Changing trends in diagnostic categories used for U.S. special education students, particularly the increased use of the autism category, have drawn substantial research interest in recent years (e.g., MacFarlane & Kanaya, 2009; Newschaffer, Falb, & Gurney, 2005; Shattuck, 2006). A review of the U.S. Department of Education's Office of Special Education Programs data from 1975 to 1995 concluded that more recent birth cohorts had increasing autism prevalence and that the increases were not associated with decreases in the categories of intellectual disabilities or speech–language impairment (Newschaffer et al., 2005). By contrast, an analysis of U.S. special education administrative prevalence data from 1994 to 2003 for children ages 6 to 11 years concluded that increases in the use of the autism category (and the use of the other health impairment categories) were offset in some states by corresponding declines in the use of other diagnostic categories (specifically intellectual and learning disabilities; Shattuck, 2006). A review of 2008 state special education eligibility criteria for autism identified variability in state eligibility criteria that might explain discrepancies between reported prevalence rates of autism and the typically lower rate at which children receive special education services for autism (MacFarlane & Kanaya, 2009).

Table 1 and Figure 1 summarize changes in the use of special education diagnostic categories for students ages 6 to 21 years between 1999 and 2008 for the United States based on public use files posted at the U.S. Department of Education's Data Accountability Center (http://www.ideadata.org). They show the number of special education students classified in the categories mental retardation (the term intellectual disability is used hereafter), autism, and other intellectual or developmental disabilities (“other IDD”), the latter term including students with developmental delay, orthopedic impairments, traumatic brain injury, multiple disabilities, or deaf/blindness.

Table 1.

Changes in the Number of Special Education Students Ages 6 to 21 Years With IDD in Special Education by Type of Disability, 1999–2008

Changes in the Number of Special Education Students Ages 6 to 21 Years With IDD in Special Education by Type of Disability, 1999–2008
Changes in the Number of Special Education Students Ages 6 to 21 Years With IDD in Special Education by Type of Disability, 1999–2008
Figure 1.

Students with intellectual or developmental disabilities (ID/DD) ages 6 to 21 years in special education by primary diagnosis, 1999–2008.

Figure 1.

Students with intellectual or developmental disabilities (ID/DD) ages 6 to 21 years in special education by primary diagnosis, 1999–2008.

Each year, the states are required to describe all special education students as having 1 of 13 mutually exclusive primary disability classifications (Data Accountability Center, 2006, 2010a). Data files for 2008 suppressed some state child counts for low-incidence disabilities, resulting in U.S. totals being larger than the sum of individual state totals in the other IDD category.

Overall, the number of students with intellectual or developmental disabilities (IDD) receiving special education services increased from 885,696 in 1999 to 1,078,053 in 2008 (21.7%). During these years, the total number of students in elementary and secondary schools in the United States increased 5.0%. The number of students with a primary diagnosis of intellectual disability decreased 20.4% and the number with a primary diagnosis of orthopedic impairment declined 12.0%. The number of students in all the other categories increased, with large increases noted for autism (349.2%), traumatic brain injury (83.2%), and developmental delay (390.6%). The statistics on developmental delay are difficult to interpret because the category, applicable only to students ages 9 years and younger, is optional for states, with 17 states reporting 19,743 students in 1999 and 33 states reporting 96,854 students in 2008.

The proportion of students with IDD who had intellectual disability declined from 67.5% in 1999 to 44.1% in 2008 while the proportion who had autism increased from 7.4% to 27.1% and the proportion who had a developmental delay increased from 2.2% to 9.0%. Although the number of students with traumatic brain injury (TBI) was smaller, the proportion with TBI increased notably from 1.5% to 2.3%.

National changes in the primary diagnosis of special education students were also evident in individual states (see Table 2). Overall, the number of students with IDD increased in all but 11 states, with 6 states reporting increases of more than 10,000 students (Illinois, Minnesota, Texas, New Jersey, Massachusetts, and California). The number of students who had intellectual disability decreased in 44 of 50 states, whereas the number who had autism increased in all states. In 34 states, the number of students with autism increased more than the number of students with intellectual disability declined. All states reported increases in the proportion of students with IDD who had autism, with 5 states reporting increases of 30% or more (Rhode Island, Maryland, Minnesota, Nevada, and Connecticut). In 1999, no state labeled more than 30% of its students with IDD as having autism; by 2008, 14 states did. In fact, by 2008, the number of students with autism exceeded the number of students with intellectual disability in 12 states.

