an unaffected population numbering 260 million people (like the
US) with an average IQ of 100 and a standard deviation of 15 (right,
above). In that population there would be 6 million people with
IQs above 130 and 6 million below 70.
decrease in average IQ of 5 points would shift the distribution
to the left (right, below). The number of people scoring above
130 would decline by 3.6 million while the number below 70 would
increase by 3.4 million.
actual data are available that would allow a test of this "Weiss
Effect" at a population level. At least two pervasive contaminants
known to impair intelligence, lead and certain PCBs, are sufficiently
widespread to raise concerns.
for example, reveal that over 60% of inner city children in Philadelphia
carry lead levels above criteria established by the US CDC for
lead poisoning. Lead contamination in many major cities in developing
countries is much worse, with millions of children affected (see
discussion in Williams 1997). Other
research documents significant declines in IQ in children
exposed in utero to levels of PCBs experienced by many
people in the United States.
are just two of many neurotoxic chemicals. Vast quantities are
released by accident or design from industrial facilities, or
released purposively (e.g., pesticides) each year. Given what
is already known about the exposures these releases create, especially
given emerging science on interactions among diverse chemicals
in mixtures, (e.g., Porter
et al. 1997), it is scientifically indefensible to assume
there are no effects.