Heritability is the proportion of variability in a phenotype
that is “accounted for” (in the usual regression
sense) by variation in genotype. Most studies estimate that
the heritability of IQ is somewhere between .4 and .8 (and
generally less for children), but it really makes no sense to
talk about a single value for the heritability of intelligence.
The heritability of a trait depends on the relative variances
of the predictors, in this case genotype and environment.
The concept of heritability has its origins in animal breeding,
where variation in genotype and environment is under
the control of the experimenter, and under these conditions
the concept has some real-world applications. In freeranging
humans, however, variability is uncontrolled, there
is no “true” degree of variation to estimate, and heritability
can take practically any value for any trait depending on the
relative variability of genetic endowment and environment
in the population being studied. In any naturally occurring
population, the heritability of intelligence is not zero (if
genotype varies at all, it will be reflected in IQ scores) and
it is not one (if environment varies at all, it will be reflected
in IQ scores). That the heritability of intelligence is between
zero and one has one important consequence: Without
additional evidence, correlations between biologically