>>40765851
http://www.lagriffedulion.f2s.com/retard.htm

" In 1959, AAMD set the IQ threshold for mental retardation at < 85. The civil rights movement of the next decade forced psychologists to rethink this boundary, because half the African American population fell below it. In 1973, responding to this concern, AAMD (by then AAMR) changed the threshold for retardation from IQ < 85 to IQ < 70. The boundary moved south by one standard deviation! The proportion of blacks below the threshold instantly dropped from about 50 percent to 12 percent. Subsequent refinements made it still more difficult to meet the criteria for retardation."