Prevalence of Intellectual Disabilities

Estimates of the prevalence of ID among children in the United States based on epidemiologic studies range widely, from 8.7 to 36.8 per 1,000 (Boyle and Lary, 1996; Camp et al., 1998) (see Table 15-1). A major source of this wide range in prevalence is variation in the inclusion of mild ID (often defined to include individuals with IQs in the range of 50–70 and deficits in adaptive behavior).¹ While the prevalence of serious ID (IQ <50 with deficits in adaptive behavior) in the United States and other developed countries is consistently found to be in the range of 2.5 to 5 per 1,000 children, that of mild ID ranges from as low as 2 to more than 30 per 1,000. The risk of mild ID is highest among children of low socioeconomic status (Durkin et al., 2007; Maulik et al., 2011). From the evidence reviewed, disparities in the rates of ID by race and ethnicity are also reported. Socioeconomic status is a confounder for these disparities; however, there is also evidence that test bias and diagnostic bias contribute to these differences (Jencks and Phillips, 1998).

TABLE 15-1

Prevalence of Intellectual Disability per 1,000 Children.

In addition to epidemiologic studies, evidence from the Metropolitan Atlanta Developmental Disabilities Surveillance Program (MADDSP) and two national surveys, the National Health Interview Survey (NHIS) and the National Survey of Children's Health (NSCH) as well as receipt of special education services will be discussed.

National Health Interview Survey

The NHIS provides national-level data on the frequency of ID among children (ages 3–17 years) in the United States for the period 1997 to 2008. The NHIS data, which are based on parent-reported diagnoses of “mental retardation” or ID, are summarized in Table 15-2.

TABLE 15-2

NHIS Prevalence of Intellectual Disability/Mental Retardation.

Prevalence Trends for Intellectual Disabilities

It is important to acknowledge that there are no perfect sources of data for evaluating trends in the prevalence of ID in the United States. An ideal source that would allow evaluation of trends over time might be generated by large-scale, nationally representative, population-based epidemiologic studies conducted periodically, and using validated and comparable methods and diagnostic criteria over time. No such data source exists. The data sources used for the purposes of this study have complementary strengths and weaknesses. For example, the epidemiologic data source described below, from the Centers for Disease Control and Prevention's (CDC's) MADDSP, has the advantages of being population-based, including one large and diverse populations of U.S. children, relying on validated methods for classifying cases of ID that were comparable over a multiple-year period (1991–2010), and allowing evaluation of trends over time. The MADDSP data are, however, not nationally representative and they rely exclusively IQ scores recorded in the records of health care providers or schools, without incorporating information on adaptive behavior. In contrast, the national survey data have the advantage of being based on national probability samples, but the disadvantages of relying on parental reports rather than diagnostic assessments, and of response rates less than 100 percent. Special education “child count” data have the advantages of being nationally representative and available annually, but do not necessarily rely on standard case definitions or diagnostic criteria that are comparable over time and across states and school districts. Despite the limitations each data source, taken together they provide valuable and complementary evidence for evaluating trends over time in the frequency of ID in U.S. children.

Special Education Services

As reported previously in comparison with prevalence trends for autism spectrum disorder, the rate of the receipt of special education services for ID among school-aged children in the United States declined steadily from 9.1 per 1,000 in 2004 to 6.6 per 1,000 in 2012. Table 15-3 shows the decreasing trend in the rate of children receiving special education services for ID from 2004 to 2012.

TABLE 15-3

Prevalence of Receipt of Special Education Services for Intellectual Disability in Children of Ages 6–17 Years, per 1,000 Children in the United States, 2004–2012.

