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Intellectual Disability


Stephen Brian Sulkes

, MD, Golisano Children’s Hospital at Strong, University of Rochester School of Medicine and Dentistry

Reviewed/Revised Feb 2022
Topic Resources

Intellectual disability is characterized by significantly subaverage intellectual functioning (often expressed as an intelligence quotient < 70 to 75) combined with limitations of adaptive functioning (ie, communication, self-direction, social skills, self-care, use of community resources, and maintenance of personal safety), along with demonstrated need for support. Management consists of education, family counseling, and social support.

Intellectual disability is considered a neurodevelopmental disorder. Neurodevelopmental disorders are neurologically based conditions that appear early in childhood, typically before school entry, and impair development of personal, social, academic, and/or occupational functioning. They typically involve difficulties with the acquisition, retention, or application of specific skills or sets of information. Neurodevelopmental disorders may involve dysfunction in one or more of the following: attention, memory, perception, language, problem-solving, or social interaction. Other common neurodevelopmental disorders include attention-deficit/hyperactivity disorder Attention-Deficit/Hyperactivity Disorder (ADD, ADHD) Attention-deficit/hyperactivity disorder (ADHD) is a syndrome of inattention, hyperactivity, and impulsivity. The 3 types of ADHD are predominantly inattentive, predominantly hyperactive/impulsive... read more , autism spectrum disorders Autism Spectrum Disorders Autism spectrum disorders are neurodevelopmental disorders characterized by impaired social interaction and communication, repetitive and stereotyped patterns of behavior, and uneven intellectual... read more , and learning disorders Overview of Learning Disorders Learning disorders are conditions that cause a discrepancy between potential and actual levels of academic performance as predicted by the person’s intellectual abilities. Learning disorders... read more (eg, dyslexia Dyslexia Dyslexia is a general term for primary reading disorder. Diagnosis is based on intellectual, educational, speech and language, medical, and psychologic evaluations. Treatment is primarily educational... read more ).

Intellectual disability must involve early-childhood onset of deficits in both of the following:

  • Intellectual functioning (eg, in reasoning, planning and problem solving, abstract thinking, learning at school or from experience)

  • Adaptive functioning (ie, ability to meet age- and socioculturally appropriate standards for independent functioning in activities of daily life)

Basing severity on IQ alone (eg, mild, 52 to 70 or 75; moderate, 36 to 51; severe, 20 to 35; and profound, < 20) is inadequate. Classification must also account for the level of support needed, ranging from intermittent to ongoing high-level support for all activities. Such an approach focuses on a person’s strengths and needs, relating them to the demands of the person’s environment and the expectations and attitudes of the family and community.

About 3% of the population functions at an IQ of < 70, which is at least 2 standard deviations below the mean IQ of the general population (IQ of 100); if the need for support is considered, only about 1% of the population has severe intellectual disability. Severe intellectual disability occurs in families from all socioeconomic groups and educational levels. Less severe intellectual disability (requiring intermittent or limited support) occurs most often in lower socioeconomic groups, paralleling with observations that IQ correlates best with success in school and socioeconomic status rather than specific organic factors. Nevertheless, recent studies suggest that genetic factors play roles even in milder cognitive disabilities.

Etiology of Intellectual Disability

Intelligence is both genetically and environmentally determined. Children born to parents with intellectual disability are at increased risk of a range of developmental disabilities, but clear genetic transmission of intellectual disability is unusual. Although advances in genetics, such as chromosomal microarray analysis and whole genome sequencing of the coding regions (exome), have increased the likelihood of identifying some causes of intellectual disability, a cause of intellectual disability in a specific person often cannot be identified. A cause is most likely to be identified in severe cases. Deficits in language and personal–social skills also may be due to emotional problems, environmental deprivation, learning disorders Overview of Learning Disorders Learning disorders are conditions that cause a discrepancy between potential and actual levels of academic performance as predicted by the person’s intellectual abilities. Learning disorders... read more , or deafness Hearing Impairment in Children Common causes of hearing loss are genetic defects in neonates and ear infections and cerumen in children. Many cases are detected by screening, but hearing loss should be suspected if children... read more Hearing Impairment in Children rather than intellectual disability.

