Intellectual Disability

Incidence of ID refers to the number of new cases identified in a specified time period. Prevalence of ID refers to the number of people who are living with the condition in a given time period. The incidence/prevalence data in this section pertain to U.S.-based populations.

The determination of incidence and prevalence of ID is complicated because researchers of ID do not use the same operational definition when selecting and identifying individuals with ID. In some cases, an IQ cutoff score is used (e.g., 70) as a criterion for diagnosis, whereas in other cases, the diagnostic criteria are more qualitative in nature (e.g., onset in childhood with limitations in adaptive behavior and intellectual functioning). Variations in study design, terminology definitions, sample size and characteristics, and diagnostic tools can also affect incidence and prevalence data. For example, some study authors use the terms intellectual disability and developmental disability interchangeably, the latter of which can include conditions like ASD and developmental language disorder. Keep these factors in mind when reviewing the incidence and prevalence data below.

Overall Population

  • A 2011 meta-analysis of international studies found the ID prevalence of individuals across the life span to be 10.37/1000 or 1.04% (Maulik, Mascarenhas, Mathers, Dua, & Saxena, 2011).
  • A follow-up meta-analysis of international studies, extending the work of Maulik et al. (2011), found the ID prevalence of children/adolescents and adults to range from .05 to 1.55% (McKenzie, Milton, Smith & Ouellette-Kuntz, 2016).

Children and Adolescents: Overall

  • Data from the 2011 meta-analysis of international studies found the ID prevalence of children and adolescents to be 18.3/1000 or 1.83% (Maulik et al., 2011).
  • The follow-up meta-analysis of international studies reported the prevalence range for children and adolescents to be from 0.22 to 1.55% (McKenzie et al., 2016).
  • Data from the 2006-2010 National Health Interview Survey indicated that ID prevalence in children under 18 years of age in the United States was 0.5% (Schieve et al., 2012).
  • The 2009/2010 National Survey of Children With Special Health Care Needs revealed that approximately 5.8% of children aged 2-17 years in the United States had an ID.
  • In a 2010 U.S. Census Bureau study, ID was diagnosed in approximately 154,000 (0.4%) children under 15 years of age (Brault, 2012).
  • During the 2014-2015 school year, approximately 0.12% of 3- to 5-year-old children and 0.62% of 6- to 21-year-old students who were served under the Individuals with Disabilities Education Act, Part B, had a diagnosis of ID (U.S. Department of Education, 2015).

Children and Adolescents: Gender

  • Based on data from the 2011 meta-analysis of international studies, the female-to-male ratio of children and adolescents with ID varied between 0.4 and 1.0 (i.e., four to 10 females with ID for every 10 males with the condition; Maulik et al., 2011; Maulik, Mascarenhas, Mathers, Dua, & Saxena, 2013).

Children and Adolescents: Co-Morbidities

  • The Autism and Developmental Disabilities Monitoring (ADDM) Network of the Centers for Disease Control (CDC) noted a prevalence of 4 per 1,000 children aged 8 years with ASD who also had ID (Christensen et al., 2016).
  • The ADDM report on the prevalence of co-occurring ASD and ID also revealed a greater male-to-female prevalence ratio of 3.7 males to 1.0 female in children 8 years old (Christensen et al., 2016).
  • Data from the 2009-2010 Annual Survey of Deaf and Hard of Hearing Children and Youth revealed an ID prevalence of 15.5% in children who are deaf with ASD and 8.2% in children who are deaf without ASD (Szymanski, Brice, Lam, & Hotto, 2012).

Children and Adolescents: Co-Morbidity And Race/Ethnicity

  • Co-morbid ASD and ID was significantly lower in non-Hispanic White children (3.3 per 1,000) than non-Hispanic Black children (5.8 per 1,000) who were 8 years old (Christensen et al., 2016).

Adults: Overall

  • Data from the 2011 meta-analysis of international studies found the ID prevalence of adults to be 4.94/1,000 or .49% (Maulik et al., 2011).
  • The follow-up meta-analysis of international studies found the ID prevalence of adults to range from .05 to .08% (McKenzie et al. 2016).
  • A study by the U.S. Census Bureau revealed that approximately 1.2 million (0.5%) civilian, noninstitutionalized adults had ID in 2010 (Brault, 2012).

Adults: Gender

  • Based on data from the 2011 meta-analysis of international studies, the female-to-male ratio of adults with ID varied between 0.7 and 0.9 (i.e., seven to nine females with ID for every 10 males with the condition; Maulik et al., 2011; Maulik et al., 2013).

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