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This meant that a great number of people could be considered "sub-average". In the general population, this might mean 16% of the people. It would be even higher in population groups where other factors lower intelligence test scores (though not necessarily intelligence) – minority status, language, or socioeconomic status.

The developmental period began at birth and ended at 16 years. In 1973, the end of the developmental period would change from 16 to 18 years. In 1983, the beginning developmental period would change from birth to conception.

In 1962, the President's Panel on Mental Retardation chose to use the four level classification and did not speak of "borderline". It adjusted the IQ scores associated with the four levels – mild (IQ usually 50 to 70), moderate (IQ usually 35 to 50), severe (IQ from about 20 to 35) and profound (IQ usually below 20). Interestingly enough, the 1971 report of the President's Committee on Mental Retardation used the five level AAMD scheme again.