Autism and Intellectual Disability: Two Distinct Things That Can Co-Occur

Autism and intellectual disability are frequently confused with each other, but they are distinct conditions, measured differently, that can occur separately or together. Understanding the difference matters for getting the right support and avoiding assumptions that don’t fit a specific person.

They are not the same thing

Autism is defined by differences in social communication and the presence of restricted or repetitive behaviors and interests. Intellectual disability is defined by limitations in intellectual functioning (reasoning, learning, problem-solving) and adaptive behavior (everyday practical and social skills), typically identified through standardized assessment. A person can have either one without the other, or both together.

How common is true co-occurrence

Historical estimates suggested the large majority of autistic people also had an intellectual disability, in the 1980s, figures as high as 69 percent were reported. As autism diagnostic criteria have become more refined and better understood, more recent research puts co-occurring intellectual disability at closer to 25 to 30 percent of autistic people, a meaningful and important shift, not just a statistical footnote. This change reflects better diagnostic precision, not a change in autism itself.

Why the two get confused so often

Standard intelligence assessments often assume strong verbal communication and comfortable social engagement with the person administering the test, both areas where autistic people frequently have a different developmental trajectory, independent of actual cognitive ability. This means an autistic person’s intelligence can be significantly underestimated by a test that wasn’t designed with their communication style in mind, leading to inaccurate assumptions about intellectual disability where none may exist.

Why presuming competence matters here specifically

Presuming competence, assuming a person has the capacity to think, understand, and learn, is essential when working with autistic people, people with intellectual disability, and people who have both. Communication differences, including being nonspeaking or minimally speaking, are not reliable indicators of intellectual ability, and assuming otherwise has historically led to people being denied access to appropriate education and the assumption that they cannot understand what’s happening around them, an assumption research and lived experience have repeatedly shown to be wrong.

What accurate assessment requires

A thorough evaluation should consider communication style and access needs directly, rather than scoring communication differences as evidence of lower cognitive ability. This may mean using assessment tools designed for nonspeaking or minimally speaking individuals, allowing extra processing time, or using a person’s preferred communication method (including AAC) during testing itself, not only outside of it.

Support that fits the actual person

Two autistic people with the same diagnosis, with or without intellectual disability, can have completely different support needs, strengths, and ways of engaging with the world. Support plans should be built around the specific individual’s actual profile, not a generic template based on either label alone.

Key words to know

Intellectual disability: A diagnosis involving limitations in both intellectual functioning and adaptive (everyday practical and social) skills.

Adaptive behavior: Practical, everyday skills like communication, self-care, and social functioning, assessed alongside cognitive testing.

Presuming competence: Assuming a person has the capacity to think, understand, and learn, regardless of how they communicate.