Intellectual Disability

What is Intellectual Disability?

Intellectual disability (ID) — also called intellectual developmental disorder in DSM-5 — is defined by significant limitations in two areas:

1. Intellectual functioning — reasoning, problem-solving, planning, abstract thinking, judgment, learning from experience — typically reflected in an IQ score below approximately 70 (more than two standard deviations below the mean)
2. Adaptive behaviour — the conceptual, social, and practical skills that people use every day to function effectively in their communities

By definition, ID originates during the developmental period (before age 18). This distinguishes it from conditions such as dementia, where intellectual function is lost after a period of typical development.

ID is classified by severity:

• Mild (IQ approx. 55–70): The large majority (about 85%) of people with ID; most can live independently or with minimal support
• Moderate (IQ approx. 40–55): May need more support for daily living and community participation
• Severe (IQ approx. 25–40): Significant support needs across most areas of daily life
• Profound (IQ below 25): Often accompanied by significant physical and sensory impairments; requires intensive support

Causes include genetic conditions (Down syndrome, Fragile X syndrome), metabolic disorders, prenatal exposure to toxins or alcohol (fetal alcohol spectrum disorder), infections during pregnancy, birth complications, and environmental causes such as severe early deprivation or lead poisoning. In many cases, no specific cause is identified.

How It Presents

ID affects different people very differently:

• People with mild ID often develop reading and communication skills and can live and work in the community with appropriate support
• Across severity levels, adaptive skills in communication, self-care, social skills, and community use vary considerably
• Co-occurring conditions are common, including epilepsy, ADHD, autism, mental health conditions, sensory impairments, and physical disabilities

The concept of adaptive functioning is as important as IQ in understanding a person's needs and strengths. People with ID are individuals with unique personalities, relationships, interests, and gifts.

Assistive Technology and Supports

• Easy Read materials — written documents reformatted with simple language, short sentences, and supporting images to improve comprehension
• AAC — for people with limited speech, picture-based or symbol-based communication systems (PECS, SymbolStix, Boardmaker)
• Visual schedules and reminder apps — support independence in daily routines
• Video modelling — video demonstrations of tasks to support skill learning
• Cognitive accessibility features — simplified UI modes, large text, and predictable navigation in digital tools
• Supported decision-making frameworks — approaches that support people with ID to make their own choices rather than having decisions made for them

Common Misconceptions

• "People with intellectual disabilities cannot make decisions." Supported decision-making is a human rights-based approach that recognises the right of people with ID to make choices with appropriate support.
• "Intellectual disability means the same level of need for everyone." ID ranges from mild (most people with ID) to profound; the experiences and needs are vastly different.
• "People with ID cannot have relationships, jobs, or meaningful lives." With appropriate support and an accessible community, people with ID participate in all aspects of community life.

Language and Identity

Person-first language ("person with an intellectual disability") is the widely accepted standard in disability services, education, and advocacy in most English-speaking countries. The R-word ("retarded" and its derivatives) is considered a slur and should never be used. Self-advocacy movements, such as People First, have been led by people with intellectual disabilities advocating for their own rights since the 1970s.