Prenatal factors

A number of chromosomal anomalies and genetic metabolic and neurologic disorders can cause intellectual disability ( see Table: Some Chromosomal and Genetic Causes of Intellectual Disability* Some Chromosomal and Genetic Causes of Intellectual Disability* Some Chromosomal and Genetic Causes of Intellectual Disability* ).

Congenital infections Overview of Neonatal Infections Neonatal infection can be acquired In utero transplacentally or through ruptured membranes In the birth canal during delivery (intrapartum) From external sources after birth (postpartum) Common... read more that can cause intellectual disability include rubella Congenital Rubella Congenital rubella is a viral infection acquired from the mother during pregnancy. Signs are multiple congenital anomalies that can result in fetal death. Diagnosis is by serology and viral... read more and those due to cytomegalovirus Congenital and Perinatal Cytomegalovirus Infection (CMV) Cytomegalovirus infection may be acquired prenatally or perinatally and is the most common congenital viral infection. Signs at birth, if present, are intrauterine growth restriction, prematurity... read more Congenital and Perinatal Cytomegalovirus Infection (CMV) , Toxoplasma gondii Congenital Toxoplasmosis Congenital toxoplasmosis is caused by transplacental acquisition of Toxoplasma gondii. Manifestations, if present, are prematurity, intrauterine growth restriction, jaundice, hepatosplenomegaly... read more Congenital Toxoplasmosis , Treponema pallidum Congenital Syphilis Congenital syphilis is a multisystem infection caused by Treponema pallidum and transmitted to the fetus via the placenta. Early signs are characteristic skin lesions, lymphadenopathy... read more Congenital Syphilis , herpes simplex virus Neonatal Herpes Simplex Virus (HSV) Infection Neonatal herpes simplex virus infection is usually transmitted during delivery. A typical sign is vesicular eruption, which may be accompanied by or progress to disseminated disease. Diagnosis... read more Neonatal Herpes Simplex Virus (HSV) Infection , or HIV Human Immunodeficiency Virus (HIV) Infection in Infants and Children Human immunodeficiency virus (HIV) infection is caused by the retrovirus HIV-1 (and less commonly by the related retrovirus HIV-2). Infection leads to progressive immunologic deterioration and... read more Human Immunodeficiency Virus (HIV) Infection in Infants and Children . Prenatal Zika virus Zika Virus (ZV) Infections The Zika virus is a mosquito-borne flavivirus that is antigenically and structurally similar to the viruses that cause dengue, yellow fever, and West Nile virus. Zika virus infection is typically... read more infection causes congenital microcephaly Microcephaly Microcephaly is a head circumference < 2 standard deviations below the mean for age. (See also Introduction to Congenital Craniofacial and Musculoskeletal Disorders and Overview of Congenital... read more Microcephaly and associated severe intellectual disability.

Severe undernutrition during pregnancy may affect fetal brain development, resulting in intellectual disability.

Perinatal factors

Complications related to prematurity Preterm Infants An infant born before 37 weeks of gestation is considered preterm. In 2021 in the United States, 10.48% of births were preterm ( 1), and in 2018, 26.53% of births were early term (significantly... read more , central nervous system bleeding Intracranial Hemorrhage in Neonates The forces of labor and delivery occasionally cause physical injury to the infant. The incidence of neonatal injury resulting from difficult or traumatic deliveries is decreasing due to increasing... read more Intracranial Hemorrhage in Neonates , periventricular leukomalacia, breech Breech presentation Abnormal fetal lie or presentation may occur due to fetal size, fetal anomalies, uterine structural abnormalities, multiple gestation, or other factors. Diagnosis is by examination or ultrasonography... read more or high forceps delivery, a multifetal pregnancy Multifetal Pregnancy Multifetal pregnancy is presence of > 1 fetus in the uterus. Multifetal (multiple) pregnancy occurs in up to 1 of 30 deliveries. Risk factors for multiple pregnancy include Ovarian stimulation... read more , placenta previa Placenta Previa Placenta previa is implantation of the placenta over or near the internal os of the cervix. It typically manifests as painless vaginal bleeding after 20 weeks gestation; the source of bleeding... read more , preeclampsia Preeclampsia and Eclampsia Preeclampsia is new-onset or worsening of existing hypertension with proteinuria after 20 weeks gestation. Eclampsia is unexplained generalized seizures in patients with preeclampsia. Diagnosis... read more , and perinatal asphyxia may increase the risk of intellectual disability. The risk is increased in small-for-gestational-age infants Small-for-Gestational-Age (SGA) Infant Infants whose weight is < the 10th percentile for gestational age are classified as small for gestational age. Complications include perinatal asphyxia, meconium aspiration, polycythemia... read more ; intellectual impairment and decreased weight share similar causes. Very low- and extremely low-birth-weight infants have variably increased chances of having intellectual disability, depending on gestational age, perinatal events, and quality of care.

