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Onset and Prevalence of Intellectual Disabilities

Tammy Reynolds, B.A., C.E. Zupanick, Psy.D. & Mark Dombeck, Ph.D.

Onset

Intellectual disability (ID formerly mental retardation) is not a specific disease. It is a specific disability with many causes. Therefore, there is no specific time of onset for intellectual disability. However, diagnostic criteria require childhood onset, before the age of 18. If the problems begin after age eighteen, the correct diagnosis is neurocognitive disorder. The symptoms may be identical to an intellectual disability. The difference is whether someone has lost a prior level of functioning. In this case, it is called a neurocognitive disorder (formerly dementia). If someone never developed this level of functioning in the first place, it is called an intellectual disability. It is possible to have both diagnoses. For instance, suppose a young man with an ID, suffers a head injury at age 21. If this injury caused a loss of prior functioning, then he would have both and ID and neurocognitive disorder.

Prevalence

An estimated seven to eight million Americans have an intellectual disability (ID). This means about one in ten families in the United States are affected by an ID. A 2010 study estimated that 5 million children in Europe have an intellectual disability and Australia reports that about 3% of their population has an intellectual disability and that it is the most common primary disability reported in that country.

Previously, the American Psychiatric Association's Diagnostic and Statistical Manual (DSM-IV, APA, 2000), reported 2-3% of people meet the criteria for ID. This was because the diagnostic criterion required an IQ score of approximately 70 or below. A score of 70 is two standard deviations below the average score for all people. Statistically, two standard deviations below average equal 2.5% of the population. However, DSM-5 (APA 2013) has moved away from relying on specific IQ scores. The estimates in DSM-5 report a prevalence rate of approximately one percent. IQ testing is discussed here.


Behavioral and Psychological Features of Intellectual Disabilities
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Life Expectany and Severity

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Resources

  • Articles

    • Introduction to Intellectual Disabilities
    • Causes of Intellectual Disabilities
    • Diagnosis of Intellectual Disabilities
      • The Diagnosis of Intellectual Disabilities
      • Psychological Tests and Intellectual Disabilities
      • Psychological Tests and Intellectual Disabilities Continued
      • Tests of Adaptive Functioning
      • Diagnostic Criteria for Intellectual Disabilities: DSM-5 Criteria
      • The American Association on Intellectual and Developmental Disabilities (AAIDD) Diagnostic Criteria for Intellectual Disability
      • Comparing the APA and the AAIDD Diagnostic Criteria for Intellectual Disability
      • Intellectual Disability and Severity Codes
      • Intellectual Disability and Other Psychiatric Disorders
    • Historical & Contemporary Perspectives of Intellectual Disabilities
      • Historical And Contemporary Perspectives on Intellectual Disabilities
      • Early Medical Explanations of Intellectual Disability
      • History of Stigmatizing Names for Intellectual Disabilities
      • History of Stigmatizing Names for Intellectual Disabilities Continued
      • Reducing the Stigma of Intellectual Disabilities: The Evolution of Modern Medical Explanations
      • Paving the Way to a Modern Conception of Intellectual Disability: Advancements in Intelligence Testing
      • Advancements in Genetic Research
      • Social and Political Controversies Associated with Intellectual Disabilities
      • Reproductive Rights for People with Intellectual Disabilities
    • Intellectual Disabilities & Supportive Rehabilitation
      • Intellectual Disabilities and Supportive Rehabilitation: Developing an Individualized Support Plan (ISP)
      • Educational Supports and Individual Educational Plans (IEPs)
      • The Choice of Educational Settings: The Pros and Cons of Mainstreaming Children With Intellectual Disabilities
      • Effective Teaching Methods for People With Intellectual Disabilities
      • Applied Behavioral Analysis (ABA) and Intellectual Disabilities
      • Physical Therapy and Sensory Skills Training
      • Individualized Support Plans: Adaptive Functioning & Life Skills
      • Social Skills Training
      • Supported Employment and Integrated Work Sites
      • Supported Housing and Community Integration
      • Therapies for Intellectual Disabilities and Outdated/Unproven Treatments
    • Support for Families of People with Intellectual Disabilities
      • Additional Support Services for People with Intellectual Disabilities and Their Families: Community Supports
      • Additional Support Services: Financial Supports
      • Additional Support Services: Family Supports
      • Additional Support Services: Advocacy and Legal Supports
    • Intellectual Disabilities Summary & Conclusion
      • Intellectual Disabilities Summary and Conclusion
    • Intellectual Disabilities Resources & References
      • Intellectual Disabilities Resources and References
      • Intellectual Disabilities Document Revision History
  • Book & Media Reviews

    • A Special Education
    • Living with Mild Cognitive Impairment
  • Videos

    • What is Intellectual Disability?
    • Let's Talk About Intellectual Disabilities: Loretta Claiborne
    • What Causes an Intellectual Disability?
    • What Is An Intellectual Disability?
    • What's disability to me? Mia's story.
    • Primary Care of Children and Young Adults with Down Syndrome
    • Dr. Tim Shriver: Intellectually Different ... Not Disabled
    • Living with Down Syndrome: Parents, Health Professionals and Personal Perspectives