SSI

Within the SSI program, the number of allowances and recipients for ID substantially and continuously decreased between 2004 and 2013. The proportion of allowances and recipients attributable to ID among the 10 major mental disorders also decreased. In spite of these decreases, in 2013 ID was the basis for the third highest number of allowances (12,470) of the major mental disorders, the fifth highest number (13,613) of determinations, and the third highest number (120,248) of recipients. In addition, the allowance rate for ID remained above 90 percent for each year from 2004 to 2013, indicating that nearly all children with ID who applied for benefits were deemed eligible. Based on these findings, it appears that fewer children are being adjudicated as having a disability due to ID. The decreasing numbers of ID allowances and recipients is unique; decreasing trends of similar magnitude are not observed for any of the other selected mental disorders.

Table 15-4 presents the SSI administrative data on ID in children. Column 1 shows the number of child allowances made on the basis of ID at the initial level for each year. Column 2 shows the number of child recipients who received SSI benefits on the basis of ID in December of each year. Column 3 shows the estimated number of children in households with incomes under 200 percent of the federal poverty level (FPL) for each year. To control for the changes in the magnitude of child poverty, allowances and recipients are shown as a percentage of the number of children in households under 200 percent FPL for each year. Column 4 shows the percentage of children in households under 200 percent FPL who were allowed benefits for ID (i.e., were found to be severely impaired, with a primary diagnosis of ID) in each year. Column 5 shows the percentage of children in households under 200 percent FPL who were recipients (including newly allowed and existing recipients) of SSI payments for ID in December of each year. Figure 15-1 plots the percentages from columns 4 and 5, along with the 10-year average of percentage of allowances and recipients for ID as a visual reference point.

TABLE 15-4

SSI Child Initial Allowances and Recipient Numbers for Intellectual Disability.

FIGURE 15-1

Percentages of SSI child initial allowances and recipients for intellectual disability below 200 percent FPL. NOTE: The Current Population Survey table creator was used to generate numbers of children below 200 percent of the federal poverty level. Parameters (more...)

Over the 10-year period from 2004 to 2013, the rates of child SSI ID allowances and recipients were decreasing. The rate of child ID allowances among children in households less than 200 percent FPL decreased between 2004 and 2013 by approximately 56 percent, from 0.09 to 0.04 percent. Similarly, the rate of child ID recipients decreased between 2004 and 2013 by 49 percent, from 0.75 to 0.38 percent.

Medicaid

Table 15-5 shows the percentage of children who were diagnosed with ID in two different groups of Medicaid enrollees for each year from 2001 to 2010. Column 1 shows the percentage of ID diagnoses among all Medicaid enrollees² for each year. Column 2 shows the percentage of ID diagnoses among the smaller subpopulation of Medicaid enrollees who were eligible to be enrolled in Medicaid on the basis receiving SSI benefits.³

TABLE 15-5

Percentage of Child Medicaid Enrollees and SSI Medicaid Enrollees Diagnosed with Intellectual Disability.

As shown in Figure 15-2, the rate of ID diagnoses among all children enrolled in Medicaid was mostly flat, remaining very close to the 10-year average each year. The rate of ID diagnoses among all Medicaid enrollees decreased by 16.67 percent from 2001 to 2010, from 0.6 to 0.5 percent. The rate of ID diagnoses among the SSI-eligible subpopulation of Medicaid enrollees decreased by −3.03 percent, from 6.6 to 6.4 percent. There was a peak in the rates of ID diagnoses among SSI-eligible Medicaid enrollees in 2008 which does not seem to appear in any of the other trends observed.

FIGURE 15-2

Percentage of child Medicaid enrollees and SSI Medicaid enrollees diagnosed with intellectual disability. SOURCE: MAX data.

For the years in which the Medicaid data and the SSI administrative data overlapped, 2004–2010, there was no change in the rate of ID diagnoses among all Medicaid enrolled children, the rate of ID diagnoses among SSI eligible Medicaid enrolled children increased 3.23 percent, and the prevalence of SSI recipients for ID among children in households below 200 percent FPL decreased by 41 percent. A clear decreasing trend is observed in the SSI program, while no clear decreasing trend is observed in the Medicaid population.