The rise and fall of IQ in special ed: Historical trends and their implications

Using IQ records from 10 sites around the country, we examined longitudinal trends in IQ among mental retardation (MR) students during the Wechsler Intelligence Scale for Children (WISC) to WISC-Revised (R) transition in the 1970s and learning disability (LD) students during the WISC-R to WISC-3rd Revision (III) transition in the 1990s. Based on the work by Flynn [American Journal of Mental Deficiency 90 (1985) 236; Psychology, Public Policy, and Law 6 (2000) 191], we predicted a rise in IQ followed by a fall each time a newly normed IQ test is introduced into the schools and used to diagnose children as MR or LD. As expected, we found that mean IQs of MR and LD students followed a nearly identical parabolic trajectory, and differed depending on what year they were tested. IQs from the older norms were higher than IQs from the newly introduced norms. This systematic shift had a significant impact on the likelihood of a MR diagnosis. The broader implications of this pattern are discussed.