i8 6 THE MISMEASURE OF MAN
The equation of "heritable" with "inevitable." To a biologist,
heritability refers to the passage of traits or tendencies along family
lines as a result of genetic transmission. It says little about the range
of environmental modification to which these traits are subject. In
our vernacular, "inherited" often means "inevitable." But not to a
biologist. Genes do not make specific bits and pieces of a body; they
code for a range of forms under an array of environmental condi-
tions. Moreover, even when a trait has been built and set, environ-
mental intervention may still modify inherited defects. Millions of
Americans see normally through lenses that correct innate defi-
ciencies of vision. The claim that IQ is so-many percent "heritable"
does not conflict with the belief that enriched education can
increase what we call, also in the vernacular, "intelligence." A par-
tially inherited low IQ might be subject to extensive improvement
through proper education. And it might not. The mere fact of its
heritability permits no conclusion.
The confusion of within- and between-group heredity. The
major political impact of hereditarian theories does not arise from
the inferred heritability of tests, but from a logically invalid exten-
sion. Studies of the heritability of IQ, performed by such tradi-
tional methods as comparing scores of relatives, or contrasting
scores of adopted children with both their biological and legal par-
ents, are all of the "within-group" type—that is, they permit an
estimate of heritability within a single, coherent population (white
Americans, for example). The common fallacy consists in assuming
that if heredity explains a certain percentage of variation among
individuals within a group, it must also explain a similar percentage
of the difference in average IQ between groups—whites and
blacks, for example. But variation among individuals within a
group and differences in mean values between groups are entirely
separate phenomena. One item provides no license for speculation
about the other.
A hypothetical and noncontroversial example will suffice.
Human height has a higher heritability than any value ever pro-
posed for IQ. Take two separate groups of males. The first, with
an average height of 5 feet 10 inches, live in a prosperous Ameri-
can town. The second, with an average height of 5 feet 6 inches,
are starving in a third-world village. Heritability is 95 percent or so
in each place—meaning only that relatively tall fathers tend to have