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

104 Educability and Group Differences
twins reared apart (MZA). The two twin studies which obtained
both intelligence and achievement scores on sufficiently large
samples of MZT and MZA are those by Burt (1966) and by
Newman, Freeman, and Holzinger (1937). Since intelligence tests
are usually composed of a number of subtests of various types, it
is best to compare them with the composite score on a scholastic
achievement test made up of subtests of reading, spelling, arithmetic,
etc. The correlations of MZT and MZA, along with dizygotic
twins reared together (DZT) and unrelated children reared
together (UT), on group and individual intelligence tests and on
a composite achievement test are as follows:
MZT
Burt
MZA DZT
UT
Newman et al.
MZT MZA DZT
Number of Pairs 95 53 127 136 50 19 51
IQ
Group Test 0-94 0-77 0-55 0-28 0-92 0-73 0-62
Individual Test 0-92 0-86 0-53 0-25 0-88 0-67 0-63
Scholastic
Achievement 0-98 0-62 0-83 0-54 0-89 0-51 0-70
The pattern of these correlations is highly instructive. Note that
in the case of MZ twins, being reared apart lowers the correlation
between the twins much more for scholastic achievement than for
IQ. And being reared together makes for a much higher correlation
between unrelated children in achievement than in IQ.
Dizygotic twins, with only half their genetic variance in common,
when reared together are more alike in achievement than MZ twins
reared apart, but this is not the case with IQ. This is strong
evidence that the family environment exerts a greater influence on
scholastic attainment than on IQ. Furthermore, the family environmental
influences are greatest on the simpler school subjects such
as spelling and arithmetic computation, which therefore have the
lowest heritability. (When such subjects are tested outside the
school context, however, they may reflect to a larger extent the
consolidated aspects of the person’s learning and would therefore
have higher heritability.) The reason is quite easy to understand.
Simple circumscribed skills can be more easily taught, drilled, and