Challenges in Measurement: The Prevalence of Intellectual Disabilities across the Life Course

Background/Purpose:  Individuals with intellectual disabilities have long been considered a particularly vulnerable population. Social workers play a crucial role in connecting these individuals to needed services and supports, giving them the opportunity for increased wellness as well as meaningful community participation. One of the challenges to supporting individuals with intellectual disabilities across the lifespan is the lack of data about the prevalence of intellectual disability, particularly as it relates to older age cohorts.  In the absence of valid and appropriate survey measures, this exploratory study uses current survey and publically-available administrative data to examine trends over time; we aim to provide estimates of the prevalence of intellectual disability for individuals who receive specific federal benefits and services. Given the over-representation of children and adolescents in available data, we focus on the prevalence of intellectual disability for older age cohorts.

 Methods:  We use publically-available data from two federal agencies. Specifically, we use data from the Social Security Administration (SSA) and the Department of Education.

 SSA: We use data from four waves of the National Beneficiary Survey (NBS) (2004, 2005, 2006, 2010) to examine individuals with a primary diagnosis of intellectual disability by age cohort.  We also use microdata files to determine the number of Supplemental Security Income (SSI) recipients in 2010 with a primary diagnosis of intellectual disability by age group.

Department of Education: We use data from 2005 to present to identify individuals who received special education services under the Individual with Disabilities Education Act (IDEA). By providing an estimate of prevalence in a younger cohort, we aim to provide useful evidence to help shape services and policies that will support these individuals as they age.

For the SSI and IDEA files, we combine data with population estimates from the US Census Bureau to approximate population trends in the number of individuals with intellectual disabilities who receive services.

Results:  Our study is indicative of the larger difficulty in measuring prevalence and trends in intellectual disability across the lifespan, particularly among middle-aged and older adults.  Though individuals remain “counted” in public data while they receive services through the special education system, many disappear from data sources during adulthood.  Our findings provide some indication that this may be particularly concerning.  Data from all waves of the NBS indicate that approximately 80% of the beneficiary population with intellectual disabilities are over age 25, and, in later waves as many as 48% may be above age 40.

Conclusions/Implications: For practitioners and researchers invested in issues that affect individuals with intellectual disability, it is crucial to have some base of knowledge about the number of individuals affected, services and policies they may rely upon, and how this may vary across the lifespan. This study provides an important first step in gathering evidence of intellectual disability. Most importantly, this work underscores the dire need for an interdisciplinary push toward a population-based survey of this community. Without data, social work cannot hope to solve the grand challenge of ensuring social justice and equity for this marginalized population.