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

Social Class Differences in Intelligence 155
polation from our experiment, or indeed from any other animal
experiment, must be made only with extreme caution.
Nevertheless, we do feel that our experiment is relevant to
the human situation inasmuch as it strengthens the expectation
that social mobility related to a heritable variable will give rise
to some genetic difference between class means despite strong
parent-offspring environmental correlation. We therefore believe
that our experimental results support those who hold the view
that neither cultural nor genetic approaches alone are likely
to lead to adequate explanations of social class phenomena.
(Thoday & Gibson, 1970, p. 992)
If the environmentalist hypothesis that there is no genetic component
to social class intelligence differences were true, it would
mean that all the factors involved in social mobility, educational
attainments, and the selection of persons into various occupations
have managed scrupulously to screen out all variance associated
with genetic factors among individuals in various occupational
and social strata. The probability that the selection processes have
led to there being only environmental variance in intelligence
among various socioeconomic groups and occupations - a result
that could probably not be accomplished even by making an
explicit effort toward this goal - is so unlikely that the argument
amounts to a reductio ad absurdum. If individual differences in
intelligence are due largely to genetic factors, then it is virtually
impossible that the average intelligence differences between social
classes (defined by educational and occupational criteria) do not
include a genetic component.
The statistical argument goes as follows: The correlation between
phenotypes (the measurable characteristic) and genotypes (the
genetic basis of the phenotype) is the square root of the heritability,
or h. An average estimate of h for intelligence in European and
North American Caucasian populations is 0-90. An estimate of the
average correlation between occupational status and IQ is 0-50. A
purely environmentalist position says that the correlation between
IQ and occupation (or SES) is due entirely to the environmental
component of IQ variance. In other words, this hypothesis requires
that the correlation between genotypes and SES be zero. So we
have correlations between three sets of variables: (