[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Behavioral science](https://durgical.com/en/behavioral-science/) › [Other mental disorders](https://durgical.com/en/behavioral-science/areas/disorders/) › Intellectual disability

# 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)](https://durgical.com/en/conditions/down-syndrome/) | 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](https://durgical.com/en/conditions/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](https://durgical.com/en/conditions/fetal-alcohol-spectrum-disorder/) | Alcohol exposure during pregnancy | Most common toxic prenatal cause; common and preventable |
| [Inborn errors of metabolism, such as phenylketonuria](https://durgical.com/en/conditions/phenylketonuria/) | Developmental regression, unusual urine odor | Review newborn screening results; metabolic tests according to the findings |
| [Congenital infections (CMV, rubella, toxoplasma, Zika)](https://durgical.com/en/conditions/congenital-infections-syphilis-toxoplasmosis-cmv-and-zika/) | Microcephaly with Zika; intracranial calcifications with toxoplasma or CMV | Vaccines have all but eliminated congenital rubella |
| [Prader-Willi and Angelman syndromes](https://durgical.com/en/conditions/prader-willi-and-angelman-syndromes/) | Phenotypic features on examination | Imprinting disorders; found with genetic testing |
| [Lead poisoning](https://durgical.com/en/conditions/lead-poisoning/) | History of exposure | A postnatal, preventable cause; screening for lead exposure is often appropriate |
| [Perinatal asphyxia and prematurity](https://durgical.com/en/conditions/perinatal-asphyxia-and-hypoxic-ischaemic-encephalopathy/) | 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.

## Memory aid

Three causes, three Cs: Chromosome (Down), the other Chromosome X (fragile X) and the Cup of alcohol (fetal alcohol, the preventable one).

## 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.

[Add to my cards (5)](https://durgical.app/estudiar/?add=/en/behavioral-science/intellectual-disability/)

## Elsewhere in DurgiPedia

- [Alcohol: fetal alcohol spectrum disorder](https://durgical.com/en/embryology/alcohol-fetal-alcohol-spectrum-disorder/)
- [Trinucleotide repeat expansion disorders](https://durgical.com/en/biochemistry/topics/trinucleotide-repeat-expansion-disorders/)
- [Decision-making capacity (competence) and how to assess it](https://durgical.com/en/medical-ethics/decision-making-capacity-and-how-to-assess-it/)
- [Hearing loss in children and language development](https://durgical.com/en/ent/hearing-loss-in-children-and-language-development/)

## Connections

## Related 7

- [Intelligence testing and IQ](https://durgical.com/en/behavioral-science/intelligence-testing-and-iq/)
- [Psychological testing: reliability, validity and norms](https://durgical.com/en/behavioral-science/psychological-testing-reliability-validity-and-norms/)
- [Developmental milestones: domains, corrected age and red flags](https://durgical.com/en/behavioral-science/developmental-milestones-domains-corrected-age-and-red-flags/)
- [Assessing children and adolescents in mental health](https://durgical.com/en/behavioral-science/assessing-children-and-adolescents-in-mental-health/)
- [Communication disorders: language, speech sound, stuttering and social communication](https://durgical.com/en/behavioral-science/communication-disorders-language-speech-sound-stuttering-and-social-communicatio/)
- [Specific learning disorder (dyslexia, dyscalculia)](https://durgical.com/en/behavioral-science/specific-learning-disorder/)
- [Developmental coordination disorder and stereotypic movement disorder](https://durgical.com/en/behavioral-science/developmental-coordination-disorder-and-stereotypic-movement-disorder/)

## Related conditions 12

- [Down syndrome (trisomy 21)](https://durgical.com/en/conditions/down-syndrome/)
- [Fragile X syndrome](https://durgical.com/en/conditions/fragile-x-syndrome/)
- [Fetal alcohol spectrum disorder](https://durgical.com/en/conditions/fetal-alcohol-spectrum-disorder/)
- [Congenital hypothyroidism](https://durgical.com/en/conditions/congenital-hypothyroidism/)
- [Phenylketonuria (PKU)](https://durgical.com/en/conditions/phenylketonuria/)
- [Congenital (TORCH) infections: syphilis, toxoplasmosis, CMV and Zika](https://durgical.com/en/conditions/congenital-infections-syphilis-toxoplasmosis-cmv-and-zika/)
- [Lead poisoning](https://durgical.com/en/conditions/lead-poisoning/)
- [Prader–Willi and Angelman syndromes (imprinting)](https://durgical.com/en/conditions/prader-willi-and-angelman-syndromes/)
- [Autism spectrum disorder (ASD)](https://durgical.com/en/conditions/autism-spectrum-disorder/)
- [Attention-deficit/hyperactivity disorder (ADHD)](https://durgical.com/en/conditions/attention-deficit-hyperactivity-disorder/)
- [Cerebral palsy](https://durgical.com/en/conditions/cerebral-palsy/)
- [Epilepsy and seizures](https://durgical.com/en/conditions/epilepsy-and-seizures/)

[← All of behavioral science](https://durgical.com/en/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](https://www.msdmanuals.com/professional/pediatrics/learning-and-developmental-disorders/intellectual-disability) (MSD Manual Professional Edition, 2026); [A clinical primer on intellectual disability](https://pmc.ncbi.nlm.nih.gov/articles/PMC7082244/) (Translational Pediatrics (PubMed Central), 2020); [Down Syndrome (Trisomy 21)](https://www.msdmanuals.com/professional/pediatrics/chromosome-and-gene-anomalies/down-syndrome-trisomy-21) (MSD Manual Professional Edition, 2026); [Fragile X Syndrome](https://www.msdmanuals.com/professional/pediatrics/chromosome-and-gene-anomalies/fragile-x-syndrome) (MSD Manual Professional Edition, 2026).

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/behavioral-science/intellectual-disability/