Intellectual disability

Disorder

Lifelong limits in reasoning and learning together with limits in everyday living skills, starting in childhood; support needs set its severity.

Updated Nov 13, 2026

Key points

Severity (mild, moderate, severe, profound) is judged by the level of support needed in adaptive functioning, not by IQ alone.
Measured intelligence is usually about two standard deviations or more below the mean, allowing for test error; see intelligence testing.
Down syndrome is the most common chromosomal abnormality, fragile X the most common inherited cause and prenatal alcohol a common and preventable cause.
Fragile X testing belongs in every work-up, and boys with autism and intellectual disability should always be tested for it.
Every child with developmental delay needs hearing and vision checked; chromosomal microarray is the standard first genetic test.
Challenging behavior is often situational or a sign of physical discomfort or trouble communicating; look for these before blaming the disability.

What it is

Intellectual disability (also called intellectual developmental disorder) means deficits in intellectual functioning, such as reasoning, problem solving, abstract thinking and learning from experience, together with deficits in adaptive functioning, all beginning during the developmental period. It affects roughly 1 to 3 in 100 people, about twice as many males as females, and about 85 in 100 affected people have the mild form. Developmental regression points to another cause, such as an inborn error of metabolism, and early intervention can lessen how severe the disability becomes.

Key facts

Mild: academic skills can reach about a fourth- to fifth-grade level; some support is needed, for example with money management.
Moderate: academic skills stay around an early elementary level; relative independence in self-care can be reached.
Severe: communication is limited to single words or phrases, and the person needs constant supervision.
Profound: communication is mostly nonverbal, and a high level of support is needed for all activities of daily living.
Co-occurring conditions are common, especially ADHD, and also autism, cerebral palsy, anxiety and depression; people with mild disability are at higher risk of being manipulated by others.

Step by step

Take a full history: pregnancy, alcohol and drug exposure, birth, milestones, regression and family history.
Examine for dysmorphic features, head size, skin signs and neurological signs; plot growth.
Check hearing and vision in every child, and review the newborn screening results.
Order chromosomal microarray first, with fragile X testing; use exome sequencing when no cause is found and offer genetic counselling.
Add brain MRI when the neurological history or examination is abnormal, and metabolic tests according to the findings, such as regression.
Confirm with standardized intelligence tests given by a qualified psychologist, together with an assessment of adaptive functioning.

Causes worth knowing, with the clue that points to each

Cause	Clue	Why it matters
Down syndrome (trisomy 21)	Hypotonia, eyes angled upward at the outer edge, single palmar crease, heart defect in about half	Most common chromosomal abnormality; Alzheimer disease at an early age
Fragile X syndrome	Large, protruding ears, large testes after puberty, social anxiety and autistic features	Most common inherited cause; CGG repeat in FMR1, X-linked
Fetal alcohol syndrome	Alcohol exposure during pregnancy	Most common toxic prenatal cause; common and preventable
Inborn errors of metabolism, such as phenylketonuria	Developmental regression, unusual urine odor	Review newborn screening results; metabolic tests according to the findings
Congenital infections (CMV, rubella, toxoplasma, Zika)	Microcephaly with Zika; intracranial calcifications with toxoplasma or CMV	Vaccines have all but eliminated congenital rubella
Prader-Willi and Angelman syndromes	Phenotypic features on examination	Imprinting disorders; found with genetic testing
Lead poisoning	History of exposure	A postnatal, preventable cause; screening for lead exposure is often appropriate
Perinatal asphyxia and prematurity	Difficult birth or very premature birth	Raise the risk; cerebral palsy may come with it

Clinical notes

Treat any treatable cause that is found, such as hydrocephalus on imaging, and screen for lead exposure when appropriate.
Early intervention and special education in the least restrictive, most inclusive setting possible give the best results.
A psychologist plans behavioral interventions for challenging behavior, together with changes to the environment.
Psychoactive medicines used without behavioral therapy and changes to the environment rarely help; treat co-occurring conditions such as ADHD.
Explain things simply and check understanding; many people need assistance with health care and legal decisions.
Support the family and plan the transition to adult services, with supported living and work settings.

Test yourself

Think of the answer, then tap the question to see it.

What sets the severity level of intellectual disability?

The level of support the person needs in adaptive functioning, not the IQ score alone.

Which common cause of intellectual disability is preventable?

Prenatal alcohol exposure (fetal alcohol syndrome).

Which cause of intellectual disability is the most common inherited one?

Fragile X syndrome, an X-linked CGG repeat expansion in the FMR1 gene.

Which two tests should every child with developmental delay have regardless of other findings?

A hearing test and a vision check, which should be done in every child with developmental delay.

An adult with severe intellectual disability starts hitting himself. What should come first?

Look for physical discomfort from a coexisting medical problem, or a change in his situation, before attributing the behavior to the disability.

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Connections

← All of behavioral science

Educational content on behavior and the clinical relationship; not a diagnostic or treatment guide and no substitute for assessment by a health professional. If you or someone near you is at risk, seek help now from an emergency service or a mental-health helpline.

Sources: Intellectual Disability (MSD Manual Professional Edition, 2026); A clinical primer on intellectual disability (Translational Pediatrics (PubMed Central), 2020); Down Syndrome (Trisomy 21) (MSD Manual Professional Edition, 2026); Fragile X Syndrome (MSD Manual Professional Edition, 2026).

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