State-Specific Rates of Mental Retardation -- United States, 1993

Mental retardation (MR) is the most common developmental disability and ranks first among chronic conditions causing major activity limitations among persons in the United States (1). National and state-specific surveillance to measure the prevalence of MR can assist in targeting areas of need and allocating resources. State-specific prevalences for MR can be determined by using data about persons who receive specialized services for MR through entitlement programs. To estimate state-specific prevalences of MR in 1993, data were analyzed from the U.S. Department of Education (DOE) for children with MR who were enrolled in special education programs and from the Social Security Administration (SSA) for adults with MR. * This report summarizes the findings, which suggest substantial state-specific variation in the prevalence of MR in the United States.

For children, the analysis was based on data in reports from the DOE, which included the number of children aged 6-17 years who because of MR were enrolled in special education programs (either Chapter 1 or Part B) during school year 1993-94. For this data set, MR was defined as "... a significantly subaverage general intellectual functioning, with deficits in adaptive behavior" (2,3).

For adults aged 18-64 years, the analysis was based on SSA data from 1993. The SSA defines MR as "... significantly subaverage general intellectual functioning, with deficits in adaptive behavior initially manifested during the developmental period (before age 22)" (4). The SSA database includes adults with MR who received Supplemental Security Income (SSI) and/or Social Security Disability Insurance (SSDI). To be eligible to receive SSA benefits for MR (and, therefore, be included in the SSA database), adults must have had an intelligence quotient (IQ) of less than or equal to 59 or an IQ of 60-70 with other physical or mental impairment(s) resulting in additional and substantial work-related limitations of function. All persons receiving SSA benefits also must meet income-resource eligibility requirements (4).

The numbers of children and adults identified through DOE reports and the SSA database in each state and the District of Columbia were combined to estimate the total population with MR. Prevalences of MR were calculated for children by using the total number of children aged 6-17 years in each state and for adults, by using the total number of persons aged 18-64 years. The 1990 census was used as a source for state population estimates and demographic data (i.e., median household income, percentage of total births to teenaged mothers, and percentage of adults aged greater than or equal to 18 years with less than a ninth-grade education). Multiple linear regression was used to determine the amount of variability in the state MR rates that could be attributed to those three socioeconomic factors.

In 1993, an estimated 1.5 million persons aged 6-64 years in the United States had MR, and the overall rate of MR was 7.6 cases per 1000 population. State-specific rates varied approximately fivefold (range: 3.0 in Alaska to 16.9 in West Virginia) (Table_1). The 10 states with the highest overall rates of MR were contiguous and located in the East South Central (Alabama, Kentucky, Mississippi, and Tennessee), South Atlantic (West Virginia, North Carolina, and South Carolina), West South Central (Arkansas and Louisiana), and East North Central (Ohio) regions. The states with the lowest rates were in the Pacific and Mountain regions.

For children, the MR rate was 11.4 per 1000 and varied approximately ninefold (range: 3.2 in New Jersey to 31.4 in Alabama) (Table_1). For adults, the rate was 6.6 and varied approximately sixfold (range: 2.5 in Alaska to 15.7 in West Virginia). In most (42 {84%}) states, the rate for children was higher than that for adults; in seven (14%) states, the rate for adults was higher, and in two states, both rates were similar. The correlation between state-specific rates for children and for adults was 0.66. Overall, 69% of the state-specific variation in prevalence rates for adults was accounted for by median household income, the percentage of total births to teenaged mothers, and the percentage of the population with less than a ninth-grade education. Low educational attainment was the most important correlate of MR rates among adults.

Reported by: PS Massey, PhD, South Carolina Dept of Disabilities and Special Needs; S McDermott, PhD, Interagency Disability Prevention Program, and Dept of Family and Preventive Medicine, Univ of South Carolina School of Medicine, Columbia. Disabilities Prevention Program, Office of the Director, and Developmental Disabilities Br, Div of Birth Defects and Developmental Disabilities, National Center for Environmental Health, CDC.

