Study Lead: Marc J. Tassé, PhD

Plain Language:


Intellectual disability (ID) confers increased risk of mental illness; however, a robust prevalence study of the occurrence of mental illness in adults with intellectual disability has not been conducted in the United States. Prevalence estimates are based on previous research in the UK (Cooper, Smiley, Morrison, Williamson, & Allan, 2007; StrØmme & Diseth, 2000) and on US studies that have used convenience samples (Fletcher, Barnhill, & Cooper, 2016; Reiss, 1994; Rojahn & Tassé, 1996; Smiley, 2005;) suggesting that mental illness occurs in 30% to 50% of adults with intellectual disability. Co-occurring mental illness is associated with an increased intensity and complexity of support needs compared to adults with ID without comorbid psychiatric disorders (Borthwick-Duffy, 1994; Rojahn, Matson, Naglieri, & Mayville, 2004; Tassé & Wehmeyer, 2010). A systematic national prevalence study is needed to guide federal policy and programs to address mental health problems in adults with intellectual disability.

The RRTC on health and function will address two major barriers in this research:

  1. We will develop self-report instruments to measure health, mental health, and health related quality of life in adults with ID.
  2. We will use these instruments in a national health surveillance protocol to establish the prevalence of mental illness among adults with intellectual disability in the U.S.


Sample We will recruit a sample of N=1,945 adults (18+ years old) with intellectual disability from across the United States. We will reach this group of adults with ID through National Core Indicators surveyed individuals. Our sample will be representative of the race and ethnic diversity of the United States per the 2018 US Census data.

In the procedure to estimate prevalence of mental illness, we will leverage National Core Indicators framework and data from 5 states (MI, NC, OH, OR, VA) that also have a University Center for Excellence in Developmental Disabilities (UCEDD) involved in coordinating the NCI data collection. The identified NCI states will yield a sufficient pool of randomly selected individuals with ID for us to recruit our target sample of 1,945. Importantly, the mean age, sex, race, ethnicity, and ID level proportions will be representative of the population of adults with ID surveyed by NCI data (P>.10), thus helping ensure the generalizability of findings from this study to the entire known population of adults with ID in the US.

We will work with these states/UCEDDs to leave behind after their NCI interview/conversation a project post-card briefly describing our prevalence study and inviting the adult with ID and/or their guardian to contact the RRTC study team if they want to participate or learn more about our project. Our recruitment plan is to recruit n=800 in 2021, n=800 in 2022, and n=345 in 2023 across MI, NC, OH, OR, and VA.


Mental Illness: The Psychiatric Diagnostic Interview – Adolescents with Intellectual Disability (PDI-AID) is a comprehensive mental illness assessment that is relatively easy to administer and interpret by a qualified bachelor-level mental health professional. The PDI-AID will be validated for use with adults with ID in years 1 and 2 of the RRTC project (2018-2019 & 2019-2020). In years 3-5 (2020-2021, 2021-2022, & 2022-2023) we will conduct the prevalence study and administer the PDI-AID to 1,945 adults with ID and to their support person (parent/staff) to assess for symptoms and signs of mental illness.

We will further validate the PDI-AID diagnoses using a Diagnostic Manual for Persons with Intellectual Disability, 2nd Edition (DM-ID:2) informed comprehensive clinical assessment conducted by an experienced NADD-certified mental health professional on a subsample (n=200) of the 1,945 individuals. The comprehensive clinical assessment with a qualified mental health professional is considered the “Gold Standard” for obtaining a reliable diagnosis of mental illness. We will use experienced ID/mental health professionals, who have received the NADD Competency-Based Clinical Certification to conduct these comprehensive clinical assessments. These comprehensive clinical assessments will help establish the accuracy (i.e., validity) of the diagnostic information obtained from the PDI-AID interviews with the adult with ID and their support person (i.e., sensitivity and specificity of the mental health diagnosis).

Characterization of our sample: In addition to basic demographic information (e.g., age, sex, race, ethnicity, etc.; caregiver – 15 minutes) we will also assess every participant’s intellectual functioning and adaptive behavior. We will administer the Wechsler Abbreviated Scale of Intelligence, 2nd Edition (person assessed – 30 minutes) and the Adaptive Behavior Assessment System, 3rd Edition (caregiver – 30 minutes) on all 1,945 participants.


With a target sample size of N=1,945, we will have sufficient statistical power to be able to detect a mental illness prevalence rate (percentage of individuals with ID who also have a psychiatric disorder) with 99% confidence. Our national sample is both representative of the NCI data and the larger population of adults with intellectual and developmental disabilities receiving paid services in the United States. We will be able to report on the most frequent type of mental illness in our sample. This study will also provide prevalence estimates by sex (men/women), chronological age, and across severity levels of ID (mild, moderate, severe/profound).

Expected Outcome

The expected outcomes of the prevalence study will include a point prevalence estimate of the percentage of adults with ID who present a co-occurring mental illness, prevalence rate by sex, prevalence rate by severity level of ID, and prevalence rate by mental illness. This study will contribute a reliable and valid measure of mental illness among adults with ID. Being able to accurately and reliably identify the presence of a mental health diagnosis is the first step towards getting appropriate care, services, and supports as well as avoiding ineffective interventions and treatments. We know that untreated mental health problems can have deleterious effects on overall health and well-being; quality of life; productivity and ability to work; and opportunities to live, work, and play in the community of our choice.

See Study 1: Measure Adaptations – Intellectual Disability and Self-Report 

See Study 3: Clinical Guidelines – Intellectual Disability and Psychotherapy

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