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

290 Educability and Group Differences
recent common ancestor with homonids). In terms of ‘fitness’ in the
genetic sense, single births have a selective advantage. Thus the
tendency for single births, like the prolongation of immaturity, may
be a reflection of evolutionary age. It is interesting therefore that
one of the best established differences among the major human races
is the large difference in the frequencies of multiple births, particularly
dizygotic twinning, being most common in Negroes and least
common in Orientals, with Caucasians more or less intermediate
(Harrison, Weiner et al., 1964, p. 148; Bulmer, 1970, pp. 83-91).
3. Intellectually gifted children are sometimes reported to have been
prococious in various infant behaviors such as walking and talking
(e.g., Terman & Oden, 1959, p. 7), on the basis of self-report and/or
parental reports. Such after-the-fact retrospective self-reports of
individuals (or their parents) who already know they have superior
IQs would hardly seem reliable. It is quite well-established, on the
other hand, that mental defectives begin talking at a later age than
most children and show retarded language development (for a
review see McCarthy, 1946, pp. 546-9).
4. Apparently the sensori-motor capability for manipulating objects in
the environment also is not crucial for normal cognitive development.
The children who are probably the most disadvantaged in this respect
are the limbless thalidomide babies, who, despite their severe motor
handicap, are reported to show no deficit in cognitive development
(Bower, 1971).