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

Between-Groups Heritability 145
could be accompanied by lowered within-group heritabilities. But
the opposite relationship is more likely. It is much like the probabilistic
but not necessary relationship between the number of years
since a person’s birth and whether or not he is living or dead. If,
in a city’s birth register, we look up a particular person, John Doe,
and know only his birthdate, we can consult a life insurance
company’s actuarial tables and determine the probability of living
to the age of John Doe. The probability of living to John Doe’s
age will be some value between 0 and 1. But no actuarial probability
tables can help us in establishing that John Doe is in fact living
or dead. The more years that have elapsed since his date of birth,
the greater is the likelihood that we will be right if we assume (i.e.,
hypothesize) that John Doe is dead. We could always be wrong,
but the likelihood of this decreases the greater the time elapsed
since the birth of John Doe. And so it is with the likelihood of
there being a genetic difference between subpopulations; the greater
the heritability of a trait within groups, the greater is the likelihood
that between-group differences in the trait involve genetic factors.
But proof of the genetic hypothesis for any particular trait depends
upon other evidence, just as proof that a particular John Doe is
alive cannot b&proved by the actuarial tables. But if John Doe was
born in 1870, how many of us would want to place our bet on the
hypothesis that he is alive today Of essentially the same nature
is our acceptance of the Watusi-Pygmy difference as genetic
without formal proof.
The same thing holds true, though to a lesser degree of likelihood,
in the case of intelligence differences between social classes
and racial groups. This valid limitation of generalizing heritability
studies within populations to differences between populations is
vigorously and correctly insisted upon by some writers (who,
however, usually fail to point out the increasing likelihood
relationship) while at the same time they show no hesitancy
whatever about generalizing environmental sources of variance
from one population to another. Hirsch (1970), for example, cites
a study8 of mid-Eastern Jewish children in Israel brought up in
a kibbutz who had IQs 30 points higher than similar children
brought up in individual homes. Hirsch concludes: ‘There is no
basis for expecting different overall results for any population in
our species’ (Hirsch, 1970, p. 101). How does this differ from
generalizing the heritability of intelligence from, say, Cyril Burt’s