Direct answer
Approximately 31% of autistic individuals also have an intellectual disability (IQ below 70) according to CDC 2023 prevalence data. In Ontario, dual diagnosis requires assessment by a psychologist using both autism diagnostic tools (ADOS-2) and cognitive measures (WISC-V or Leiter-3). Children with dual diagnosis qualify for both OAP services and developmental services through the Ministry of Children, Community and Social Services. At age 18, they may transition to adult developmental services under the Services and Supports to Promote the Social Inclusion of Persons with Developmental Disabilities Act.
Start with the short answer, then reveal deeper context where helpful.
Diagnosing autism in the presence of intellectual disability requires specialized expertise. Standard autism assessment tools (ADOS-2) have modules designed for individuals with limited language. Cognitive assessment must use appropriate tools — the Leiter-3, a nonverbal intelligence test, may be more appropriate than the WISC-V for children with limited expressive language. Adaptive behaviour assessment (Vineland-3) is essential for both diagnoses.
Children with dual diagnosis in Ontario can access both OAP clinical services and developmental services simultaneously. OAP provides ABA, speech therapy, and occupational therapy. Developmental Services Ontario (DSO) coordinates access to residential, employment, day program, and respite services for individuals with developmental disabilities.
Diagnosing autism in the presence of intellectual disability requires specialized expertise. Standard autism assessment tools (ADOS-2) have modules designed for individuals with limited language. Cognitive assessment must use appropriate tools — the Leiter-3, a nonverbal intelligence test, may be more appropriate than the WISC-V for children with limited expressive language. Adaptive behaviour assessment (Vineland-3) is essential for both diagnoses.
Diagnostic overshadowing is a significant risk: clinicians may attribute autism-related social communication differences to intellectual disability alone, or vice versa. Both conditions must be independently assessed. The DSM-5 requires that social communication deficits exceed what would be expected based on cognitive level alone for a dual autism diagnosis.
Children with dual diagnosis in Ontario can access both OAP clinical services and developmental services simultaneously. OAP provides ABA, speech therapy, and occupational therapy. Developmental Services Ontario (DSO) coordinates access to residential, employment, day program, and respite services for individuals with developmental disabilities.
The transition from child to adult services at age 18 is a critical planning point. Adult developmental services have their own waitlists and eligibility criteria. Passport Program funding replaces OAP at 18 for eligible individuals. Begin transition planning by age 14: connect with DSO, apply for ODSP, consider guardianship or supported decision-making options, and develop a person-directed plan.
CDC
Centers for Disease Control and Prevention — ADDM Network Autism Prevalence Report (2023)
Ontario
Services and Supports to Promote the Social Inclusion of Persons with Developmental Disabilities Act, S.O. 2008, c. 14
A comprehensive autism assessment includes developmental history, standardized testing (ADOS-2, ADI-R), cognitive assessment, and clinical observation.
DSM-5 classifies autism into 3 support levels. Level 1 requires support, Level 2 substantial support, Level 3 very substantial support. Learn the differences.
Children age out of OAP at 18 with no automatic transfer to adult services. Learn about the transition gap, Passport, ODSP, and planning ahead.
Commitment to Accuracy: Our data is verified against official government reports (FAO, MCCSS), peer-reviewed scientific literature, and accessible public records. Last updated: March 24, 2026.
Written by Spencer Carroll
Founder & Autism Advocate