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.
When Binet in 1905 produced the first IQ test, it promised to revolutionize the diagnosis and treatment of mental retardation. A half century later it came under attack for reasons Binet could not have imagined. Could any of the pioneer psychometricians have foreseen Larry P. v. Riles (1979), a California class-action suit that focused on IQ testing of young black children? The court held that IQ tests were not valid for African Americans.
It banned California from using the tests for placing black students in classes for the educable mentally retarded or equivalent categories on the grounds that the tests were biased. After a series of appeals, the district court ruled that no special education related purposes exist for which IQ tests could be administered to black pupils.
Though only a California ruling, the case began a political assault on standardized testing that has spread beyond the IQ test to college entrance exams, promotional exams and more.
http://www.lagriffedulion.f2s.com/retard.htm
However in 2016 America, I believe we suffer more from a MQ (morality quotient) deficit.