275; Burgess & Jahoda,
1970). Because twin differences in IQ have thus been held up as
one of the major arguments against the hypothesis of genetic
differences between racial groups, we must carry this analysis
through to its logical conclusion.
Two major criticisms may be made of the argument as it has
been presented by its proponents (viz., all of those cited above).
First, they pick and choose among the twin differences that they
wish to consider. In not one of the comparisons has the total
available evidence been examined. Instead, selected cases of the
most extreme twin differences on record are compared with the
average Negro-white difference in IQ. Second, it is never noted
that measurement error constitutes part of the average twin difference
but does not enter into the mean difference between population
groups. Therefore comparisons are always made between twin
differences including measurement error and group differences free
of measurement error.2 Let us remedy these two faults and see
what conclusions can validly follow from a comparison of twin
differences with race differences.
Only monozygotic (MZ) twins are of interest to us here, since
they have identical genotypes and any difference between them