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

18 Sensori-motor differences
There has never been any real disagreement about genetically
determined physical differences - biochemical, physiological, and
anatomical - between racial groups. It is in the realm of intellectual
functions that we see so much a priori resistance to rejection of
the null hypothesis concerning genetic racial differences. But
actually there is a continuum between the physical and the
intellectual; there are no discernible discontinuities; and racial
differences are found at all points along the continuum going from
strictly physical characteristics to behavioral characteristics, including
those processes we identify as mental ability. So the point
at which one draws the line of resistance to entertaining a genetic
hypothesis of racial differences is usually quite arbitrary.
Sensory capacities are intimately related to physical structures
and processes and are undoubtedly conditioned by genetic factors.
And we find marked racial differences in certain sensory capacities.
(An excellent detailed review of much of this evidence is found
in Spuhler & Lindzey, 1967). For example, the ability to taste the
synthetic chemical substance phenylthiocarbamide (PTC) is
known to be genetically determined, probably by a single gene.
There are striking race and subpopulation differences in the
frequencies of tasters and non-tasters of PTC, going from 0 to 57
percent in the various populations which have been studied (see
Spuhler & Lindzey, 1967, pp. 381-4). There are marked racial
differences in the incidence of various types of sex-linked color
blindness which are completely genetic; and there are differences
in ability for color discrimination. Negroes have better visual
acuity than whites - only 65 percent of whites in the armed forces
pre-induction examination have 20/20 vision, as compared with