Fig. 16.   An imaginary experiment in which a population is exposed to an agent that reduces the IQ by 5 points. If assessments are limited to people falling within the "normal" range (55-145), there is virtually no change in outcome. If, however, assessments are made in the tails of the distribution, focusing on rarely occurring values, the small change in average IQ produces disproportionately large changes, with a 3-fold increase in retardation and a loss of two-thirds of the extremely gifted group. Neurobehavioral teratogenesis by drugs of abuse or environmental factors might be studied better in human populations by concentrating on these small subpopulations, rather than by focusing on changes in average scores.