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

162 Educability and Group Differences
must be due entirely to non-genetic factors. Moreover, since our
interest here is in the effects of environmental differences on the
IQs of genetically identical persons, we shall look only at MZ twins
w'ho have been separated early in life and have been reared apart.
MZ twins reared apart show larger IQ differences than MZ twins
reared together. Most of the MZ twins reared apart who have
been reported in the world literature on twins and on whom IQs
are available are contained in four studies, by Burt (1966), Juel-
Nielsen (1965), Newman, Freeman and Holzinger (1937), and
Shields (1962). There are 122 twin pairs in all. I have re-analyzed
the original data from all these published studies of the IQs of
MZ twins reared apart (Jensen, 1970a). The essential statistics
are summarized in Table 7.1. The analysis shows that the mean
Table 7.1 Statistics on IQs of MZ twins reared apart (from Jensen,
1970a)
Study N (Pairs) Mean IQ SD \d\ SDW ri r,i
Burt 53 97-7 14-8 5-96 4-44 0-88 0-88
Shields 38 93-0 13-4 6-72 5-80 0-78 0-84
Newman et al. 19 95-7 13-0 8-21 6-65 0-67 0-76
Juel-Nielsen 12 106-8 9-0 6-46 3-22 0-68 0-86
Combined 122 96-8 14-2 6-60 5-20 0-82 0-85
Tests: Burt - ‘final assessments’, a composite score of two or more verbal
and non-verbal group and individual intelligence tests scaled to a
population mean = 100, SD = 15; Shields - composite of the Mill
Hill Vocabulary Test and the Domino D-48 (non-verbal reasoning)
Test; Newman et al. - Stanford-Binet (1916); Juel-Nielsen - Danish
adaptation of the Wechsler-Bellevue (Form I).
\d\ = mean absolute difference between co-twins
r. I = intraclass correlation
rd = correlation based on the twin differences, i.e.,
r‘ =1_(§
where \dp\ = mean absolute difference between all possible paired
comparisons in the general population
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