Postnatal factors


Symptoms and Signs of Intellectual Disability

The primary manifestations of intellectual disability are

  • Slowed acquisition of new knowledge and skills

  • Immature behavior

  • Limited self-care skills

Some children with mild intellectual disability may not develop recognizable symptoms until preschool age. However, early identification is common among children with moderate to severe intellectual disability and among children in whom intellectual disability is accompanied by physical abnormalities or signs of a condition (eg, cerebral palsy Cerebral Palsy (CP) Cerebral palsy refers to a group of nonprogressive conditions characterized by impaired voluntary movement or posture and resulting from prenatal developmental malformations or perinatal or... read more ) that may be associated with a particular cause of intellectual disability (eg, perinatal asphyxia). Delayed development is usually apparent by preschool age, often manifesting more as delay in communication than in motor skills. Among older children, hallmark features are a low IQ combined with limitations in adaptive behavior skills (eg, communication, self-direction, social skills, self-care, use of community resources, maintenance of personal safety). Although developmental patterns may vary, it is much more common for children with intellectual disability to experience slow progress than developmental arrest.

Behavioral problems are the reason for most psychiatric referrals and out-of-home placements for people with intellectual disability. Behavioral problems are often situational, and precipitating factors can usually be identified. Factors that predispose to unacceptable behavior include

  • Lack of training in socially responsible behavior

  • Inconsistent limit setting

  • Reinforcement of faulty behavior

  • Impaired ability to communicate

  • Discomfort due to coexisting physical problems and mental health disorders such as depression or anxiety

In institutional settings (now uncommon in the US), overcrowding, understaffing, and lack of activities contribute to both behavior challenges and to limited functional progress. Avoidance of long-term placement in large congregate care settings is extremely important in maximizing the individual's success.

Comorbid disorders

Some children have comorbid motor or sensory impairments, such as cerebral palsy Cerebral Palsy (CP) Cerebral palsy refers to a group of nonprogressive conditions characterized by impaired voluntary movement or posture and resulting from prenatal developmental malformations or perinatal or... read more or other motor deficits, language delays, or hearing loss. Such motor or sensory impairments can mimic cognitive impairment but are not in themselves causes of it. As children mature, some develop anxiety or depression if they are socially rejected by other children or if they are disturbed by the realization that others see them as different and deficient. Well-managed, inclusive school programs can help maximize social integration, thereby minimizing such emotional responses.

Diagnosis of Intellectual Disability

  • Prenatal testing

  • Intelligence and developmental assessment

  • Imaging of the central nervous system

  • Genetic testing

Prenatal testing can be done to determine whether the fetus has abnormalities, including genetic disorders, that predispose to intellectual disability.

From birth on, growth and development Childhood Development Descriptions of development are often divided into specific domains, such as gross motor, fine motor, language, cognition, and social/emotional growth. These designations are useful, but substantial... read more , including cognitive ability, are routinely assessed at well-child visits. For suspected cases of intellectual disability, development and intelligence are assessed in greater detail, typically by early intervention or school staff.

Establishing intellectual disability is followed by efforts to determine a cause, often including central nervous system (CNS) imaging and genetic and metabolic testing. Accurate determination of the cause may provide a developmental prognosis, suggest plans for educational and training programs, help in genetic counseling, and relieve parental guilt.

Prenatal testing

Genetic counseling Prenatal Genetic Counseling Prenatal genetic counseling is provided for all prospective parents, ideally before conception, to assess risk factors for genetic disorders. In addition, prenatal counseling provides information... read more may help high-risk couples understand possible risks. If a child has intellectual disability, evaluation of the etiology can provide the family with appropriate risk information for future pregnancies.