Editorial Note

Editorial Note: This analysis of data from entitlement service programs suggests wide variation in state-specific rates of MR for children and adults in the United States. Use of this method of monitoring the prevalence of MR can assist in evaluating temporal trends and in identifying high-risk areas within states. The high rates of MR documented in the South Atlantic and East South Central regions are consistent with rates for disabilities from all causes, which also indicate wide variations among the states (5). In addition, the finding that a substantial proportion of state-specific variation was associated with differences in median income, percentage of births to teenaged mothers, and percentage of adults with less than a ninth-grade education is consistent with previous reports documenting the relation between the prevalance of MR and socioeconomic factors (6), particularly low maternal education levels (7).

The findings in this report are subject to at least four limitations. First, although national guidelines determine the eligibility requirements for entitlement programs, these programs are administered locally, and guidelines are subject to local interpretations and modifications that can influence the numbers of persons served. Second, the DOE data do not include those who drop out of school and those who never enroll in a public education program. Dropout rates and enrollment in private schools can vary substantially among states and can affect the numbers of children identified through this method. Third, the eligibility data for SSA services is based on both personal income and the presence of a disability. Financial eligibility is based on the adult's own income, and an adult with MR can qualify for SSI benefits regardless of family income or assets. However, some adults with MR who meet the disability eligibility requirements may not be eligible because their earned income or other assets exceed eligibility requirements. Reduced participation in these programs in states with higher median household incomes could lower the MR rate for adults in those states; however, the incentive to apply for SSI or SSDI to ensure health benefits and financial support probably ensures consistent participation in this program among all states. Finally, small rate differences among states can result from other data limitations that reflect the problems intrinsic to complicated state and federal cooperative arrangements.

The large state-to-state differences in MR rates in this analysis probably reflect at least some real differences in MR rates (e.g., related to income and educational attainment). State-specific variations in the prevalence of MR should be assessed using multiple data sources, and further efforts should seek to explain the largest differences in rates among states and the difference between the rates for children and adults within states. Some states (e.g., South Carolina and Alabama) are examining variations in rates among counties or local school districts to determine factors possibly influencing their local and state rates. CDC's Metropolitan Atlanta Developmental Disabilities Surveillance Program tracks MR rates for children aged 3-10 years using multiple data sources and can be used as a model for other areas (8). Improved understanding of the risk factors for MR and the factors influencing rate variations can assist in developing and targeting prevention strategies and efforts.

References

1. Pope AM, Tarlov AR. Disability in America: toward a national agenda for prevention. Washington, DC: National Academy Press, 1991.

2. US Department of Education. To assure the free appropriate public education of all children with disabilities: sixteenth annual report to Congress on the implementation of the Individuals with Disabilities Education Act. Washington, DC: US Department of Education, 1994.

3. Frankenberger W, Harper J. State definitions and procedures for identifying children with mental retardation: comparisons of 1981-1986 guidelines. Ment Retard 1988;26:133-6.

4. Social Security Administration. Disability evaluation under Social Security. Washington, DC: US Department of Health and Human Services, Public Health Service, 1994; publication no. (SSA) 64-039.

5. McCoy JL, Davis M, Hudson RE. Geographic patterns of disability in the United States. Soc Secur Bull 1994;57(no. 1):25-36.

6. McDermott S. Explanatory model to describe school district prevalence rates for mental retardation and learning disabilities. Am J Ment Retard 1994;99:175-85.

7. Drews CD, Yeargin-Alsopp M, Decoufle P, Murphy CC. Variation in the influence of selected socio-demographic risk factors for mental retardation. Am J Public Health 1995;85:329-34.

8. Decoufle P, Yeargin-Alsopp M, Boyle CA, Doernberg NS. Developmental disabilities. In: Wilcox LS, Marks JS, eds. From data to action -- CDC's public health surveillance for women, infants, and children. Atlanta, Georgia: US Department of Health and Human Services, Public Health Service. (CDC monograph).