Between 1999 and 2008, there was a steady and substantial shift in the primary diagnoses reported for students with IDD receiving special education services. Overall, the number of students who had intellectual disability decreased by 121,900, whereas the number who had a developmental delay increased by 77,100 and the number who had autism increased by 227,500. The number of students in the other IDD categories increased by 9,637 (5%), similar to the overall increase in the total elementary and secondary student population.

Although it is not possible to say from these data what caused these dramatic changes, two things are clear. First, schools labeled far fewer students as having intellectual disability in 2008 than they did in 1999. If the number of students with intellectual disability had increased by 5% (mirroring the change in the general school population), there would have been 627,474 students with the intellectual disability label in 2008. Instead, there were 475,713 (151,760 fewer students than expected). Some of this difference was likely due to increased use of the developmental delay label for students ages 6 to 9 years (76,124 more students than expected). More was probably due to increased use of the autism label (224,233 more students than expected). State special education criteria for autism may have also influenced these changes (MacFarlane & Kanaya, 2009).

Second, growth in number of special education students with autism was greater than can be explained by the decline in the number with intellectual disability. If the number of students with autism had increased by 5% between 1999 and 2008, 68,405 students would have been labeled as having autism in 2008. Even if 100% of the decline in the use of the intellectual disability label between 1999 and 2008 was due to reassignment to the autism label, that would account for only 151,760 students. The remaining increase of 72,473 students would have to be explained by something else. It could reflect a true increase in the number of students with autism or it could reflect changes in labels in categories other than those typically associated with intellectual or developmental disabilities (e.g., specific learning disabilities, emotional disturbance, or speech or language impairments). Regardless, between 1999 and 2008, the proportion of special education students with autism increased dramatically and the proportion of special education students with intellectual disability declined nearly as dramatically.

(Sources: Data Accountability Center. (2006). IDEA Part B data fact sheet child count. Individuals with Disabilities Education Act (IDEA) data. Retrieved May 1, 2010, from http://https://www.ideadata.org/618DataCollection.asp. Data Accountability Center. (2010a). Individuals with Disabilities Education Act (IDEA) Part B, data collection history. Rockville, MD: Author. Retrieved May 1, 2010, from http://https://www.ideadata.org/618DataCollection.asp. Data Accountability Center. (2010b). Part B educational environments state files 1999 to 2008. Individuals with Disabilities Education Act (IDEA) data. Retrieved May 1, 2010, from http://https://www.ideadata.org/PartBEnvironments.asp. MacFarlane, J. R., & Kanaya, T. (2009). What does it mean to be autistic? Inter-state variation in special education criteria for autism services. Journal of Child and Family Studies, 18, 662–669. Newschaffer, C. J., Falb, M. D., & Gurney, J. G. (2005). National autism prevalence trends from United States special education data. Pediatrics, 115, e277–e282. Shattuck, P. T. (2006). The contribution of diagnostic substitution to the growing administrative prevalence of autism in US Special Education. Pediatrics, 117, 1028–1037. U.S. Department of Education, National Center for Education Statistics. (2010). Digest of education statistics: 2009 Retrieved May 11, 2010, from http://nces.ed.gov/programs/digest/d09/tables_1.asp.)

Table 2.

Changes in Primary Disability for Special Education Students Ages 6 to 21 Years With Intellectual or Developmental Disabilities Between 1999 and 2008

Changes in Primary Disability for Special Education Students Ages 6 to 21 Years With Intellectual or Developmental Disabilities Between 1999 and 2008
Changes in Primary Disability for Special Education Students Ages 6 to 21 Years With Intellectual or Developmental Disabilities Between 1999 and 2008

Preparation of this report was supported by a grant from the Administration on Developmental Disabilities (Grant 90D0217/01) of the U.S. Department of Health and Human Services and a cooperative agreement with supplemental support from the National Institute on Disabilities and Rehabilitation Research, U.S. Department of Education (Agreement H133B080005-09) to the University of Minnesota's Research and Training Center on Community Living.