  • Amniocentesis or chorionic villus sampling

  • Quad screen

  • Ultrasonography

  • Maternal serum alpha-fetoprotein

  • Noninvasive prenatal screening

Amniocentesis Amniocentesis Prenatal procedures that provide a definitive diagnosis of genetic disorders are invasive and involve some fetal risk. Women may choose to have prenatal procedures to know of fetal abnormalities... read more or chorionic villus sampling Chorionic Villus Sampling Prenatal procedures that provide a definitive diagnosis of genetic disorders are invasive and involve some fetal risk. Women may choose to have prenatal procedures to know of fetal abnormalities... read more may detect inherited metabolic and chromosomal disorders, carrier states, and CNS malformations (eg, neural tube defects, anencephaly) and may be considered for all pregnant women > 35 years of age because their risk of having an infant with Down syndrome is increased and for pregnant women with a family history of inherited metabolic disorders.

The quad screen (ie, measurement of maternal levels of beta-hCG, unconjugated estriol, alpha-fetoprotein, and inhibin A) is recommended for most pregnant women to evaluate risk of Down syndrome, trisomy 18, spina bifida, or abdominal wall defects.

Maternal serum alpha-fetoprotein is a helpful screen for neural tube defects, Down syndrome, and other abnormalities.

Noninvasive prenatal screening Screening Chromosomal abnormalities cause various disorders. Abnormalities that affect autosomes (the 22 paired chromosomes that are alike in males and females) are more common than those that affect... read more (NIPS) methods can be used to identify numerical chromosome abnormalities and have been used to identify some larger microdeletion syndromes such as 22q11 deletion.

Intelligence and developmental assessment

Standardized intelligence tests can measure subaverage intellectual ability but are subject to error, and results should be questioned when they do not match clinical findings; illness, motor or sensory impairments, language barriers, and/or cultural differences may hamper a child’s test performance. Such tests also have a middle-class bias but are generally reasonable in appraising intellectual ability in children, particularly in older ones.

Developmental screening tests such as the Ages and Stages Questionnaire (ASQ) or the Parents’ Evaluation of Developmental Status (PEDS) provide gross assessments of development for young children and can be given by a physician or others. Such measures should be used only for screening and not as substitutes for standardized intelligence tests, which should be given by qualified psychologists. A neurodevelopmental assessment should be initiated as soon as developmental delays are suspected.

A developmental pediatrician or pediatric neurologist should investigate all cases of

  • Moderate to severe developmental delays

  • Progressive disability

  • Neuromuscular deterioration

  • Suspected seizure disorders

Diagnosis of cause

History (including perinatal, developmental, neurologic, and familial) may identify causes. (See also the 2011 evidence report on genetic and metabolic testing on children with global developmental delay from the American Academy of Neurology and the Practice Committee of the Child Neurology Society.)

Cranial imaging (eg, MRI) can show central nervous system malformations (as seen in neurodermatoses such as neurofibromatosis or tuberous sclerosis), treatable hydrocephalus, or more severe brain malformations such as schizencephaly.

Genetic tests may help identify disorders:

Chromosomal microarray analysis is the preferred investigative tool; it can be used to identify specifically suspected syndromes and also when no specific syndrome is suspected. It affords opportunities for identifying otherwise unrecognized chromosome disruptions but requires parental testing to interpret positive findings. Whole genome sequencing Genetic Diagnostic Technologies Genetic diagnostic technology is rapidly improving. A small amount of DNA can be amplified using the polymerase chain reaction (PCR) process, which can produce millions of copies of a gene or... read more of the coding regions (whole exome sequencing) is a newer, more detailed method that may uncover additional causes of intellectual disability.

Specific laboratory tests are done depending on the suspected cause ( see Table: Tests for Some Causes of Intellectual Disability Tests for Some Causes of Intellectual Disability Tests for Some Causes of Intellectual Disability ). Visual and auditory assessments should be done at an early age, and screening for lead poisoning is often appropriate.


Prognosis for Intellectual Disability

Many people with mild to moderate intellectual disability can support themselves, live independently, and be successful at jobs that require basic intellectual skills.

Life expectancy may be shortened, depending on the etiology of the disability, but health care is improving long-term health outcomes for people with all types of developmental disabilities. People with severe intellectual disability are more likely to require life-long support. The more severe the cognitive disability and the greater the immobility, the higher the mortality risk.

