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

Technical Misconceptions and Obfuscations 45
demonstrate) a perfect relationship between characteristics or
measurements of the environment and the effects of these environmental
differences upon the phenotype can we directly equate
7 1 with the variance of some independent measurement of environments.
The model as presented here may assume or imply a one-toone
correspondence between the objective environment and the
e ffects of the environment on the phenotype.
Heritability, then, is defined as the proportion of total phenotypic
variance (individual differences) shown by a trait that can be
attributed to genetic variation in the population:
(7 2 0-2
h2 = = i f (2.4)
al + a2 a2
What are the limitations of this h2} First of all, since we usually
cannot measure a trait in every member of the population, we have
to be content with a sample from the population which provides
us with an estimate of h2. From the sample we get an approximation
of the value of h2 in the entire population, and the larger the
sample, the better is the approximation. Conversely, the larger
the sample, the smaller is the margin of error in our estimate. So
we must be aware that estimates of h2 all involve sampling error,
more or less, depending upon the adequacy of the sample.
Secondly, since populations can change over time, the obtained
estimate of h2 pertains only to the population that was actually
sampled. How much h2 varies from time to time or from one
population to another can only be answered empirically. It has
often been noted, however, that much of the interest and value in
heritability estimates derives from the fact that h2 for a given
characteristic is not a highly unstable statistic but under natural
conditions generally remains very much in the same range across
time and across a variety of populations. This, of course, means no
more than the fact that the populations and conditions under which
heritability estimates are generally made do not differ all that much.
It is entirely possible, on the other hand, to find marked exceptions,
and it is also possible experimentally to produce great changes in
h2 by radically increasing or decreasing the variation in environmental
factors, or by increasing or decreasing the genetic variance
through selective breeding. Since h2 reflects a ratio of genetic
to phenotypic variance, a change in either source of variance will
alter the value of h2. It is possible to go wrong by believing that