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Defining intellectual disability

This section will focus on intellectual disabilities – sometimes called learning disabilities and formerly referred to as “mental retardation”.

This section will focus on intellectual disabilities – sometimes called learning disabilities and formerly referred to as “mental retardation”. Throughout this course we will use the term intellectual disability or ID.

Definition of ID

Individuals with ID have deficits in intellectual and adaptive functioning, which are observed during development (generally, before the age of 18). Intellectual functioning includes the ability to reason, solve problems, plan, think abstractly, exercise judgement, and learn. Adaptive functioning refers to the skills needed to live in an independent and responsible manner, including communication, social skills, and self-help skills (for example, getting dressed, feeding, money management, and shopping).


Estimates of how many people have a diagnosis of ID tend to be in the region of 1-2%. A 2011 meta-analysis of international studies found the ID prevalence of individuals across the lifespan to be 1%, the rate was 1.8% when only considering children and adolescents (ie not adults). The Foundation for People with Learning Disabilities (or ID) estimates 1.5 million people in England have an ID.

Sex Differences

Based on data from the 2011 meta-analysis of international studies, the female-to-male ratio of children and adolescents with ID varied between 0.4 and 1.0 (ie 4 females to 10 males with ID).

Probable causes

ID can result when something interferes with brain development although specific causes are only established in 1/3 of individuals living with ID. Causes include:

  • an injury to the brain/head
  • problems during pregnancy/birth
  • genetic conditions such a Fragile X syndrome, Down Syndrome or Williams Syndrome

There are also higher rates of ID found with some developmental disabilities (eg autism) and certain medical conditions (eg epilepsy).


It is difficult to quantify the costs of ID but an estimate in the USA was an average lifetime cost per person of around one million dollars. Greater costs are associated with increased severity and more challenging behaviours.

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