Treatment of Intellectual Disability

  • Early intervention program

  • Multidisciplinary team support

Treatment and support needs depend on social competence and cognitive function. Referral to an early intervention program during infancy may prevent or decrease the severity of disability resulting from a perinatal insult. Realistic methods of caring for affected children must be established.

Family support and counseling are crucial. As soon as intellectual disability is confirmed or strongly suspected, the parents should be informed and given ample time to discuss causes, effects, prognosis, education and training of the child, and the importance of balancing known prognostic risks against negative self-fulfilling prophecies in which diminished expectations result in poor functional outcomes later in life. Sensitive ongoing counseling is essential for family adaptation. If the family’s physician cannot provide coordination and counseling, the child and family should be referred to a center with a multidisciplinary team that evaluates and serves children with intellectual disability; however, the family’s physician should provide continuing medical care and advice.

A comprehensive, individualized program is developed with the help of appropriate specialists, including educators.

A multidisciplinary team includes

Affected children with concomitant mental health disorders such as depression may be given appropriate psychoactive drugs in dosages similar to those used in children without intellectual disability. Use of psychoactive drugs without behavioral therapy and environmental changes is rarely helpful.

Every effort should be made to have children live at home or in a community-based residence. Living at home with the family is usually better for the child than alternative placements unless marked behavior difficulties require a higher level of supervision than the family can provide. The family may benefit from psychologic support and help with daily care provided by day care centers, homemakers, and respite services. The living environment must encourage independence and reinforce learning of skills needed to accomplish this goal.

Whenever possible, children with intellectual disability should attend an appropriately adapted day care center or school with peers without cognitive disability. The Individuals with Disabilities Education Act (IDEA), the primary US special education law, stipulates that all children with disabilities should receive appropriate educational opportunities and programs in the least restrictive and most inclusive environments. The Americans with Disability Act and Section 504 of the Rehabilitation Act also provide for accommodations in schools and other public settings.

As people with intellectual disability reach adulthood, an array of supportive living and work settings is available. Large residential institutions are being replaced by small group or supported individual residences matched to the individual's functional abilities and needs.

Prevention of Intellectual Disability

Fetal alcohol syndrome Fetal Alcohol Syndrome Alcohol exposure in utero increases the risk of spontaneous abortion, decreases birth weight, and can cause fetal alcohol syndrome, a constellation of variable physical and cognitive abnormalities... read more is a highly common and totally preventable cause of intellectual disability. Because it is unknown when during pregnancy alcohol is most likely to harm the fetus and whether there is a lower limit of alcohol use that is completely safe, pregnant women should be advised to avoid all alcohol intake.

Folate supplementation (400 to 800 mcg orally once a day) in women beginning 3 months before conception and continuing through the 1st trimester reduces the risk of neural tube defects (see prevention of congenital neurologic anomalies Prevention Spina bifida is defective closure of the vertebral column. Although the cause is often unknown, low folate levels during pregnancy increase risk. Some children are asymptomatic, and others have... read more ).

Continuing improvements in, and increased availability of, obstetric and neonatal care and the use of exchange transfusion and Rho(D) immune globulin to prevent hemolytic disease of the newborn have reduced the incidence of intellectual disability; the increase in survival of very low-birth-weight infants has kept the prevalence constant.

Key Points

  • Intellectual disability involves slow intellectual development with subaverage intellectual functioning, immature behavior, and limited self-care skills that in combination are severe enough to require some level of support.

  • A number of prenatal, perinatal, and postnatal factors can cause intellectual disability, but a specific cause often cannot be identified.

  • Deficits in language and personal–social skills may be due to emotional problems, environmental deprivation, learning disorders, or deafness rather than intellectual disability.

  • Screen using tests such as the Ages and Stages Questionnaire (ASQ) or the Parents’ Evaluation of Developmental Status (PEDS) and refer suspected cases for standardized intelligence testing and neurodevelopmental assessment.

  • Search for specific causes with cranial imaging, genetic tests (eg, chromosomal microarray analysis, exome sequencing), and other tests as clinically indicated.

  • Provide a comprehensive, individualized program (including family support and counseling) using a multidisciplinary team.

More Information

The following are English-language resources that may be useful. Please note that THE MANUAL is not responsible for the content of these resources.

Drugs Mentioned In This Article

Drug Name Select Trade
Dilantin, Dilantin Infatabs, Dilantin-125, Phenytek
NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
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