Mental functions and intellectual disability

Why it matters

Cognitive and intellectual conditions are among the most heterogeneous categories in the BoK. They affect how people understand, remember, and act on information — which means web content, interfaces, and printed materials all carry potential barriers. Understanding mental functions and their variations is foundational before examining specific conditions like dyslexia or ADHD. (BoK p. 20)

Mental functions

The BoK identifies several broad categories of mental function relevant to disability: (BoK p. 20)

• Global mental functions: consciousness, orientation, sleep, temperament, energy/drive.
• Specific mental functions: attention, memory, language, calculation, higher-level cognition (reasoning, abstraction), perception, thought (including executive function), sequencing, and time-based functions.

These functions can be affected by internal factors — neurological differences, genetic conditions, or brain injury — or by external factors, including environmental stress, inadequate education, or socioeconomic disadvantage. The biopsychosocial model (earlier lesson) applies directly: cognitive function sits at the intersection of biology, psychology, and social environment.

Working memory

Working memory — the ability to hold and use a limited amount of information in mind — is particularly relevant to web accessibility. Interfaces that require users to remember large amounts of information from one screen to another, or that impose complex multi-step tasks without progress indicators, create barriers for users with reduced working memory capacity. (BoK p. 21)

Intellectual disability

Intellectual disability (ID) is defined by the DSM-5 using three criteria, all of which must be present: (BoK p. 21)

1. Significant limitations in intellectual functioning — typically an IQ score approximately two or more standard deviations below the mean (IQ ~70 or below), but the BoK emphasizes that IQ alone is insufficient.
2. Significant limitations in adaptive behavior — covering conceptual skills (literacy, numeracy, time), social skills (interpersonal communication), and practical skills (self-care, work, health management).
3. Onset during the developmental period — before age 18.

Causes and range of severity

Intellectual disability spans a wide range of severity, from mild to profound. Causes include chromosomal conditions (Down syndrome), genetic disorders, prenatal exposures (fetal alcohol syndrome), birth complications, and early childhood brain injury or infections. The cause often coexists with other conditions — sensory, physical, or behavioral.

Adaptive behavior: the key complement to IQ

The DSM-5’s inclusion of adaptive behavior as a required criterion reflects the functional solutions model: the question is not only what score a person gets on an IQ test, but how well they can manage daily life. Adaptive behavior includes skills that can be improved through instruction and supported environments — the distinction between intellectual potential and daily functioning matters for accessibility and support planning. (BoK p. 21)

Real-world example

A state agency redesigns its benefits portal. Users with intellectual disability may have difficulty reading complex sentences, navigating multi-step processes without guidance, and remembering account details between pages. Plain language, step-by-step navigation with clear progress indicators, and easy-to-reach help contacts reduce these barriers — without removing value for any other user.