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

368 Educability and Group Differences
The covariance of genotypes and environments, Co\GE, arises
when genotypic values and environmental values are correlated in
the population. An example is children with genotypes for high
intelligence who are reared in homes with superior environmental
advantages for intellectual development, or a musically talented
child who is given music lessons, phonograph records, and taken
to concerts more than a child who evinces little or no sensitivity
to music. Some part of the covariance is, in a sense, truly created
by the genotype, as when an intellectually gifted child spontaneously
spends more time in reading or other intellectual activities,
or a musically talented child of his own accord spends many hours
a day practising a musical instrument. Similarly, in nature, the
genotypically larger and stronger animal will get more food in its
particular ecological niche and will therefore be favored also
nutritionally. This covariance increases the total population
variance in the trait (in this case body size). Because the covariance
is so closely linked to genotypic characteristics when conditions
are such that individuals can choose or create those features of the
environment in accord with their genotypic proclivities, some
geneticists (e.g., Roberts, 1967, p. 217) include CovGE as part of
the total genetic variance rather than as part of the environmental
variance, and they define the environmental variance component,
VE, as those environmental effects which are independent of the
genotype.
Thus heritability can be defined differently for various purposes,
depending upon the components that enter into the numerator
and denominator.
Heritability in the narrow sense, h2, is the proportion of additive
genetic variance, thus:
K = ~
V P
Heritability in the broad sense, h2, is:
VA + VB+V'. + VA„
Vo
(A.4)
h - v„ - VP (A'5)
Some estimates of heritability in the broad sense include CovGE
in the numerator:
h2 = -^ ± £ °-vG£ (A.6)