Large state-level fluctuations in mental retardation classifications related to introduction of renormed intelligence test.

Oppositely valenced forces may be at work to influence rates of placement of children into mental retardation programs. On one hand, educational policies regarding intellectual disability and concerns about overrepresentation of minorities in special education may contribute to lower placement rates; on the other hand, more difficult intelligence test norms may be a countervailing force, increasing placement rates. An analysis of longitudinal data on state and national level placement rates reveals that a lengthy and steep 12-year decline in students receiving mental retardation services reversed shortly after the introduction of the WISC-III in 1991. This phenomenon has relevance for death-penalty cases, because this historical pattern may affect the ability to establish whether an adult meets the developmental period onset criterion for mental retardation.