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

48 Educability and Group Differences
the se for the genetic value. An individual’s estimated environmental
value, E., would be
£t = rlP(Pi-P„) + Pp (2.8)
so that a person with IQ 140, for example, would have an estimated
environmental value of 0-20 (140 —100) +100 = 108 IQ, and someone
with an IQ of 70 would have an estimated environmental value
of 94 IQ. An h2 of 0’80 means that genetic factors outweigh environmental
factors by 2 to 1, in determining individual differences in IQ,
since the ratio of SJ7P' to —h2 is 0-894 to 0*447 or 2 to 1. Note
also that if we express an individual’s values for P if Gh and Et all
as deviations from the population mean Pp, the estimated deviation
values of P t = Gt + Et are:
Pt-P , = (Gt-P,)+(Bi-P,) (2-9)
which in our hypothetical case of the person with an IQ of 140
are estimated as
140-100 = (132 —100) + (108 —100)
40 = 32 + 8.
And thus we are back to formula 1, with which we started, showing
that the individual’s phenotypic score (expressed as a deviation
from the population mean) is equal to the sum of the individual’s
genetic value and his environmental value, i.e., Pi = Gi + E{. This
detailed exposition has made clear the relationship between heritability
as a population parameter and heritability as a means of
estimating an individual’s genetic value. Only if heritability were
complete (h2 = 1-00) would the phenotype be a perfect indicator
of the genotype. The fact that h2 for intelligence is less than
1*00 (being around 0-70 to 0-80), means that an individual’s
test score estimates his genetic value only with some margin of
error, the magnitude of which is indicated by the standard error
of estimate, SEG.
THE MEANING AND NON-MEANING OF ‘INTERACTION’
There are three main wrays in which the term ‘interaction’ is used
in discussions of heredity and environment. The meanings that
make sense must be distinguished from the more commonly
encountered abuse of the term, which only confuses discussion.