Intellectual Disability

What intellectual disability means

Three criteria define intellectual disability:

1. Limits in intellectual functioning: typically an IQ score around 70 or below, about two standard deviations below average, on standardized testing.
2. Limits in adaptive behavior: difficulties with conceptual skills (language, reading, money, time), social skills (relationships, judgment, following rules) and practical skills (self-care, safety, travel, work).
3. Onset during the developmental period, before about age 18 to 22.

Current definitions emphasize adaptive functioning and the level of support needed rather than IQ alone.

Levels of intellectual disability

Signs at different ages

Infants and toddlers

• late sitting, crawling or walking
• delayed babbling, words or understanding
• limited interest in play or interaction for age

When a child under 5 is delayed in several areas, doctors often use the term global developmental delay, since IQ cannot be reliably measured yet.

School-age children

• learning much more slowly than peers in most subjects
• difficulty with memory, problem-solving and abstract ideas
• trouble understanding social rules or consequences
• needing help with self-care tasks that peers manage alone

Causes

Genetic

• Down syndrome: the most common genetic cause
• Fragile X syndrome: the most common inherited cause
• Williams syndrome, Angelman syndrome, Prader-Willi syndrome, Rett syndrome and many other single-gene and chromosomal conditions
• metabolic disorders such as phenylketonuria, which newborn screening detects so that diet can prevent disability

During pregnancy

• alcohol (fetal alcohol spectrum disorders) is a leading preventable cause
• infections such as cytomegalovirus, rubella, Zika and toxoplasmosis
• certain medicines and drugs
• severe malnutrition and iodine deficiency

Around birth

• very premature birth and very low birth weight
• lack of oxygen around delivery

After birth

• meningitis and encephalitis
• traumatic brain injury and near-drowning
• lead poisoning
• untreated congenital hypothyroidism
• severe neglect and malnutrition

Associated conditions

People with intellectual disability have higher rates of:

• epilepsy
• autism spectrum disorder and ADHD
• cerebral palsy
• vision and hearing problems
• anxiety, depression and other mental health conditions, which may show up as behavior changes
• thyroid problems, heart defects and other conditions related to specific syndromes

Diagnosis

• developmental screening at well-child visits
• standardized intelligence tests and adaptive behavior assessments by a psychologist
• hearing and vision tests
• genetic testing: chromosomal microarray and fragile X testing are first-line. Exome sequencing finds a cause in many more children
• metabolic tests, brain MRI and EEG when there are specific features such as seizures or regression

Finding a cause can guide medical care, connect families with support groups and inform family planning.

Support and treatment

There is no cure, but support helps people reach their potential:

• early intervention services from birth to age 3 (in the US, through IDEA Part C)
• special education and an individualized education program (IEP) at school
• speech, occupational and physical therapy
• behavior support using positive approaches
• treatment of associated conditions, such as seizures, mental health problems and sensory impairments
• family support, respite care and parent training
• transition planning from about age 14 for adult life: vocational training, supported employment and housing options
• supported decision-making or guardianship arrangements when needed

Regular health checks matter because people with ID may not report pain or symptoms in typical ways.

Outlook

Most people with mild intellectual disability live independently or semi-independently, work and have relationships. People with more severe disability need ongoing support but can enjoy a good quality of life. Life expectancy has improved significantly but depends on associated conditions.

Frequently asked questions

What is intellectual disability?

Intellectual disability is a condition that starts in childhood and involves significant limits in thinking skills, such as reasoning and learning, and in everyday adaptive skills, such as communication, self-care and social skills. It ranges from mild to profound.

What causes intellectual disability?

Causes include genetic conditions like Down syndrome and fragile X syndrome, alcohol or infections during pregnancy, premature birth or birth complications, and childhood brain infections, injuries or lead poisoning. In many people, no specific cause is found.

What are the signs of intellectual disability in a child?

Signs include delays in sitting, walking and talking, slower learning than peers across many areas, difficulty with memory and problem-solving, trouble understanding social rules, and needing more help with daily tasks. A developmental evaluation can clarify the cause.

What is the difference between intellectual disability and learning disability?

In the US, intellectual disability means overall limits in intellectual and adaptive functioning. A learning disability, such as dyslexia, affects specific skills like reading or math in people with average or above-average intelligence. In the UK, "learning disability" is used to mean intellectual disability.

Can a person with intellectual disability live independently?

Many people with mild intellectual disability live independently or with some support, work and form relationships. People with moderate to profound disability usually need more ongoing help, often in supported living settings. Early support and skills training increase independence.