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

340 Educability and Group Differences
deviation of 15, to make them all comparable to the IQ scale.
The results, shown in Table 19.1, indicate that there is no appreciable
or systematic Negro-white disparity in the magnitudes of
the sibling differences and sibling correlations. (The overall
Negro-white difference in the value of \d\ is 0-15 or 0-01 SD .) A
nutritional deprivation hypothesis should predict significantly
larger sibling differences (and lower correlations) for Negroes
than for whites. This prediction clearly is not borne out by the
data. Yet these racial groups differ more than 1 SD in both verbal
and non-verbal IQ.
LEAD POISONING
This has been hypothesized increasingly of late as a cause of
lower Negro IQs. Physical and mental symptoms of lead poisoning
typically depend upon the ingestion of excessive quantities of lead
over a period of time. Cases of lead poisoning in children are
found almost exclusively in those afflicted by pica, a habit of
eating non-food substances, occurring most frequently in young
children. Nearly all discovered cases of lead poisoning have
resulted from children with pica eating the paint peeling off the
walls in deteriorating pre-World War II dwellings, usually in
urban slums. Almost no post-World War II dwellings have lead
paint, and such paint has long been outlawed in the manufacturing
of children’s toys. Therefore, although the incidence of lead
poisoning is not established, it is regarded as a very rare condition
as compared with many other health hazards. It has attracted
attention largely because of the rather close association that has
been found between lead poisoning, pica, and mental retardation.
Pica has a much higher incidence among retarded than among
normal children, and lead poisoning is highly associated with pica,
so the cause-and-effect relationship between lead poisoning and
mental retardation remains problematic.
Although the seriousness of lead poisoning, where it occurs,
should not be minimized and all possible measures should be taken
to prevent its occurrence, the actual known frequencies of the
condition appear to be so low that there is no subpopulation whose
mean would be shifted appreciably by lead poisoning occurring
at such low frequencies. Interest in this problem in New York
City in recent years has led to increased efforts to discover cases