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

8 Multiple and partial correlation
methods
The literature on social class and racial differences in intelligence
is replete with reports of correlations between innumerable
environmental, attitudinal, and personal variables and IQ. Correlations
have been reported between IQ and variables such as family
size, absence of father, prematurity, child-rearing practices, birth
weight, family income, parental education, protein intake, books
in the home, mother’s age, position in family - the list is almost
endless. Each variable may yield some non-zero correlation with
IQ, however small, and since some racial groups differ on many
of these variables in the same direction as they differ in IQ and
in the same direction that the variables are correlated with IQ, it
creates the impression that the racial IQ difference must be easily
explainable in terms of the racial differences on all of these
environmental factors. The fallacy in this, of course, is that the IQ
variance (either within or between racial groups) accounted for
by all these environmental factors is not equal to the sum of all
the various environmental influences. In accounting for IQ
variance, environmental measures, not being independent sources
of variance, must be added up in the fashion of a multiple regression
equation. That is to say, the contribution to IQ variance of each
environmental factor must be added up after removing whatever
IQ variance it has in common with all the previous factors added
in. The greater the correlation between the environmental factors,
the smaller is the contribution to IQ variance of each successive
factor which is added to the composite of environmental variables.
Because of the lack of independence among environmental