• SSA data for children also were available but were not included in this analysis because a child's eligibility is, in part, based on total household income. For persons aged greater than or equal to 18 years, eligibility is based on that person's own income without regard to family assets. Therefore, persons with MR are more likely to be included in the SSA database after they reach age 18 years.

  
  
  Table_1
  Note: To print large tables and graphs users may have to change their printer settings to landscape and use a small font size.
  

  
  TABLE 1. Prevalence rate * of mental retardation, by state --
  United States, 1993 +
  ==================================================================
                          Children          Adults
  Region/State         aged 6-17 yrs    aged 18-64 yrs  Total
  ------------------------------------------------------------------
  New England
   Connecticut              7.1             5.1        5.5
   Maine                    6.2             7.1        6.9
   Massachusetts           13.8             5.1        6.7
   New Hampshire            4.0             3.7        3.8
   Rhode Island             5.9             5.9        5.9
   Vermont                 11.8             6.6        7.7
   Total                    9.8             5.3        6.1
  
  Mid Atlantic
   New Jersey               3.2             4.6        4.3
   New York                 5.7             6.2        6.1
   Pennsylvania            14.0             6.4        7.9
   Total                    7.9             5.9        6.3
  
  East North Central
   Illinois                10.4             6.6        7.5
   Indiana                 17.8             7.6        9.9
   Michigan                10.3             7.7        8.2
   Ohio                    22.5             8.7       11.7
   Wisconsin                4.6             6.9        6.4
   Total                   13.8             7.6        8.9
  
  West North Central
   Iowa                    21.2             7.3       10.5
   Kansas                  10.9             6.8        7.7
   Minnesota               11.1             5.1        6.5
   Missouri                12.5             8.8        9.6
   Nebraska                15.3             4.7        7.2
   North Dakota             8.9             6.9        7.4
   South Dakota             9.3             5.9        6.7
   Total                   13.3             6.8        8.3
  
  South Atlantic
   District
    of Columbia            13.5             6.0        7.1
   Delaware                14.4             7.4        8.8
   Florida                 14.8             4.8        6.8
   Georgia                 20.0             8.4       10.9
   Maryland                 6.5             4.8        5.2
   North Carolina          19.5             8.9       11.0
   South Carolina          21.9             8.3       11.3
   Virginia                11.8             6.1        7.2
   West Virginia           21.1            15.7       16.9
   Total                   10.4             6.9        7.6
  
  East South Central
   Alabama                 31.4            11.2       15.7
   Kentucky                25.5            13.5       16.2
   Mississippi             12.9            13.9       13.7
   Tennessee               14.3            11.9       12.4
   Total                   21.3            12.4       14.4
  
  West South Central
   Arkansas                23.1            10.4       13.4
   Louisiana               12.9            12.5       12.6
   Oklahoma                19.7             6.4        9.5
   Texas                    6.4             5.0        5.3
   Total                   10.4             6.8        7.6
  
  Mountain
   Arizona                  7.7             4.3        5.1
   Colorado                 4.2             4.7        4.6
   Idaho                   12.1             5.0        6.9
   Montana                  7.1             6.2        6.4
   Nevada                   6.6             3.4        4.0
   New Mexico               5.6             5.8        5.7
   Utah                     6.9             4.3        5.1
   Wyoming                  5.7             5.0        5.2
   Total                    6.8             4.7        5.2
  
  Pacific
   Alaska                   5.0             2.5        3.0
   California               4.5             4.5        4.5
   Hawaii                   8.0             3.2        4.2
   Oregon                   7.6             5.5        6.0
   Washington               8.3             5.1        5.8
   Total                    5.3             4.6        4.7
  
  Total                    11.4             6.6        7.6
  ------------------------------------------------------------------
  * Per 1000 population.
  + For children, the analysis was based on data in reports from the
    Department of Education and for adults, on data from the Social
    Security Administration.
  ==================================================================
  

  
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